New York State Invasive Species Information
Garlic mustard (Alliaria petiolata) is an invasive herb that has spread throughout much of the United States over the past 150 years, becoming one of the worst invaders of forests in the American Northeast and Midwest. While it is usually found in the undergrowth of disturbed woodlots and forest edges, recent findings have shown that garlic mustard has the ability to establish and spread even in pristine areas. This spread has allowed it to become the dominant plant in the undergrowth of some forests, greatly reducing the diversity of all species. Garlic mustard is one of very few non-native plants to be able to successfully invade forest understories.
Garlic mustard has a biennial life cycle, that is, it takes two years to fully mature and produce seeds. Seeds germinate in February to early March of the first year and grow into a short rosette by the middle of the summer. In the plant's second year, a stalk develops, flowers form, and the plant dies by June. Siliques, four-sided seedpods, develop in May, containing small black seeds lined up in a row. On average, a garlic mustard plant will produce 22 siliques, each of which can contain as many as 28 seeds. A particularly vigorous plant may produce as many as 7,900 seeds (Nuzzo, 1993) although the average is more likely to be in the 600 seed range. The seeds generally germinate within one to two years, but may remain viable for up to five years in the seed bank. Seed dispersal is mainly by humans or wildlife carrying the seeds.
Characteristics and Identification
Identification of first year plants can be difficult; the task is made easier by smelling the garlic odor produced when the leaves of the plant are crushed. The basal leaves of an immature plant are dark-green and kidney shaped with round teeth (scalloped) along the edges; average size of the leaves is 6 to 10 cm in diameter. The petiole, or leaf stalk, of first year plants are 1 to 5 cm long. In its second year, the alternating stem leaves become more triangular shaped, 1 to 5 cm long, and have sharper teeth, with leaves becoming gradually smaller towards the top of the stalk. Leaf stalks of mature plants are hairy. As with the younger plants, second year plants have a garlic odor when crushed but the odor is less obvious with increasing age.
Garlic mustard flowers arrive in early April and die by June. Flowers develop on an unbranched (occasionally weakly branched) stalk and have 4 small white petals arranged symmetrically. Flowers are approximately 6 to 7 mm in diameter with 3 to 6 mm petals. Individual flowers contains six stamens, two shorter and four longer. Mature flowering plants reach 3.5 feet tall, although shorter flowering specimens may be found.
Garlic mustard is a non-native species originating from Europe and parts of Asia. It is believed that garlic mustard was introduced into North America for medicinal purposes and food. The earliest known report of it growing in the United States dates back to 1868 on Long Island, NY. It has since spread throughout the eastern United States and Canada as far west as Washington, Utah, and British Columbia.
Garlic mustard has the potential to form dense stands that choke out native plants in the understory by controlling light, water, and nutrient resources. Plants most affected by these dense stands are herbaceous species that occur in similar moist soil forest habitats and grow during the spring and early summer season. Although unsupported by the lack of long-term research into garlic mustard impacts, the plant has been circumstantially tied to decreased native herbaceous species richness in invaded forests. Researchers have found that garlic mustard is allelopathic (it releases chemicals that hinder the growth of other plant species) and has inhibited growth of both grasses and herbs in laboratory settings (Michigan State University, 2008). Some researchers also believe that these compounds may hinder the beneficial relationships some plant species have with soil fungi (Roberts and Anderson, 2001). Experimental trials have shown that removal of garlic mustard leads to increased diversity of other species, including annuals and tree seedlings (MSU, 2008).
Other aspects of the forest ecosystem may be altered due to the change in the vegetative community tied to garlic mustard invasion. While the impacts to wildlife are not completely understood, altering the plant diversity can cause a change in leaf litter availability, potentially impacting salamanders and mollusks (MSU, 2008). Insects, including some butterflies, may be affected through the lost diversity in plants and loss of suitable egg-laying substrate (MSU, 2008). Garlic mustard may also affect the tree composition by creating a selective barrier that some seedlings, such as the chestnut oak (Quercus prinus), may not be able to overcome (MSU, 2008). These changes in tree composition could have significant long-term effects.
Prevention, Control and Management
There are few effective natural enemies of garlic mustard in North America. Herbivores, or animals that eat plant material, such as deer (Odocoileus virginianus) and woodchucks (Marmota monax) only remove up to 2% of the leaf area in a stand of garlic mustard (Evans et al. 2005). This level of herbivory is ineffective in controlling reproduction or survival of garlic mustard. Although 69 herbivorous insects have been found to be associated with garlic mustard in Europe, less than a dozen have been found on North American infestations of the species (Hinz and Gerber, 1998).
Manual removal of plant has been shown to prevent the spread of garlic mustard. Pulling by hand must remove at least the upper half of the root to prevent a new stalk from forming; this is most easily accomplished in the spring when the soil is soft. Hand-pulling should be performed before seeds are formed and needs to be continued for up to five years in order to deplete any established seed bank. This method works best in smaller pockets of invasion or in areas recently invaded to help prevent the development of a seed bank.
Chemical applications can also be effective for controlling garlic mustard, particularly in areas too large for removal by hand. In dense stands where other plant species are not present, a glyphosate-based herbicide such as Roundup® can be an effective method for removal. Glyphosate herbicides are non-selective, so caution must be used when non-target species are in the area. Chemical applications are most affective during the spring (March-April) when garlic mustard is one of the few plants actively growing. Fall applications may be used; however other plant species still in their growing season may be harmed. Readers are advised to check with local regulatory agencies to determine the regulations involved with chemical treatments.
The best method for controlling garlic mustard, or any other invasive plant, is to prevent its establishment. Disturbances in the forest understory that would allow for rapid invasion should be minimized. This would include limiting foot traffic, grazing, and erosion-causing activities. Monitoring the forest understory and removing any garlic mustard plants as soon as they are introduced will help to prevent the establishment and spread of this invader.
History in the United States
Garlic mustard was first recorded in the United States about 1868, from Long Island, New York. It was likely introduced by settlers for food or medicinal purposes.
History in the United States
Garlic mustard (Alliaria petiolata) is an obligate biennial herb of the cabbage family (Brassicaceae), also known as the mustard flowers. It has dark-green, kidney-shaped basal leaves with scalloped edges, 6-10 cm diameter. Stem leaves are alternate, sharply-toothed, triangular or deltoid, and average 3-8 cm long and wide, gradually reducing in size towards the top of the stem. All leaves have pubescent petioles 1-5+ cm long. New leaves produce a distinct garlic odor when crushed. The fragrance fades as leaves age, and is virtually non-existent by fall.
Plants usually produce a single unbranched or few-branched flower stalk, although robust plants have been recorded with up to 12 separate flowering stalks. Flowers are produced in spring (usually April to May) in terminal racemes, and occasionally in short axillary racemes. Some plants produce additional axillary racemes in mid-summer. Flowers are typical of the mustard family, consisting of four white petals that narrow abruptly at the base, and 6 stamens, two short and four long. Flowers average 6-7mm in diameter, with petals 3-6mm long.
Seedlings emerge in spring and form basal rosettes by midsummer. Immature plants overwinter as basal rosettes. In the spring of the second year the rosettes (now adult plants) produce flower stalks, set seed, and subsequently die.
Alliaria petiolata invades forested communities and edge habitats. The plant has no known natural enemies in North America, is self-fertile, and is difficult to eradicate once established. Thus, the best and most effective control method for Alliaria petiolata is to prevent its initial establishment.
Cutting flowering Alliaria petiolata plants at ground level results in 99% mortality, and eliminates seed production. Cutting at 10 cm above ground level results in 71% mortality and reduces seed production by 98% (Nuzzo 1991). Cutting is most effective when plants are in full bloom and/or have developed siliques; plants cut earlier in the flowering period may have sufficient resources to produce additional flowerstems from buds on the root crown (Nuzzo, personal observation).
Range and Habitat in Illinois
Regularity: Regularly occurring
Regularity: Regularly occurring
Global Range: Alliaria is native "throughout Europe from about 68o north southwards, but less common in the extreme south" (Tutin et al. 1964), occurring from England (Martin 1982) east to Czechoslovakia (Lhotska 1975), and from Sweden and Germany south to Italy, but is noticeably absent from Iceland, the Azores, Sardinia and Spitsbergen (Tutin et al. 1964). From this native range Alliaria has spread to North Africa, India, Sri Lanka (Cavers et al. 1979), and New Zealand (Bangerter 1985), as well as Canada (Cavers et al. 1979) and the United States (Gleason and Cronquist 1991, Nuzzo 1993a).
The North American range extends from British Columbia (Cavers et al. 1979, White et al. 1993) to New England (Gleason and Cronquist 1991), and from Ontario (Cavers et al. 1979) to Tennessee (Nuzzo 1993a). Alliaria was first recorded in North America in 1868 on Long Island NY, and by 1991 had spread to 30 states and 3 provinces (Nuzzo 1993a). This plant has spread exponentially since introduction in both Illinois (Nuzzo 1992b) and North America (Nuzzo 1993a).
In the United States Alliaria is most abundant in the New England and Midwestern states, but also has populations established as far west as North Dakota and Kansas, and south to Tennessee and North Carolina. Infrequent collections from western states indicate the plant may be a sporadic rather than established component of the regional flora, and/or in the process of becoming established in Utah (1971, 1983, 1984) and eastern Colorado (1952, 1958, 1966) (dates of herbarium collections). As of 1991 Alliaria had not been recorded west of the Rocky Mountains in the United States, with the exception of an 1892 record from Idaho, and a 1959 record from Portland Oregon (population absent in 1991). Alliaria is well established in Victoria B.C. and Vancouver in western Canada (Cavers et al. 1979, White et al. 1993).
In Canada Alliaria occurs in Victoria, British Columbia, and in the St. Lawrence Valley from Point Pelee in Ontario to Quebec City in Quebec (Cavers et al. 1979). Alliaria is especially abundant in southwestern Ontario, and near Toronto and Ottawa (White et al. 1993). White et al. (1993) recorded the plant as common in deciduous woods on the Canadian Shield, although 25 years earlier Cavers et al. (1979) stated the plant was noticeably absent from the region.
Regional Distribution in the Western United States
This species can be found in the following regions of the western United States (according to the Bureau of Land Management classification of Physiographic Regions of the western United States):
BLM PHYSIOGRAPHIC REGIONS :
Occurrence in North America
Distribution in the United States
Garlic mustard ranges from eastern Canada, south to Virginia and as far west as Kansas and Nebraska.
Distribution and Habitat in the United States
Garlic mustard is an established, cool-season, monocarpic, taprooted, herbaceous biennial [6,15,26,31,49,61] or occasional winter annual [15,31,61]. The common name is derived from the scent of garlic, which is noticeably exuded by its aboveground plant parts, especially foliage [15,31,73,82,86].
Seedlings develop into rosettes 0.8-4 inches (2-10 cm) in diameter during the 1st growing season. Mature plants produce erect flowering stems up to 4.13 feet (1.25 m) high . Each rosette usually produces a single flowering stem, although multiple stems from a single rosette occur occasionally . Flowers are borne in racemes, with fully expanded corollas 0.12-0.48 inches (3-12 mm) across [6,15,17,26,27,61,68,73,74,82,86]. Average plant biomass is quite variable within a habitat, between habitats, or between generations within the same habitat, and is strongly influenced by light. Plants grown under higher irradiance levels typically produce greater biomass per plant .
Seeds are produced in pods (siliques) up to 6 inches (15 cm) in length [15,27,31,74,82]. Fully developed siliques typically contain 12-19 seeds, and the number of siliques per plant can vary greatly from 1 to more than 200 . Seeds are oblong to nearly cylindrical [15,61] and about 0.12 inch (3 mm) long [27,31,61].
Garlic mustard is a cool season biennial herb with stalked, triangular to heart-shaped, coarsely toothed leaves that give off an odor of garlic when crushed. First-year plants appear as a rosette of green leaves close to the ground. Rosettes remain green through the winter and develop into mature flowering plants the following spring. Flowering plants of garlic mustard reach from 2 to 3-½ feet in height and produce buttonlike clusters of small white flowers, each with four petals in the shape of a cross.
Recognition of garlic mustard is critical. Several white-flowered native plants, including toothworts (Dentaria), sweet cicely (Osmorhiza claytonii), and early saxifrage (Saxifraga virginica), occur alongside garlic mustard and may be mistaken for it.
Beginning in May (in the mid-Atlantic Coast Plain region), seeds are produced in erect, slender pods and become shiny black when mature. By late June, when most garlic mustard plants have died, they can be recognized only by the erect stalks of dry, pale brown seedpods that remain, and may hold viable seed, through the summer.
Description and Biology
- Plant: biennial herb in the mustard family (Brassicaceae); first-year plants are low rosettes of kidney shaped leaves; second-year plants produce single or multiple flowering stalks 1-4 ft. high and, then die back by late spring; dried fruiting stalks may persist for many months.
- Leaves: crushed leaves and stems smell like garlic; first-year leaves are kidney-shaped with scalloped margins; leaves of mature, second year plants are heart-shaped with toothed margins and pointed tips.
- Flowers, fruits and seeds: flowers occur in small button-shaped clusters, flowers have four petals in the shape of a cross; fruits are slender, erect capsules (siliques); seeds are 2½-3 mm long, slender and tan to dark.
- Spreads: a single plant can produce hundreds of seeds, most of which fall nearby but may be carried further by wind, water, wildlife and people.
- Look-alikes: toothworts (Cardamine or Dentaria), sweet cicely (Osmorhiza claytonii), wild anise (Osmorhiza longistylis) and early saxifrage (Saxifraga virginiensis).
Alliaria petiolata is an obligate biennial herb of the mustard family (Brassicaceae). Seedlings emerge in spring and form basal rosettes by midsummer. Immature plants overwinter as basal rosettes. In the spring of the second year the rosettes (now adult plants) produce flower stalks, set seed, and subsequently die.
Basal leaves are dark-green and kidney-shaped with scalloped edges, 6-10 cm diameter. Stem leaves are alternate, sharply-toothed, triangular or deltoid, and average 3-8 cm long and wide, gradually reducing in size towards the top of the stem. All leaves have pubescent petioles 1-5+ cm long. New leaves produce a distinct garlic odor when crushed. The fragrance fades as leaves age, and is virtually non-existent by fall.
Plants usually produce a single unbranched or few-branched flower stalk, although robust plants have been recorded with up to 12 separate flowering stalks. Flowers are produced in spring (usually May) in terminal racemes, and occasionally in short axillary racemes. Some plants produce additional axillary racemes in mid-summer. Flowers are typical of the mustard family, consisting of four white petals that narrow abruptly at the base, and 6 stamens, two short and four long. Flowers average 6-7mm in diameter, with petals 3-6mm long. Fruits are linear siliques, 2.5-6cm long and 2mm wide, held erect on short (5mm), stout, widely divergent pedicels. Individual plants produce an average of 22 siliques (range 2 to 422; Nuzzo unpublished). Siliques contain an average of 16 seeds (range 3- 28; Nuzzo unpublished), arranged alternately on both sides of a papery sinus. Seeds are black, cylindrical (3mm x 1mm) and transversely ridged, and range in weight from 1.62-2.84mg.
Adult plants range in height from 0.05m to 1.5m, and average 1.0m, at the time of flowering. As plants of all sizes are found in the same cluster, plant height is likely a response to competition rather than genetically determined.
Immature plants can be confused with other rosette forming species, especially violets (Viola sp.), white avens (Geum canadense), and Cardamine sp. Alliaria petiolata can be distinguished from these plants by the strong garlic odor in spring and summer. In fall and winter Alliaria can be distinguished by examining the root system. Alliaria has a slender, white, taproot, with a distinctive "s" curve at the top of the root, just below the root crown (Nuzzo, personal observation). Axillary buds are produced at the root crown and along the upper part of the "s".
Chromosome number of 2n=36 has been recorded for European material, and 2n=24 for North American and European material (Cavers et al. 1979).
Excellent illustrations are contained in Cavers et al. (1979). Descriptive characteristics derived from Cavers et al. (1979) and Gleason and Cronquist (1991) except where otherwise noted. There is one other species in this genus (Gleason and Cronquist 1991).
Range and Habitat in Illinois
In its native Europe Alliaria is an edge species, growing in hedges and fencerows (Fitter et al. 1974, Martin 1982) and in open woods (Wilmanns and Bogenrieder 1988). Alliaria is disturbance adapted, and is frequently a component of ruderal communities (Swies and Kucharczyk 1982), including open, highly disturbed forests (Klauck 1986).
In North America Alliaria invades wet to dry-mesic deciduous forest (Cavers et al. 1979, Nuzzo 1992a, 1993a), and also occurs in the partial shade characteristic of oak savanna, forest edges, hedgerows, shaded roadsides, and urban areas, and occasionally in full sun (Nuzzo 1991a). Alliaria is rarely found under coniferous trees in the Midwest, but has been reported from under seven species of coniferous trees in Ontario (Cavers et al. 1979). Alliaria grows on sand, loam, and clay soils, and on both limestone and sandstone substrates, but has been observed only once growing on a drained peat soil, and does not occur on muck soils. Alliaria frequently grows in well-fertilized sites (Cavers et al. 1979), and is described as a nitrophile by Passarge (1976) and Wilmanns and Bogenrieder (1988). In Europe, Alliaria increased in cover with deposition of air-borne industrial emissions, which increased soil nitrogen, nitrate, phosphorous and pH (Wilmanns et al. 1986, Wilmanns and Bogenrieder 1988).
Alliaria is common in river-associated habitat, particularly in the Northeast (Nuzzo 1993a). It may preferentially invade drier forest communities in the Midwest than it does in the northeast (Nuzzo 1993a). This is supported by the higher presence along railroads in the Midwest (Nuzzo 1993a), which are generally indicative of drier habitats. Byers and Quinn (1987) reported that Alliaria, once considered a plant of floodplains and moist woods in New Jersey, had become common in a wider range of habitats. In the Great Plains Alliaria is most frequently recorded from moist, usually riverine, habitat and waste ground (Kansas and Oklahoma), while on the eastern edge of the Rocky Mountains Alliaria has been recorded along hiking trails (Utah), and on hotel grounds and around a beaver pond (Colorado).
Garlic mustard has a wide tolerance of environmental conditions for growth and reproduction, including moisture regimes ranging from periodically flooded areas to dry sand forest [15,42], light environments ranging from open fields to shaded forest interior [12,14], and a range of various soil characteristics including texture [14,15,57], nutrient level , organic matter content [14,15], and pH [4,14]. It is apparently not found on acid soils in Indiana, Kentucky, Massachusetts, or the Canadian Shield region , and is absent from undrained peat and muck soils .
Garlic mustard may be less competitive in areas with low soil pH, as evidenced by an experiment demonstrating a significant positive correlation (r = 0.98; p < 0.001) between plant dry weight and soil pH. This has been hypothesized as a contributing factor in the limited colonization of garlic mustard in the southern third of Illinois, where soils are more acidic than in the more heavily colonized central and northern sections of the state . Inhibition of garlic mustard by acidic soils may explain its apparent absence from conifer-dominated communities .
Garlic mustard appears to favor shaded sites , and is often found in dense groups of nearly pure stands, sometimes covering large areas, particularly under moist shaded conditions such as mature eastern deciduous woodlands. In drier or more open areas plants increase allocation to fruit production, perhaps in response to observed declines in seed weight, seed germination, and seedling survivorship [14,46]. While biomass production may be greatest under full sun , and garlic mustard plants can also be found under dense shade, they are most commonly found in woodland understories with partial shade and are probably less invasive under extreme conditions of light or shade . Nuzzo  describes typical habitat in Illinois as mesic upland or floodplain forest, usually shaded, and often associated with some type of disturbance. Despite its apparent affinity for moist shaded environments, garlic mustard is not tolerant of growing season inundation, which may limit its ability to invade wetland communities .
Most populations of garlic mustard appear to be connected to some form of disturbance [14,49]. Garlic mustard is often associated with anthropogenic disturbance such as trails, roads, or railroads [49,50], and less commonly, in farm fields and gardens . Garlic mustard is sometimes linked to naturally disturbed habitats such as floodplains and riverbanks, where the combination of flooding as a dispersal agent and moist, shaded conditions may promote invasion . Garlic mustard was invasive in relatively undisturbed woodlands in central Illinois. Establishment was thought to occur where small-scale anthropogenic and natural disturbance removed competing vegetation, such as areas browsed by white-tailed deer .
Experiments examining mechanisms that link disturbance and garlic mustard occurrence and spread are scarce. One study showed that disturbance of soil in a young hardwood forest in northern Kentucky resulted in lowered garlic mustard densities compared to undisturbed plots . An experiment in a southwestern Ohio deciduous forest examined the effects of small-scale litter disturbance on garlic mustard invasiveness. There were no differences (p = 0.7184) in garlic mustard germination, rosette survival, growth, or reproduction among total litter removal, partial litter removal, and control treatments, indicating forest floor disturbance alone may not be a prerequisite for invasion .
More research is needed to help understand factors that influence garlic mustard invasiveness and habitat invasibility, particularly for the role of disturbance. In particular, questions involving which life history traits are affected by disturbance seem most appropriate. Experiments that separate disturbance-mediated dispersal from other interactions between disturbance and garlic mustard invasiveness might provide important insights leading to more effective management prescriptions.
Habitat: Rangeland Cover Types
This species is known to occur in association with the following Rangeland Cover Types (as classified by the Society for Range Management, SRM):
More info for the term: cover
SRM (RANGELAND) COVER TYPES :
601 Bluestem prairie
Habitat: Cover Types
This species is known to occur in association with the following cover types (as classified by the Society of American Foresters):
More info for the term: cover
SAF COVER TYPES :
20 White pine-northern red oak-red maple
25 Sugar maple-beech-yellow birch
27 Sugar maple
31 Red spruce-sugar maple-beech
39 Black ash-American elm-red maple
42 Bur oak
50 Black locust
52 White oak-black oak-northern red oak
53 White oak
55 Northern red oak
59 Yellow-poplar-white oak-northern red oak
60 Beech-sugar maple
61 River birch-sycamore
62 Silver maple-American elm
95 Black willow
108 Red maple
110 Black oak
225 Western hemlock-Sitka spruce
Habitat: Plant Associations
This species is known to occur in association with the following plant community types (as classified by Küchler 1964):
KUCHLER  PLANT ASSOCIATIONS:
K074 Bluestem prairie
K081 Oak savanna
K082 Mosaic of K074 and K100
K098 Northern floodplain forest
K100 Oak-hickory forest
K101 Elm-ash forest
K102 Beech-maple forest
K104 Appalachian oak forest
K106 Northern hardwoods
K107 Northern hardwoods-fir forest
K108 Northern hardwoods-spruce forest
This species is known to occur in the following ecosystem types (as named by the U.S. Forest Service in their Forest and Range Ecosystem [FRES] Type classification):
FRES10 White-red-jack pine
Habitat in the United States
Garlic mustard frequently occurs in moist, shaded soil of river floodplains, forests, roadsides, edges of woods and trails edges and forest openings. Disturbed areas are most susceptible to rapid invasion and dominance. Though invasive under a wide range of light and soil conditions, garlic mustard is associated with calcareous soils and does not tolerate high acidity. Growing season inundation may limit invasion of garlic mustard to some extent.
Habitat & Distribution
Flower-Visiting Insects of Garlic Mustard in Illinois
(information is limited; insect activity is unspecified; these observations are from Krombein et al.)
Andrenidae (Andreninae): Andrena forbesii, Andrena personata, Andrena spiraeana
gregarious larva of Athalia glabricollis grazes on leaf (underside) of Alliaria petiolata
Other: major host/prey
Foodplant / open feeder
gregarious larva of Athalia liberta grazes on leaf (underside) of Alliaria petiolata
Foodplant / feeds on
larva of Ceutorhynchus alliariae feeds on Alliaria petiolata
Foodplant / feeds on
larva of Ceutorhynchus constrictus feeds on Alliaria petiolata
Foodplant / feeds on
larva of Ceutorhynchus thomsoni feeds on Alliaria petiolata
In Great Britain and/or Ireland:
Foodplant / parasite
Erysiphe cruciferarum parasitises live Alliaria petiolata
Foodplant / sap sucker
adult of Eurydema oleracea sucks sap of Alliaria petiolata
Other: major host/prey
Foodplant / pathogen
pycnidium of Phoma coelomycetous anamorph of Leptosphaeria maculans infects and damages live, lesioned leaf of Alliaria petiolata
Plant / resting place / within
puparium of Ophiomyia alliariae may be found in stem of Alliaria petiolata
Other: sole host/prey
Foodplant / parasite
colony of sporangium of Peronospora niessliana parasitises live Alliaria petiolata
Other: sole host/prey
Foodplant / parasite
colony of sporangium of Peronospora parasitica parasitises live Alliaria petiolata
Remarks: season: 1-4
Foodplant / open feeder
adult of Phaedon cochleariae grazes on live leaf of Alliaria petiolata
Remarks: season: 5-9
Foodplant / saprobe
scattered, covered then bursting through a slit pycnidium of Phomopsis coelomycetous anamorph of Phomopsis cruciferae is saprobic on dead stalk of Alliaria petiolata
Foodplant / spot causer
scattered pycnidium of Phyllosticta coelomycetous anamorph of Phyllosticta erysimi causes spots on leaf of Alliaria petiolata
Remarks: season: 9-12
Foodplant / open feeder
imago of Phyllotreta ochripes grazes on leaf of Alliaria petiolata
Foodplant / spot causer
amphigenous colony of Ramularia hyphomycetous anamorph of Ramularia armoraciae causes spots on live leaf of Alliaria petiolata
Alliaria seeds germinate in early spring, beginning in late February or early March, and concluding by mid May in northern states and Canada (Cavers et al. 1979, Kelley et al. 1991, Roberts and Boddrell 1983). In northern Illinois, germination coincides with emergence of spring beauty (Claytonia virginica) and false mermaid weed (Floerkea proserpinacoides).
Seedling density in heavily infested forests was recorded at 5,080/m2 at the cotyledon stage, and 2,235/m2 at the 2-3 leaf stage, in Illinois (Nuzzo unpublished), and approximated at 20,000/m2 in Ohio (Trimbur 1973). Seedlings undergo high mortality, declining by 41% (Trimbur 1973) to >50% (Cavers et al. 1979) by late spring.
By June seedlings develop the characteristic rosette of first year plants. First year rosettes are sensitive to summer drought (Byers 1988) and approximately 95% die by fall (Nuzzo 1993b). By mid-fall rosettes average 4-10 cm diameter and are dark green to purplish in color (range 1-15 cm). The rosettes continue to grow in winter during snow-free periods when temperatures are above freezing (Cavers et al. 1979).
Natural mortality continues through winter: in northern Illinois rosette density in November averaged 186.4/m2 (range 50-466/m2), and declined significantly to an average of 39.9/m2 (range 4-102/m2) by the following spring (Nuzzo 1993b). Rosette density varies between sites and years; mean densities range from 30/m2 to 80/m2 (Nuzzo 1991a), and reach a high of >450 adult plants/m2 (Nuzzo 1993b). Over-winter mortality is only slightly density-dependent: 9% of the variation between fall and spring densities was due to initial density in fall (Nuzzo 1993b). Total survival rate from seedling to adult stage varies from 1% (Nuzzo 1993b) to 2-4% (Cavers et al. 1979).
Alliaria is an obligate biennial: all plants that survive the winter produce flowers, regardless of size, and subsequently die (Cavers et al. 1979, Byers and Quinn 1988, Bloom et al. 1990). Plants only 5cm tall, with 3-4 leaves, have been observed with flowers and seeds. The majority of plants are taller, averaging 0.7 to 1.0 meters when in flower. Flower stalks begin to elongate in March or April, and flowers open early April through May. This is some 6-10 weeks after new seedlings germinate; in established populations generations overlap, and two cohorts can be seen from March through July. Alliaria flowers can be self-or cross-pollinated (Cavers et al. 1979, Babonjo et al. 1990). Syrphid flies, midges and bees visit flowers and may effect pollination (Cavers et al. 1979). Whether in-bred or out- bred, Alliaria plants maintain substantial genetic variation within populations (Byers 1988).
Plants usually produce 1-2 flowering stems, although a single individual may produce up to 12 separate stems. Damage to the primary flower stem stimulates growth of additional stems (Cavers et al. 1979) from axillary buds at the stem base and along the root crown, although such damage is not a prerequisite for development of multi-stem plants (Nuzzo personal observation). Some plants continue to produce flowers through August in small axillary inflorescences. Large plants produce flowers earlier and for a longer time period, and consequently produce significantly more seeds, than plants with small rosettes (Byers and Quinn 1988).
Seeds develop in a linear silique, with siliques forming on the lower part of the inflorescence while flowers are still opening on the upper part. Seeds ripen and disperse between mid-June and late September (Cavers et al. 1979, Kelley et al. 1991). Alliaria produces an average of 16.4 (+ 3.0) seeds/silique (range 3 to 28), and 21.8 (+ 22.5) siliques/plant (range 2 to 422; Nuzzo unpublished, Cavers et al. 1979). Actual production varies significantly within and between communities, with plants in drier communities tending to produce fewer seeds than plants in mesic and wet communities (Byers 1988). Plants produce an average of 360.5 seeds, ranging from 194.3 in mesic sand forest to 608.2 in mesic floodplain forest (Nuzzo unpublished). Maximum production per plant is estimated at 7,900 seeds on a plant with 12 stems, while minimum production is 14 seeds on a plant with 2 siliques (Nuzzo unpublished). Seed production is density dependent, with plants producing fewer seeds as density increases (Trimbur 1973). However, total seed production increases with increasing density (Trimbur 1973). In Illinois, seed production within dense patches of Alliaria ranged from 3,607/m2 to >22,000/m2 (Nuzzo unpublished), while in Ohio Trimbur (1973) reported 19,060 to 38,025 seeds/m2, and in Ontario Cavers et al. (1979) estimated average production at 19,800 to 107,580 seeds/m2.
Seeds are dormant at maturity and require 50 to 100 days of cold stratification to come out of dormancy (Byers 1988, Lhotska 1975, Baskin and Baskin 1992). Dormancy period lasts eight months in southern locales (Baskin and Baskin 1992, Byers 1988) and 22 months in northern areas (Cavers et al. 1979). Alliaria seeds may break dormancy more rapidly when exposed to low temperatures that fluctuate around freezing (0.5 to 10 C, as occurs in central states such as Kentucky) than under a constant temperature regime well below freezing (as occurs in northern states and Canada). This is likely a physiological rather than genetic response, as Ontario seed germinated at 20% and 50% in 3 months when moist stratified at 5 and 2 degrees C, respectively (Cavers et al. 1979).
Unlike some forest crucifers that fail to germinate under leaf cover, Alliaria seeds germinate in both light and dark after dormancy is broken (Bloom et al. 1990, Byers 1988). Light alone will not stimulate germination during cold stratification (Byers 1988). Germination rates of 12-100% have been reported (Baskin and Baskin 1992, Byers 1988, Cavers et al. 1979), but vary greatly within and between populations and habitats (Byers 1988, Cavers et al. 1979). Interestingly, substrate affects germination rate: Baskin and Baskin (1992) reported lower germination on sand substrates than on soil, as seeds on sand failed to afterripen (possibly due to water relations at the seed:soil interface). The majority of seeds germinate as soon as dormancy is broken (Roberts and Boddrell 1983, Baskin and Baskin 1992). A small percentage of seed remains viable in the seed bank for up to four years (Roberts and Boddrell 1983, Baskin and Baskin 1992).
Byers (1988) determined that seeds were concentrated in the upper 5cm of soil, and that three of four populations maintained a seed bank after germination. The fourth population, located in a floodplain, lacked a seedbank due to flooding and scouring of the surface, but was expected to gain new seeds during flood deposition.
Alliaria spreads exclusively by seed (Cavers et al. 1979). Seeds typically fall within a few meters radius of the plant. Wind dispersal is limited, and seeds purportedly do not float well, although seeds readily attach to moist surfaces (Cavers et al. 1979). Anthropogenic distribution is the primary dispersal mechanism (Lhotska 1975, Nuzzo 1992b, 1993a). Seeds are transported by natural area visitors on boots and in pant cuffs, pockets and hair, and by roadside mowing, automobiles and trains (Nuzzo 1992b). Seeds are widely dispersed in floodwaters. Seeds may be dispersed by rodents or birds; isolated plants are frequently found at the bases of large trees in forest interiors. Seeds may possibly be distributed directly or indirectly by white- tailed deer (Odocoileus virginianus).
In southern locales Alliaria populations are even-aged, alternating annually between immature plants and adult plants (Baskin and Baskin 1992), probably due to the 8 month seed dormancy. In northern climates Alliaria populations can be even-aged in early stages of invasion, and then become multi-aged as the seed bank builds up. Dormant seeds may have different longevity rates in northern and southern locales, but no work has been conducted to test this.
Alliaria is frequently overlooked at low density levels. In many sites Alliaria can be present for a number of years before appearing to "explode" in favorable years. Once Alliaria reaches this level of infestation control is difficult to achieve. At any given site Alliaria frequency and cover fluctuate annually, reflecting the biennial nature of the plant. These annual fluctuations are deceptive, as Alliaria consistently occurs with increasing frequency through time, on average doubling in four years (Nuzzo 1992a). The greatest increases in presence occur in sites subjected to large-scale natural disturbances. One site, flooded in mid- summer, experienced a 241% increase in frequency two years later (Nuzzo 1992a). In a site hit by a severe windstorm that blew down overstory trees, Alliaria frequency increased 1000% during the same time period (Nuzzo 1992a).
Alliaria is rarely if ever browsed by deer or other large herbivores. Alliaria is only rarely observed with obvious signs of insect herbivory in the U.S., although it is a preferred host plant for Pieridae butterflies in Europe (Forsberg and Wiklund 1989, Courtney and Duggan 1983, Remorov 1987, Kuijken 1987), and is utilized by a European curculionid weevil (Ceutorhynchus constrictus; Nielsen et al. 1989). In the Netherlands Alliaria is targeted by the orange- tip butterfly Anthocharis cardamines (Pieridae) when the preferred host species Arabis glabra is unavailable (Kuijken 1987). In eastern Europe Alliaria is utilized by butterflies that feed on commercial crucifers (Remorov 1987) and thus may be a threat to commercial production of cabbage. However, macerates of Alliaria leaves sprayed on cauliflower deterred oviposition by the garden pebble moth (Jones and Finch 1987). Alliaria is a preferred host of the monophagous weevil Ceutorhynchus constrictus (Nielsen er al. 1989). The general lack of insect utilization in North America may be due to Pieridae preference for open habitat (Alliaria usually occurs in forested habitat), general lack of natural enemies in the US, and Alliaria's cool season growth habit: Plants undergo rapid growth at low temperatures in late fall or early spring when insects are not very active (Cavers personal communication 1989). Cavers observed plants in Britain and Europe with greater obvious insect damage to leaves than in Ontario, suggesting that natural enemies may have some impact on the plant (Cavers personal communication 1989). Pieridae butterflies are common in North America, and Alliaria may be used by Pieridae species when the preferred host plant is unavailable.
In Ontario an unidentified virus (or several viruses) has been observed to kill flowering plants and prevent them from ripening viable seeds (Cavers personal communication 1989). Alliaria is frequently infected with a strain of turnip mosaic virus (TuMV-Al) in both Ontario and Europe, with infected plants developing a mosaic leaf pattern (Stobbs and Van Schagen 1987). The virus does not affect total seed production or seed germination, but does reduce diameter of individual seeds and average silique length (Stobbs and Van Schagen 1987). Although closely related to TuMV-Br, a virus that infects crops Brassicaceae, the two viruses are mutually exclusive: the Alliaria virus is not transmissible to commercial Brassicaceae species, specifically rutabagas and canola, nor does TuMV-Br infect Alliaria (Stobbs and Van Schagen 1987). In Europe Alliaria is a host plant for a number of viruses, including cucumber mosaic virus (CMV) and turnip mosaic virus (TuMV), that infect commercially propagated crucifers (Polak 1985). Alliaria is host for an isolate of turnip yellow mosaic virus (TYMV-A) that induces systemic infection in broccoli, turnip, and other crucifers grown in Europe (Pelikanova et al. 1990). This was the first finding of TYMV-A virus in wild growing vegetation in the former Czechoslovakia (Pelikanova 1990).
Alliaria was historically eaten as a potherb, particularly in winter and early spring when few greens were unavailable (Georgia 1920). There is no direct evidence that Alliaria was specifically imported for garden or medicinal use, although Fernald et al. (1958) state that this "old fashioned garden plant...has spread somewhat to roadsides and borders of groves", and cite earlier authors who describe the use of Alliaria as a salad plant. Zennie and Ogzewalla (1977) promote eating Alliaria for it's high Vitamin A content (8,600 units/100g in young leaves, 19,000 in basal leaves) and Vitamin C content (190mg/100g in young leaves), both substantially higher than levels in commercially grown fruits and vegetables.
Broad-scale Impacts of Plant Response to Fire
There is some indication that garlic mustard is capable of sprouting following
fire, but several questions remain. To what extent is postfire sprouting in
garlic mustard influenced by fire severity? What, if any, physiological
conditions promote or constrain postfire root crown sprouting? To what extent are
resprouting plants successful at producing seed?
Nuzzo and others  reported that a fall burn in a central Illinois black oak forest
removed 79% of the litter layer, and very few adult garlic mustard plants were encountered in
these plots the following spring. Conversely, many garlic mustard plants resprouted following
a mid-spring burn at the same site that resulted in removal of only 32% of the litter layer.
Spring burn plots retained a damp 0.4- to 0.8-inch (1-2 cm) layer of litter which
seems to have protected the root crowns of top-killed plants, fostering survival via sprouting
of multiple secondary shoots from adventitious buds located just below the soil surface .
Hintz  conducted a late-March prescribed burn in a mesic upland oak-hickory
forest in northern Illinois. Garlic mustard established following the fire,
although it is unclear whether these were sprouting burned plants or new spring
seedlings. The burn was conducted near the time when seedling emergence might be expected,
leaving some question as to which life-cycle stage was observed to be "sprouting".
There is reference to "very little" garlic mustard producing seed that summer,
intimating that at least some adult plants were present both prior to and after the fire.
Luken and Shea  conducted a prescribed fire experiment in a northern
Kentucky mesic deciduous forest in which they showed that garlic mustard plants
could be removed by a fall burn. Yet it was also apparent from this experiment that populations
can persist following even repeated burns. Garlic mustard remained the dominant species in the
herb layer of both burned and unburned plots through 3 seasons of fall burning,
and beyond. The authors proposed 3 possible explanations. First, persistence of individual garlic
mustard plants immediately following fire may result from the patchy nature of many understory
or mixed-severity burns. Under such conditions some extant plants may escape damage, and because
of its ability to self-pollinate [3,15,17], the survival of a single plant may be sufficient to
perpetuate a population. Second, the data of Luken and Shea  showed that burning resulted in
higher densities of flowering stems compared with control plots. They speculated this as being
due to either resprouting or release from competition. No observations of sprouting were reported.
Third, even if all plants are killed, the existing seed bank may remain viable for several years
[7,14], requiring subsequent annual burns to completely eradicate the population.
The Research Paper by Bowles and others 2007 provides information on postfire responses of several plant species, including garlic mustard, that was not available when this species review was originally written.
Plant Response to Fire
Garlic mustard has at least some ability to sprout from the root crown following damage by fire. By excavating charred rosettes, Nuzzo and others  found that adult plants resprouted from adventitious buds on the root crown located just below the soil surface following a mid-spring burn. In a northern Illinois oak woodland, garlic mustard reportedly resprouted several weeks following complete top removal by a prescribed fire conducted in late March . Repeated fall burning (2-3 annual burns) did not reduce abundance or relative importance of garlic mustard in an eastern mesophytic forest understory in Kentucky .
Broad-scale Impacts of Fire
It has been suggested that dense stands of garlic mustard may be able to resist low-severity fire, such that "abundant green garlic mustard plants...may literally extinguish fires" , but detailed descriptions of the direct effects of fire on garlic mustard plants (or vice versa) are scarce. Such observations may be confounded by the inherently patchy nature of mixed-severity FIRE REGIMES in many eastern deciduous forests where garlic mustard may commonly be found. For more information see the Fire Ecology section of this summary.
Immediate Effect of Fire
Garlic mustard is often top-killed when exposed to fire. A prescribed burn in the understory of a northern Illinois hardwood forest apparently removed all aboveground garlic mustard biomass . Prescribed burns in a central Illinois black oak forest conducted both in the fall and in mid-spring removed nearly all garlic mustard rosettes . Although there was no immediate postfire survey of plants mentioned in the article, Luken and Shea  suggest garlic mustard "plants are readily killed by mid-intensity dormant season fires". Emergent seedlings may also be killed by fire .
Fire Adaptations: Although garlic mustard plants are readily top-killed when exposed to fire, they may ultimately survive by sprouting from the root crown . Ecological conditions that permit sprouting are not well understood and it is unclear to what extent resprouted plants are capable of producing viable seed (see Fire Effects section of this summary).
At the population level, garlic mustard may be adapted to perpetuate itself in mixed-severity or low-severity surface FIRE REGIMES, although this has not been quantified. Even though individual plants may be killed by fire, postfire conditions may be favorable for rapid population expansion because of increases in the area of disturbed habitat and, depending on the extant community, temporary reductions in interspecific competition. Additionally, garlic mustard seed banks may facilitate rapid recolonization of disturbed areas . For example, 3 consecutive years of prescribed burning in a central Illinois black oak forest, which were described as "hot and fast" with flame lengths to 4 feet (1.2 m), failed to eradicate garlic mustard populations. This was attributable, in part, to the protection afforded a small number of plants by refugia such as the lee of a downed log or an area of damp litter . The ability of individual plants to escape mortality will depend upon fire severity and the heterogeneity of the fire landscape.
Fuels: Although it has been demonstrated that fire can top-kill garlic mustard (see IMMEDIATE FIRE EFFECT ON PLANT), it is also apparent that garlic mustard plants can be difficult to ignite. Nuzzo  noted that low fuel loads, coupled with abundant green garlic mustard plants, "which on occasion literally extinguished fires", made it difficult to achieve prescribed fire objectives.
FIRE REGIMES: Garlic mustard may be found within understory surface, stand-replacement, mixed-severity fire, and nonFIRE REGIMES . Because garlic mustard has become established only relatively recently in most areas in North America, and because natural FIRE REGIMES have been substantially altered in many of these areas, predicting the response of garlic mustard to any particular fire regime is speculative. In some areas colonized by garlic mustard, estimated mean fire return intervals may be longer than the time in which garlic mustard has been present. As natural areas and preserve managers reintroduce fire into locations where natural and anthropogenic fire has been suppressed in recent times, the response of this and many other species may become better understood. Those who intend to reintroduce fire where it has been absent for a substantial period are encouraged to plan and implement research and monitoring programs and share their findings.
FIRE REGIMES of some of the plant communities in which garlic mustard occurs are summarized below. For further information, see the FEIS summaries of the dominant species listed below.
|Community or Ecosystem||Dominant Species||Fire Return Interval Range (years)|
|silver maple-American elm||A. saccharinum-Ulmus americana|
|sugar maple||A. saccharinum||> 1000|
|sugar maple-basswood||A. saccharinum-Tilia americana||> 1000 |
|bluestem prairie||Andropogon gerardii var. gerardii-Schizachyrium scoparium||37,58]|
|sugarberry-America elm-green ash||Celtis laevigata-U. americana- Fraxinus pennsylvanica|
|beech-sugar maple||Fagus spp.-A. saccharum||> 1000|
|black ash||Fraxinus nigra||83]|
|tamarack||Larix laricina||35-200 |
|eastern white pine-northern red oak-red maple||Pinus strobus-Quercus rubra-A. rubrum||35-200|
|Virginia pine-oak||P. virginiana-Quercus spp.||10 to|
|sycamore-sweetgum-American elm||Platanus occidentalis-Liquidambar styraciflua-U. americana||83]|
|eastern cottonwood||Populus deltoides||58]|
|aspen-birch||P. tremuloides-Betula papyrifera||35-200 [21,83]|
|black cherry-sugar maple||Prunus serotina-A. saccharum||> 1000|
|northeastern oak-pine||Quercus-Pinus spp.||10 to|
|southeastern oak-pine||Quercus-Pinus spp.|
|white oak-black oak-northern red oak||Q. alba-Q. velutina-Q. rubra|
|northern pin oak||Q. ellipsoidalis|
|bur oak||Q. macrocarpa||83]|
|oak savanna||Q. macrocarpa/Andropogon gerardii-Schizachyrium scoparium||2-14 [58,83]|
|chestnut oak||Q. prinus||3-8|
|northern red oak||Q. rubra||10 to|
|post oak-blackjack oak||Q. stellata-Q. marilandica|
|black oak||Q. velutina||83]|
More info for the terms: association, cover, fire frequency, frequency, hardwood, presence, succession, tree
Garlic mustard occurs in communities that represent a wide range of successional stages, from prairie openings to understories of mature, shade-tolerant eastern hardwood forests. While garlic mustard colonizes a variety of sites, it is often mentioned with particular concern to invasiveness in the herb layer of mature eastern deciduous forests, since these communities were thought to be somewhat resistant to invasion by nonindigenous plants (see Impacts and Control). In some areas of eastern deciduous forest, dense garlic mustard stands may inhibit recruitment of woody seedlings, perhaps altering successional trajectories .
The ability of garlic mustard to invade and compete in habitats with light environments ranging from partial to deep shade may be due to its ability to acclimate to variation in irradiance [2,15]. Despite substantial plasticity in photosynthetic response to variation in irradiance, photosynthetic rates of garlic mustard at high light levels lag behind those of species typically found in unshaded environments, inhibiting the competitiveness of garlic mustard under these conditions . Nevertheless, the ability of the species to acclimate to a wide range of light environments almost certainly contributes to its ubiquitous and invasive nature .
Garlic mustard is often mentioned in association with oak savannah communities which, when viewed from the context of fire as the determinant of successional trajectory, represent a transitional state between grassland and forest. For example, garlic mustard was present mainly in areas of lower ambient light levels in a northern Illinois oak savanna remnant, invading where reduced fire frequency resulted in increased tree canopy cover . Because the presence of garlic mustard may inhibit the ability of a forest understory to carry surface fire , invasion of garlic mustard could potentially accelerate succession in these oak savannas by further suppressing fire.
Pollination: Garlic mustard is capable of self-pollinization, as well as cross-fertilization [3,15,17]: both seem equivalent in effectiveness. Self-pollination often takes place before flowers open , although variation in this ability may exist between populations [3,17]. Cross-pollination has been observed to occur via generalist insect pollinators, providing a high likelihood of pollination wherever garlic mustard occurs [3,15,17].
Seed production: Because a large percentage of flowers typically set fruit, and most ovules develop seeds, garlic mustard is a prodigious seed producer . Seed production varies between and within sites and between years, but under shaded, moist (apparently favorable) conditions, dense stands may produce > 100,000 seeds/m2 [14,15]. Seed production in Ohio ranged from 165 to 868 seeds/plant, depending on habitat and population density . The number of seeds per silique in a southern Ontario study varied from 6 to 22 with an average of 16. The number of siliques varied greatly, from 1 or 2 on small plants to up to 150 per plant . Seed production in several states was:
|Estimated Seed Production (seeds/m2)||Location|
|15,000||Central Illinois |
|19,060 - 38,025||Ohio |
|19,800 - 107,580||Southern Ontario |
|30,689 - 45,018||New Jersey |
|10,000||Northern Illinois |
Seed dispersal: In forested areas, garlic mustard is typically 1st seen along trails and streams, and can quickly spread via seeds throughout the forest within a few generations . Seeds generally fall within a few meters of the plant [50,74], and may be ballistically dispelled from siliques . Wind dispersal is doubtful. Seeds stick together when damp and adhere readily to small soil clusters . Seed dispersal rates may accelerate along river corridors [46,50], although there are conflicting reports regarding the ability of seeds to float [15,74]. Humans may also spread seeds. Garlic mustard often invades natural areas along roads and trails, purportedly via seed transport on muddy boots or pant cuffs. Seed dispersal may also be facilitated by roadside mowing, as well as on mud-encrusted automobile tires . Animals, especially white-tailed deer, may promote seed dispersal and spread of garlic mustard. Deer are thought to provide an important seed dispersal vector over short distances by transporting seeds in their fur, although this has not been tested as of this writing [3,15]. Foraging deer may create microsite disturbances favorable to garlic mustard dispersal by mixing mineral soil and garlic mustard seeds .
Germination: Seeds of garlic mustard require cold stratification before they can germinate, with 1 season's overwintering usually sufficient to break dormancy at most North American locations . An additional year of dormancy was reportedly required prior to germination in southern Ontario , and this lengthier dormancy period may be required in other northern locations [55,70]. Germination often occurs in early spring and can occur at temperatures approaching 32 degrees Fahrenheit (0 Â°C) [7,63]. Low-temperature germination is ecologically important because garlic mustard seedlings incur a competitive advantage by being the 1st germinants of the season [7,45].
Seed banking: Garlic mustard produces small but potentially important seed banks. Seed viability has been shown to drop off substantially after the 1st growing season following stratification, indicating seed banks of garlic mustard are relatively short lived [7,63]. In a study of garlic mustard seed biology, roughly 88% of seeds that germinated did so during the 1st spring following production . In a study comparing garlic mustard populations from contrasting habitats in New Jersey, 3 out of 4 populations were found to maintain a seed bank. The 4th population was located in a seasonal floodplain where flooding actions were thought to either remove the seedbank or produce a patchy distribution that was difficult to sample .
A small percentage of seeds may remain viable for 4-6 years [7,15,63]. Because garlic mustard is a prodigious seed producer, elimination of a single season's crop may not suffice to eradicate the species from an area because germination and survival of only a few individuals in subsequent years may quickly lead to repopulation at or near previous levels .
Seedling establishment/growth: Garlic mustard seedlings emerge in early spring, just before or simultaneous with germination of native spring ephemerals . They establish during periods of relatively high light availability in the forest understory prior to canopy leaf-out, typically with reduced interspecific competition and drought potential [7,15,45]. Greatest mortality rates occur in spring during the seedling stage . Seedling mortality can vary substantially, often depending on moisture availability . Initial seedling density may be very high (20,000 seedlings/m 2) [49,74]. In reports where natural spring seedling densities were approximately 3,100 to 5,600/m2, only about 1% to 16% survived to produce flowers the following year [14,15]. Two consecutive cohorts retained similar numbers of mature flowering plants during their 2nd spring, despite having initial seedling densities differing by more than 100% .
Growth Form (according to Raunkiær Life-form classification)
More info for the terms: hemicryptophyte, therophyte
RAUNKIAER  LIFE FORM:
Life History and Behavior
Biology and Spread
After spending the first half of its two-year life cycle as a rosette of leaves, garlic mustard plants develop rapidly the following spring into mature plants that flower, produce seed and die by late June. In the mid-Atlantic Coastal Plain region, seeds are produced in erect, slender, four-sided pods, called siliques, beginning in May. Siliques become tan and papery as they mature and contain shiny black seeds in a row. By late June, most of the leaves have faded away and garlic mustard plants can be recognized only by the dead and dying stalks of dry, pale brown seedpods that may remain and hold viable seed throughout the summer.
A single plant can produce thousands of seeds, which scatter as much as several meters from the parent plant. Depending upon conditions, garlic mustard flowers either self-fertilize or are cross-pollinated by a variety of insects. Self-fertilized seed is genetically identical to the parent plant, enhancing its ability to colonize an area. Although water may transport seeds of garlic mustard, they do not float well and are probably not carried far by wind. Long distance dispersal is most likely aided by human activities and wildlife. Additionally, because white-tailed deer prefer native plants to garlic mustard, large deer populations may help to expand it by removing competing native plants and exposing the soil and seedbed through trampling.
Molecular Biology and Genetics
Barcode data: Alliaria petiolata
Statistics of barcoding coverage: Alliaria petiolata
Public Records: 9
Specimens with Barcodes: 24
Species With Barcodes: 1
National NatureServe Conservation Status
Rounded National Status Rank: NNA - Not Applicable
Rounded National Status Rank: NNA - Not Applicable
NatureServe Conservation Status
Rounded Global Status Rank: GNR - Not Yet Ranked
Impacts and Control
Impacts: The control of garlic mustard may be desirable to undisturbed deciduous forests of the eastern and midwestern United States and southern Ontario [3,15,17,49,55]. In forested natural areas, garlic mustard has the potential to dominate the herb layer [41,52,56,91]. Invasion of mature eastern deciduous forests by garlic mustard is notable because these habitats were thought to be relatively resistant to nonindigenous plant invasion, particularly by herbaceous species [43,45,55,56]. From the results of a greenhouse study examining the competitive potential of garlic mustard, Meekins and McCarthy  postulated that competition for light within dense garlic mustard stands might inhibit oak regeneration in the understory of eastern deciduous woodlands. However, this same study failed to show greater levels of interspecific competition among garlic mustard, jewelweed, and box elder, 2 potential understory associates.
McCarthy  demonstrated removal of garlic mustard from a deciduous forest understory resulted in increased richness and abundance of understory species, especially annuals and woody perennials. Garlic mustard may be particularly detrimental to native spring ephemerals in eastern deciduous forest understories . McCarthy  failed to demonstrate a link between the magnitude of garlic mustard infestation and native species diversity. Removal experiments, while providing some insight into possible effects of nonindigenous plant invaders, may be inherently limited in their ability to reflect impacts of invasives on preinvasion communities . Limited and conflicting evidence surrounding the assumption that garlic mustard infestation necessarily results in reduced richness and cover of native herbaceous species points out the critical need for more research in this area.
The allelopathic potential of garlic mustard has received some study, with mixed results. McCarthy and Hanson  found little evidence of allelopathic effects of garlic mustard on several plant species studied. They attributed the success of garlic mustard invasiveness strictly to its competitive abilities. Other evidence indicates at least the possibility for allelopathic interference between garlic mustard and neighboring herbaceous plants, as well as the possibility for toxicity against mycorrhizal fungi [35,80]. Roberts and Anderson  found a significant negative correlation (r2 = 0.29; P< 0.05) between garlic mustard density in the field and the mycorrhizal inoculum potential of the soil. McCarthy  found garlic mustard inhibited establishment of seedlings of other species, yet no quantitative relationship could be discerned between garlic mustard biomass and native species diversity. This finding suggests that the mere presence of garlic mustard depresses native diversity, perhaps an allelopathic effect. Further research is needed to a) determine what mechanisms, if any, are responsible for garlic mustard allelopathy, and b) sort out the relative effects of allelopathy vs. resource competition in interactions between garlic mustard and native plants.
Control: The biology of garlic mustard presents significant challenges to its control because it simultaneously possesses characteristics of native forest herbs such as shade tolerance and relatively large seeds, as well as characteristics often ascribed to weeds such as xenogamy and autogamy, and high seed production and germination under a range of environmental conditions. It is also not impacted by its native herbivores and parasites [3,5,17,44]. While garlic mustard invades relatively undisturbed woodlands, invasion may be expedited by natural and anthropogenic disturbance that removes competing native vegetation. Once garlic mustard becomes established, further dispersal and perpetuation within a particular habitat may require little to no further disturbance [46,55].
Deciduous forest fragments that are isolated in an otherwise predominantly agricultural landscape may be more resistant to garlic mustard invasion, due to limited seed sources and inhibitive dispersal distances . However, in areas with large populations of white-tailed deer, even these insular forest remnants may become colonized by garlic mustard.
As with most invasive plants, deterrence is the most effective strategy against garlic mustard. This includes annual monitoring and removal of all invading plants prior to seed production. Garlic mustard is prolific partly because of its ability to self-pollinate. A single individual can produce large numbers of genetically similar but interfertile progeny, which in turn may colonize even small, local microsite disturbances, leading to a potential garlic mustard outbreak. Allaying invasion may require reducing habitat perturbation in susceptible areas and promoting the health of native plant communities .
Garlic mustard population densities may oscillate widely from year to year . Its biennial nature and its seed banking propensity can lead to occasions in which dense stands of garlic mustard appear where none were apparent the year before, and then seemingly disappear the following year only to reappear yet again in subsequent seasons. Further, in years where rosettes are apparently sparse and may evade detection, those monitoring such sites may easily but falsely conclude that garlic mustard is absent. In previously infested areas or areas of suspected susceptibility, careful annual monitoring may be the only way to ensure that garlic mustard is indeed absent from the site.
Once garlic mustard appears within an area, management activities should focus on preventing seed production. While most seeds of garlic mustard tend to germinate during the 1st or 2nd spring following their production, a small number of seeds remain within the seed bank and may germinate over the next several years. Because garlic mustard seed banks may remain viable for up to 6 years, long-term control for a particular stand requires vigilant attention for several consecutive seasons [3,7,14,49]. Even after successful management leads to the apparent absence of garlic mustard, continued periodic monitoring is prudent. A method for destroying seeds of garlic mustard in the soil that would not harm seeds of other species has not been determined .
Because of the biennial life-history strategy of garlic mustard, eradication treatments conducted during spring, after seedlings have germinated and before adults can produce viable seed, have the advantage of affecting 2 generations simultaneously . Ideally, this maximizes the kill of new germinants and seedlings, as well as prevents seed production in adults. Since natural mortality is greatest at the seedling stage garlic mustard may be most vulnerable to control efforts during this time . One potential downside to this strategy is that delaying treatment too late into spring risks unwanted effects on native spring emergents.
An alternative approach is to delay management activities until after the 1st growing season to take advantage of significant natural mortality of rosettes. First year garlic mustard mortality at a site in northern Illinois was estimated at greater than 95% between April and November . This strategy may be especially prudent when the control method requires intensive labor, such as cutting or hand-pulling plants, if minimizing quantities of applied chemicals is desired, or simply if costs of more intensive management activities are prohibitive.
Control of garlic mustard has been tested using several different methods. Since a single control method is rarely 100% effective, a combination of more than 1 may often be a useful strategy. Regardless of methodology, treatments for eradication of garlic mustard must be applied over the entire area of infestation to prevent seed production.
Manual or Mechanical Removal: Pulling entire plants may be an effective method for control of garlic mustard. Care should be taken to remove as much of the root system as possible, to reduce resprouting potential. Pulling can cause soil disturbance and redistribute seeds stored within the upper soil horizons. This problem may be mitigated by thoroughly tamping disturbed soil after pulling. Generally speaking, cutting results in fewer disturbances than pulling. However, pulling may be done at any time during the plant lifecycle, while cutting must be performed during the 2nd growing season while the flowering stem is elongating. Due to the labor-intensive nature of cutting and pulling plants, these practices may only be practical in small or lightly infested areas, especially where burning or herbicide application is inadvisable [49,56]. Hand removal may be most useful for preventing establishment of new garlic mustard colonies in previously uninfested areas .
Control may be accomplished by cutting flowering stems, i.e. using sickles, clippers, or string trimmers, prior to seed production and dissemination. Cutting as close to ground level as possible appears to be most effective. Nuzzo  found that cutting at ground level killed 99% of plants and resulted in virtually no seed production, while cutting at 4 inches (10 cm) resulted in 71% mortality and 98% lower total seed production. Mortality was 6% in control plants during the 3-month study period. Cutting plants prior to full flowering or the onset of seed development may result in production of additional flowering stems from buds located on the root crown . However, waiting until after plants have finished flowering risks dissemination of viable seed. Cut or pulled plant material should consequently be removed from the site and destroyed whenever possible to minimize the risk of inadvertently distributing viable seed [56,70].
Mowing may be similar in effect to cutting, but with more possible negative consequences. Mowing of flowering plants may result in regrowth of new flowering shoots, although this response reportedly diminishes as the season progresses . While mowing may be convenient in large, relatively open areas of infestation such as roadsides, this practice may be more problematic than cutting, as described above. Mowing may promote seed dispersal and is more likely to be indiscriminate regarding which plant species are destroyed. Mowing equipment may also create more disturbed habitat that is likely to be recolonized by garlic mustard .
Prescribed Fire: In areas with a fire-tolerant native flora, frequent prescribed burning may deter garlic mustard invasion by both directly killing invading plants, and perhaps in some areas by enhancing growth of native herbaceous competitors and thereby reducing habitat for garlic mustard colonization [49,88]. For more information about using prescribed fire as a management tool to control garlic mustard, see the Fire Management Considerations section of this summary.
Chemical Control: Chemical control of invasive plants such as garlic mustard can be effective, particularly against large areas of infestation or dense monotypic colonies, and especially when considered within the context of an integrated management plan [47,49]. This report briefly examines the effectiveness of selected chemicals for controlling garlic mustard, some issues involved in the timing of application, and potential effects on native plant communities. Use of herbicides in natural areas should be cautiously considered, and appropriate education and training should be sought before proceeding. Particular caution should be exercised with the use of Bentazon or Acifluorfen. Bentazon is very soluble in water and does not bind to soil well, leading to potential groundwater contamination problems. Acifluorfen is toxic to fish, is moderately persistent in soil and kills native grasses and herbs, and can cause serious eye injury . For further information regarding the use of herbicides in natural areas for control of this and other invasive plant species, see the Weed Control Methods Handbook .
Application of 1% and 2% glyphosate during the dormant season significantly (p < 0.05) reduced adult garlic mustard cover and density in mesic and dry-mesic upland forest and mesic floodplain forest in northern Illinois, but also damaged other species that were green at the time, especially sedges and white avens . Treatment with foliar-applied glyphosate also significantly (p < 0.05) reduced adult densities of garlic mustard, regardless of spring or fall application, in a northern Illinois oak woodland. Seedling frequency in these same plots was significantly (p < 0.001) reduced by spring application .
Dormant-season application of bentazon was less effective at controlling garlic mustard in northern Illinois mesic deciduous forest, but showed none of the nontarget kill associated with glyphosate. At these same sites, application of acifluorfen during dormant season was highly effective againstgarlic mustard, but also killed most native herbaceous vegetation, which was mainly dormant at the time of application.
Use of systemic, nonselective herbicides during the growing season may not be practical in some areas due to deleterious effects on native ground-layer competitors. In these cases, dormant season application may be preferable in order to maintain viable populations of native competitors . Nuzzo  found no difference in effect between single herbicide application and twice applied treatment to the same generation of plants (spring and fall of the same year, fall and the following spring, or 2 consecutive springs). It was suggested that management efforts focus on single applications to successive generations of plants. Fall herbicide application may be a prudent option when risk of negatively affecting native spring-emergent herbs exists. Higher garlic mustard rosette densities in fall may require higher volumes of applied herbicide to be effective .
Mid-summer application of bentazon reduced garlic mustard cover by 94-96% in previously dense stands of garlic mustard rosettes in northern Illinois. Similar applications of acifluoren were less effective, but still significantly reduced garlic mustard cover by 30-46%. Mortality in control plots over the same period was 15%, and not statistically significant. Chemical control activities conducted during the growing season, as above, might be justified when target species densities overwhelm the native flora .
Biological Control: Biological control methods for garlic mustard are not yet developed, but investigations are under way. Several insects that are associated with garlic mustard in its native European habitats are being tested to examine their potential effectiveness as control agents . Fungal pathogens may also have some potential use against garlic mustard. For instance, garlic mustard has shown susceptibility to a fusarium root rot (Fusarium solani) .
Prevention and Control
Relevance to Humans and Ecosystems
Importance to Livestock and Wildlife
Use of garlic mustard as a forage species by white-tailed deer is unclear [15,56]. White-tailed deer may avoid grazing garlic mustard in favor of native herbaceous plants, although this has not been empirically tested [3,49,56]. It is likely that white-tailed deer graze a variety of understory herb species in areas typically susceptible to garlic mustard invasion, and can have a dramatic negative impact on some native herb populations . Deer grazing of native herbaceous plants may enhance garlic mustard at the expense of native species by providing small-scale soil disturbance and by reducing interspecific competition. White-tailed deer may provide small-scale disturbances suitable for garlic mustard colonization within forested areas by trampling and exposing soil. In addition, selective herbivory may enhance garlic mustard at the expense of the preferred native species [3,49,56].
Garlic mustard may be deleterious to some species of butterfly. Adults of several butterfly species lay eggs on garlic mustard instead of their native plant hosts. Because larval development on garlic mustard is often fatally inhibited, this can result in garlic mustard acting as a population sink for these butterfly species, a particularly perilous problem for rare species such as the West Virginia white butterfly (Pieris virginiensis) [10,56,59].
Other uses and values
milk in dairy cattle .
Ecological Threat in the United States
Garlic mustard poses a severe threat to native plants and animals in forest communities in much of the eastern and midwestern U.S. Many native widlflowers that complete their life cycles in the springtime (e.g., spring beauty, wild ginger, bloodroot, Dutchman's breeches, hepatica, toothworts, and trilliums) occur in the same habitat as garlic mustard. Once introduced to an area, garlic mustard outcompetes native plants by aggressively monopolizing light, moisture, nutrients, soil and space. Wildlife species that depend on these early plants for their foliage, pollen, nectar, fruits, seeds and roots, are deprived of these essential food sources when garlic mustard replaces them. Humans are also deprived of the vibrant display of beautiful spring wildflowers.
Garlic mustard also poses a threat to one of our rare native insects, the West Virginia white butterfly (Pieris virginiensis). Several species of spring wildflowers known as "toothworts" (Dentaria), also in the mustard family, are the primary food source for the caterpillar stage of this butterfly. Invasions of garlic mustard are causing local extirpations of the toothworts, and chemicals in garlic mustard appear to be toxic to the eggs of the butterfly, as evidenced by their failure to hatch when laid on garlic mustard plants.
Ecological Threat in the United States
Alliaria petiolata is a biennial flowering plant in the Mustard family, Brassicaceae. It is native to Europe, western and central Asia, and northwestern Africa, from Morocco, Iberia and the British Isles, north to northern Scandinavia, and east to northern India and western China (Xinjiang). In the first year of growth, plants form clumps of round shaped, slightly wrinkled leaves, that when crushed smell like garlic. The next year plants flower in spring, producing cross shaped white flowers in dense clusters. As the flowering stems bloom they elongate into a spike-like shape. When blooming is complete, plants produce upright fruits that release seeds in mid-summer. Plants are often found growing along the margins of hedges, giving rise to the old British folk name of Jack-by-the-hedge. Other common names include Garlic Mustard, Garlic Root, Hedge Garlic, Sauce-alone, Jack-in-the-bush, Penny Hedge and Poor Man's Mustard. The genus name Alliaria, "resembling Allium", refers to the garlic-like odour of the crushed foliage. Some people give the species name Alliaria officinalis for this plant.
In 17th century Britain it was recommended as a flavouring for salt fish. It can also be made into a sauce for eating with roast lamb or salad. All parts of the plant, including the roots, give off a strong odour. The small white flowers have a rather unpleasant aroma which attracts midges and hoverflies, although the flowers usually pollinate themselves. In June the pale green caterpillar of the orange tip butterfly (Anhocharis cardamines) can be found feeding on the long green seed-pods from which it can hardly be distinguished.
It is a herbaceous biennial plant growing from a deeply growing, thin, white taproot that is scented like horseradish. In the first year, plants appear as a rosette of green leaves close to the ground; these rosettes remain green through the winter and develop into mature flowering plants the following spring. Second year plants grow from 30–100 cm (rarely to 130 cm) tall. The leaves are stalked, triangular to heart-shaped, 10–15 cm long (of which about half being the petiole) and 5–9 cm broad, with a coarsely toothed margin. The flowers are produced in spring and summer in button-like clusters. Each small flower has four white petals 4–8 mm long and 2–3 mm broad, arranged in a cross shape. The fruit is an erect, slender, four-sided pod 4 to 5.5 cm long, called a silique, green maturing pale grey-brown, containing two rows of small shiny black seeds which are released when the pod splits open. A single plant can produce hundreds of seeds, which scatter as much as several meters from the parent plant.
Depending upon conditions, garlic mustard flowers either self-fertilize or are cross-pollinated by a variety of insects. Self-fertilized seeds are genetically identical to the parent plant, enhancing its ability to colonize an area where that genotype is suited to thrive.
Cultivation and uses
Garlic mustard is one of the oldest discovered spices to be used in cooking in Europe. Evidence of its use has been found from archeological remains found in the Baltic, dating back to 6100-5750 BP.
The chopped leaves are used for flavoring in salads and sauces such as pesto, and sometimes the flowers and fruit are included as well. These are best when young, and provide a mild flavour of both garlic and mustard. The seeds are sometimes used to season food directly in France.
In Europe as many as 69 species of insects and seven species of fungus utilize Garlic Mustard as a food plant, including the larvae of some Lepidoptera species such as the Garden Carpet moth.; however, in North America, the plant offers no known wildlife benefits and is toxic to larvae of certain butterfly species that lay eggs on the plants, as it is related to native mustards.
As an invasive species
Garlic mustard was introduced in North America as a culinary herb in the 1860s and is an invasive species in much of North America. As of 2006[update], it is listed as a noxious or restricted plant in the US states of Alabama, Connecticut, Massachusetts, Minnesota, New Hampshire, Oregon, Vermont, West Virginia and Washington., and occurs in 27 midwestern and northeastern states in the US, and in Canada. Like most invasive plants, once it has an introduction into a new location, it persists and spreads into undisturbed plant communities. In many areas of its introduction in Eastern North America, it has become the dominant under-story species in woodland and flood plain environments, where eradication is difficult.
The insects and fungi that feed on it in its native habitat are not present in North America, increasing its seed productivity and allowing it to out-compete native plants.
Garlic Mustard produces allelochemicals, mainly in the form of the cyanide compounds allyl isothiocyanate and benzyl isothiocyanate, which suppress mycorrhizal fungi that most plants, including native forest trees, require for optimum growth. However, allelochemicals produced by Garlic Mustard do not affect mycorrhizal fungi from Garlic Mustard's native range, indicating that this "novel weapon" in the invaded range explains Garlic Mustard's success in North America. Additionally, because white-tailed deer rarely feed on Garlic Mustard, large deer populations may help to increase its population densities by consuming competing native plants. Trampling by browsing deer encourages additional seed growth by disturbing the soil. Seeds contained in the soil can germinate up to five years after being produced (and possibly more). The persistence of the seed bank and suppression of mycorrhizal fungi both complicate restoration of invaded areas because long-term removal is required to deplete the seed bank and allow recovery of mycorrhizae.
Garlic mustard produces a variety of secondary compounds including flavonoids, defense proteins, glycosides, and glucosinolates that reduce its palatability to herbivores. Research published in 2007 shows that, in northeastern forests, garlic mustard rosettes increased the rate of native leaf litter decomposition, increasing nutrient availability and possibly creating conditions favorable to garlic mustard's own spread.
Preventing seed production and depletion of the soil seed bank are key to eradicating infestations. Non chemical control includes removal by hand-pulling or cutting at the base; mowing; burning; or manipulation of the environment to reduce light. Pulling is more effective if the root is removed. Garlic mustard may invade forested sites where the canopy has been disturbed, so management by planting or encouraging other plants to intercept light will aid control, or prevent new infestations. Control is best in early spring before flowering. Removed plants should be bagged or burned, as seeds or roots may survive composting. Chemical control may be achieved by foliar application with a number of different herbicides. Timing herbicide applications to early spring or late fall may protect native or desirable plants in the same locations as Garlic mustard is generally active across a longer season than other plants in northern temperate climates. All methods of weed control must be repeated for 3–4 years or longer to be effective—as seeds germinate, or as surviving roots re-sprout the infestation may be quickly re-established.
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-  University of Wisconsin Extension, Management of invasive plants in Wisconsin: Garlic mustard (A3924-07)
Names and Taxonomy
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