R. M. Godman and Kenneth Lancaster
Eastern hemlock (Tsuga canadensis), also called Canada hemlock or hemlock spruce, is a slow-growing long-lived tree which unlike many trees grows well in shade. It may take 250 to 300 years to reach maturity and may live for 800 years or more. A tree measuring 193 cm (76 in) in d.b.h. and 53.3 m (175 ft) tall is among the largest recorded. Hemlock bark was once the source of tannin for the leather industry; now the wood is important to the pulp and paper industry. Many species of wildlife benefit from the excellent habitat that a dense stand of hemlock provides. This tree also ranks high for ornamental planting.
General: Pine family (Pinaceae). Native trees to 30 meters tall, with a broadly conic crown, the branches often drooping at the ends and “feathery;” twigs yellow-brown, densely pubescent. Bark brownish, scaly and fissured. Needles evergreen, flat, mostly appearing 2-ranked, (5-)15-20(-25) mm long, narrowed to a petiole-like base and set on peg-like projections, the lower surface waxy, with 2 broad, conspicuous stomatal bands (like pale lines), the upper surface shiny-green to yellow-green, the margins minutely toothed, especially toward apex. Seed cones ovoid, 1.5-2.5 cm long, borne near the branch tips, hanging. The common name pertains to its distribution in eastern North America.
Variation within the species: various studies have recorded physiological and morphological variation within the species, but no major discontinuities have been found to suggest that named entities should be recognized.
The geographic distribution of eastern hemlock completely overlaps that of Carolina hemlock (Tsuga caroliniana), which differs in its high elevation habitats, leaves mostly spreading all directions from the twigs, and more elongate seed cones.
Distribution: Eastern hemlock is a species of the northeastern and Appalachian regions of North America: from New Brunswick and Nova Scotia to southern Quebec and Ontario, south to northern Georgia and Alabama, west in the lake states to Minnesota with outliers in southern Michigan, western Ohio, and southern Indiana. Many disjunct populations, probably glacial relicts, occur east of the Appalachians in the middle Atlantic states. For current distribution, please consult the Plant Profile page for this species on the PLANTS Web site.
Canada hemlock, hemlock spruce
Global Range: Ontario to Nova Scotia south to Maryland and eastern Minnesota, and along the Appalachian mountains to Georgia and Alabama (Fernald 1950, Kartesz 1999).
Range and Habitat in Illinois
Regularity: Regularly occurring
Regularity: Regularly occurring
Occurrence in North America
MN NH NJ NY NC OH PA RI SC TN
VT VA WV WI NB NS ON PE PQ
mid-Atlantic states, and the Lake States, and extends south in the
Appalachian Mountains to northern Georgia and Alabama and west from the
mountains into Indiana, western Ohio, and western Kentucky. At its
northern limit, eastern hemlock ranges along the southern border of
Canada from southern Ontario to Cape Breton Island, Nova Scotia [20,35].
The range completely overlaps that of Carolina hemlock (Tsuga caroliniana), a closely related species limited to the slopes of the Appalachians from Virginia and West Virginia into Georgia.
Commercial volumes of eastern hemlock have been greatly reduced by harvesting. In Michigan, for example, sawtimber volume decreased 69 percent and growing stock volume decreased 71 percent between 1935 and 1955 (10). Both the type area and volume are continuing to decline because of harvesting and failure to regenerate, particularly in the western portion of the range. The remaining sawtimber is concentrated in the Northeast and the Lake States (5).
- The native range of eastern hemlock.
Eastern hemlock grows from about sea level to 1500 meters. In the more southern parts of its range, eastern hemlock occurs only where there is drainage of cool, moist air -- on moist rocky ridges, valleys and ravines, hillsides, and lakeshores. In the northern hardwood forest, it is found on a greater variety of sites, including low rolling hills and glacial ridges. Eastern hemlock most commonly grows in mixed stands with species such as white pine, northern red oak, sugar maple, American beech, yellow birch, and white ash.
upsweeping branches. At maturity, it is commonly 60 to 70 feet (18-21
m) tall and 24 to 48 inches (61-122 cm) in d.b.h. One of the largest
eastern hemlock recorded was 175 feet (53 m) tall and 76 inches (193 cm)
in d.b.h. It reaches ages in excess of 800 years. Eastern hemlock
roots are shallow and widespreading [20,26].
Habitat and Ecology
Range and Habitat in Illinois
Comments: Mostly hilly or rocky woods (Fernald, 1950); in southern portion of range, frequently in moist, shaded ravines, or other sheltered microhabitats.
At its western and southern limits, eastern hemlock is confined to moist
cool valleys, moist flats, northern and eastern slopes, coves, benches,
and ravines. In the northern part of its range, it tolerates drier and
warmer sites. Eastern hemlock also occurs at swamp borders provided
peat and muck soils are shallow [14,20,40,65].
Favorable eastern hemlock sites are moist to very moist with good
drainage. Eastern hemlock grows in a wide variety of acidic soils;
textures include sandy loams, loamy sands, and silty loams with gravel
of glacial origin in the upper profile [14,20].
While generally considered a moisture-demanding species, eastern hemlock
grows on dry sites protected from fire, such as rocky ledges . Two
types of eastern hemlock have been described: one grows in mesophytic
habitats and one on subxeric slopes . The types cannot be termed
ecotypes, however, because of incomplete habitat differentiation.
Eastern hemlock growing on "subxeric" slopes may actually be receiving
moisture from seeps .
In the northeastern United States, eastern hemlock grows at elevations
ranging from sea level to 2,400 feet (730 m). In the southern
Appalachian Mountains it grows from 2,000 to 5,000 feet (610-1,520 m).
In the Allegheny Plateau region of New York, Pennsylvania, and Ohio, it
grows from 1,000 to 3,000 feet (300-910 m) [13,20,34].
Understory associates are scarce because of acidic infertile humus, low
light, and cool conditions [14,34]. Shrub and small tree associates
that occur in canopy gaps include sweet birch (Betula lenta), striped
maple (Acer pensylvanicum), mountain maple (A. spicatum), hobblebush
(Viburnum alnifolium), mapleleaf viburnum (V. acerifolium), mountain
winterberry (Ilex montana), rhododendron (Rhododendron spp.),
mountain-laurel (Kalmia latifolia), and witch hazel (Hamamelis
virginiana). Herbs can include Canada mayflower (Maianthemum
canadense), star flower (Trientalis borealis), common woodsorrel (Oxalis
montana), and goldthread (Coptis groenlandica). Other associated
species include clubmosses (Lycopodium spp.), bracken (Pteridium
aquilinum), woodfern (Dryopteris spp.), and sedges (Carex spp.). Common
mosses include Dicranium spp. and Polytrichum spp. [14,20,32,45,65].
Key Plant Community Associations
Eastern hemlock occurs as a dominant or codominant in coniferous and
mixed-hardwood forests. It is often the only conifer present in mixed
mesophytic forests of the eastern United States .
Publications listing eastern hemlock as codominant or dominant are as
The natural forests of Maryland: an explanation of the vegetation map
of Maryland 
A multivariate analysis of forest communities in the western Great Smoky
Mountains National Park 
The vegetation of Wisconsin 
The principal plant associations of the Saint Lawrence Valley 
Field guide: forest habitat types of northern Wisconsin 
A classification of the deciduous forest of eastern North America 
The natural communities of South Carolina 
Forest associations in the Harvard Forest 
Vegetation of the Great Smoky Mountains 
Habitat: Cover Types
This species is known to occur in association with the following cover types (as classified by the Society of American Foresters):
5 Balsam fir
17 Pin cherry
18 Paper birch
19 Gray birch - red maple
20 White pine - northern red oak - red maple
21 Eastern white pine
22 White pine - hemlock
23 Eastern hemlock
24 Hemlock - yellow birch
25 Sugar maple - beech - yellow birch
26 Sugar maple - basswood
27 Sugar maple
28 Black cherry - maple
30 Red spruce - yellow birch
31 Red spruce - sugar maple - beech
32 Red spruce
33 Red spruce - balsam fir
34 Red spruce - Fraser fir
35 Paper birch - red spruce - balsam fir
37 Northern white-cedar
39 Black ash - American elm - red maple
44 Chestnut oak
52 White oak - black oak - northern red oak
53 White oak
58 Yellow-poplar - eastern hemlock
59 Yellow-poplar - white oak - northern red oak
60 Beech - sugar maple
97 Atlantic white-cedar
108 Red maple
Habitat: Plant Associations
This species is known to occur in association with the following plant community types (as classified by Küchler 1964):
K093 Great Lakes spruce - fir forest
K094 Conifer bog
K095 Great Lakes pine forest
K096 Northeastern spruce - fir forest
K097 Southeastern spruce - fir forest
K103 Mixed mesophytic 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
FRES11 Spruce - fir
FRES15 Oak - hickory
FRES18 Maple - beech - birch
FRES19 Aspen - birch
Soils and Topography
Eastern hemlock grows from sea level to about 730 m (2,400 ft) in elevation in the northeastern and northern portions of the range. Most commonly it is found on benches, flats, and swamp borders, provided the peat and muck soils are shallow (Aquic Haplorthods or Aerie Haplaquods). On the Allegheny
Plateau, especially in New York and Pennsylvania, most of the hemlock grows between 300 and 910 m (1,000 and 3,000 ft) (35). In the southern Appalachians the most frequent occurrences are at elevations of 610 to 1520 m (2,000 to 5,000 ft) and often are restricted to north and east slopes, coves, or cool, moist valleys (35). Outliers tend to be severely restricted by a combination of edaphic and climatic factors.
Fully stocked stands of eastern hemlock tend to develop similar microclimates because of their dense canopy, dense shading, deep duff layer, and subsequent retention of moisture and uniformly low temperatures. In the few stands in which understories do develop, the type of vegetation tends to be similar to other forest types in the area although fewer species become established (30).
Habitat & Distribution
Eastern hemlocks begin to produce cones at about age 15 in vigorous trees or much later in suppressed trees. There is a high frequency of cone crops and individual trees have a long duration of cone production (excellent cone production has been reported for trees more than 450 years old), but the viability of seed usually is low. Seeds are particularly easily damaged by drying. The best conditions for germination and seedling establishment are under a 70-80% crown cover on an exposed, partially decomposed layer. Otherwise, regeneration is restricted to rotten logs, stumps, and mounds that normally have warmer surfaces and better moister retention than the forest floor.
Eastern hemlock is the most shade tolerant of all tree species and individuals may remain in the understory in natural stands for 25-400 years. After a plant reaches 1-2 meters in height, the root system has reached a depth not radically afffected by surface drying, and release from overstory competition may greatly increase annual growth. In general, eastern hemlock is a slow-growing tree that may take 250 to 300 years to reach maturity and may live for 900 years or more.
Associated Forest Cover
5 Balsam Fir
17 Pin Cherry
18 Paper Birch
25 Sugar Maple-Beech-Yellow Birch
26 Sugar Maple-Basswood
27 Sugar Maple
28 Black Cherry-Maple
35 Paper Birch-Red Spruce-Balsam Fir
37 Northern White-Cedar
39 Black Ash-American Elm-Red Maple
44 Chestnut Oak
52 White Oak-Black Oak-Northern Red Oak
53 White Oak
59 Yellow-Poplar-White Oak-Northern Red Oak
60 Beech-Sugar Maple
97 Atlantic White-Cedar
108 Red Maple
Fully stocked stands of eastern hemlock form such a dense canopy that an understory seldom is able to develop. When an understory does exist, the most common herbs are false lily-of-the-valley (Mianthemum canadense), starflower (Trientalis borealis), woodfern (Dryopteris spp.), common woodsorrel (Oxalis montana), goldthread (Coptis groenlandica), clubmoss (Lycopodium spp.), and sedges (Carex spp.). Common mosses are Dicranum and Polytrichum (30,39).
Diseases and Parasites
The most damaging agents to young seedlings, other than desiccation, are damping-off fungi and root rots (16). Pythium spp. and Rhizoctonia spp. flourish in wet, poorly drained soils and in well-drained soils, respectively, and are common on eastern hemlock. At least three root rots- Cylindrocladium scoparium, Rhizina undulata (common on burn areas), and Fusarium moniliforme- are common on eastern hemlock. F. moniliforme has been isolated from embryonic tissue and seedcoats as well as in the soil (16,23).
Several diseases affect the needles and twigs of eastern hemlock. The rust caused by Melampsora farlowii is one of the most damaging. It causes shoot blight and curls and attacks the cone often resulting in cone abortion. Three rusts caused by M. abietiscanadensis, Pucciniastrum hydrangeae, and P. vaccinii spp. affect only the needles. Single needle browning throughout the crown is caused by Fabrella tsugae. Lower foliage in very wet and shady areas often has a grayish mat appearance on both the needles and twigs caused by Rosellinia herpotrichioides. Dimerosporium tsugae occasionally forms a black, sooty growth on the needles.
Living heartwood of eastern hemlock is attacked by Tyromyces borealis, particularly in the northeast, leaving white flecks in the wood. Pholiota adiposa is fairly common in the Lake States and causes a cavity along the pith axis. Other rots are the trunk rot caused by Haematostereum sanguinolentum; abrown, red ring rot caused by Phellinus pini; and a red heart rot caused by P. robustus. The red-varnish-topped fungus, Ganoderma tsugae, is the most common decayer of stumps and old logs.
Numerous fungi are associated with the root system but rarely develop conks or kill trees. The most common are the shoestring fungus, Armillaria mellea, and the velvet top fungi, Phaeolus schweinitzii, Tyromyces balsameus, and Heterobasidion annosum. At least two mycorrhiza are known to occur on the roots (16).
Although at least 24 insects attack eastern hemlock, few are economically important. The most important is the hemlock borer, Melanophila fulvoguttata, which attacks weakened trees. Symptoms usually consist of woodpecker-like holes in the bark, galleries filled with dark excrement, and yellowing shoot tips (27). Spruce budworm, Choristoneura fumiferana, defoliates and kills hemlock after defoliating all the balsam fir in the stand.
The hemlock looper, Lambdina fiscellaria fiscellaria, devours part of the needle after which the remainder turns brown. In nurseries, white grubs of the strawberry root weevil, Otiorhynchus ovatus, consume the roots, and larvae of the black vine weevil, O. sulcatus, feed on the needles (40). In the eastern States the hemlock scale, Abgrallaspis ithacae, damages young shade trees, and the gypsy moth, Lymantria dispar, kills understory trees.
Numerous animals feed on eastern hemlock and often cause serious damage, marked loss of vigor, or even death. White-tailed deer readily browse this species although it has been ranked seventh in winter food preference. In some regions, patches of regeneration have been eliminated following heavy browsing in years when deer populations are high. Although deer have been blamed for the absence of eastern hemlock in many localities, no regeneration occurred under similar conditions in fenced areas; thus, overstory-site-temperature requirements are presumably more critical (2,6,8).
Snowshoe hares and New England cottontails frequently browse eastern hemlock. Mice, voles, squirrels, and other rodents also feed on seeds and small seedlings both under natural stands and in nurseries (1). Porcupines occasionally gnaw the bark on larger trees causing serious wounds and top-kill (4). Sapsuckers have been associated with ring shake in some areas (19,21).
Small eastern hemlock trees are highly susceptible to wildfire but prescribed burns are beneficial for securing natural regeneration. The thick bark of older trees is resistant to light burns but saplings are usually destroyed. Root injury often occurs from high intensity fires because of heavy litter accumulation.
Drought is probably the most serious damaging agent to eastern hemlock, especially during the seedling stage. Winter drying caused by excessive transpiration on warm, windy days has caused severe needle injury.
In later stages of stand development, heavy cuttings predispose trees to windthrow because of their shallow rooting habit. Older trees are susceptible to radial stress cracks and ring shake, particularly in partially cut stands (19). Eastern hemlock is sensitive to salt spray or drift and sulfur fumes and is one of the species most often struck by lightning (16,25).
Number of Occurrences
Note: For many non-migratory species, occurrences are roughly equivalent to populations.
Estimated Number of Occurrences: 81 to >300
Broad-scale Impacts of Plant Response to Fire
The Research Project Summary Early postfire effects of a prescribed fire in the southern Appalachians of North Carolina provides information on prescribed fire and postfire response of plant community species, including eastern hemlock, that was not available when this species review was originally written.
Immediate Effect of Fire
Low-severity fire readily kills seedlings and saplings of eastern
hemlock, and may also kill larger trees. A low-severity ground fire in
a northern hardwoods community in south-central New York killed 93
percent of the eastern hemlock saplings. Sixty percent of the mature
eastern hemlock died or were badly injured as a result of the fire .
The presence of fire scars indicates that larger trees have thick enough
bark to survive low-severity surface fires [18,36].
Tree without adventitious-bud root crown
Secondary colonizer - off-site seed
Eastern hemlock is very susceptible to fire because of its thin bark,
shallow roots, low-branching habit, and heavy litter deposits [20,51].
It is possibly the most fire-sensitive mesophytic tree species in its
Eastern hemlock usually escapes fire because it occurs in moist habitats
and is often associated with hardwoods which do not readily burn. If a
fire starts in a cutover area, a windfall area, or an area with dead
standing timber, it may carry into a northern hardwoods forest if there
is strong wind . In Michigan, the average return time for severe
crown fires in the hemlock-white pine-northern hardwoods type is
estimated to be about 1,400 years . In northeastern Maine, the
average return interval for fire in spruce-fir forests in which eastern
hemlock is a minor component is about 800 years .
Vogl  considers eastern hemlock a fire-initiated species rather than
a fire-independent species because it benefits from fire-prepared
seedbeds. However, suggestions that fire promotes regeneration of
eastern hemlock are not well documented. Given the difficulties in
accurate age estimates because of heart rot, Rogers  suggests that
even-aged eastern hemlock forests that regenerated after fire may
actually be uneven-aged.
More info for the terms: climax, litter
Obligate Climax Species
Eastern hemlock is very shade tolerant . Seedlings survive in as
little as 5 percent of full light . Individuals are able to survive
several hundred years of suppression, and many show numerous growth
releases and suppressions . Saplings less than 2 inches (5 cm) in
d.b.h. may be more than 100 years old .
Seedlings are able to establish under the canopy of mature individuals.
Eastern hemlock establishes under dense sugar maple canopies and can
replace that species . Eastern hemlock uniquely modifies
semipermanent soil properties, such as acidity, which favors its
reproduction. Opportunities to establish in a mature forest increase
over time as nurse logs and tip-up mounds accumulate .
The general desgination of eastern hemlock as a climax species has been
questioned [22,41]. In some old-growth eastern hemlock stands, the
smaller size classes of hemlock are being replaced by American beech
(Fagus grandifolia) and sugar maple . Because of this lack of
regeneration, Hemond and others  suggest that eastern hemlock
requires disturbance to perpetuate itself.
In contrast, other authors suggest that disturbance is responsible for
the lack of regeneration in mature hemlock forests [3,6,51].
White-tailed deer populations have increased since presettlement times
because logging of virgin forests opened up habitat, predators declined,
and the deer were protected. Deer often consume all eastern hemlock
seedlings and saplings in the winter. Where deer populations are low,
eastern hemlock appears to be able to reproduce in its own shade and
become a component of a self-perpetuating homogenous climax forest .
Eastern hemlock requires partial shade for establishment and is a late
colonizer of disturbed sites . In the Pisgah Forest in southwestern
New Hampshire, 80 percent of old-growth eastern hemlock established
within 37 years of disturbance. Hardwoods grew rapidly into the canopy
while eastern hemlock grew slowly as shade-tolerant saplings. Eastern
hemlock extended into the canopy following subsequent disturbance .
The understory population of eastern hemlock readily takes advantage of
canopy gaps. Eastern hemlock increased in importance as American
chestnut (Castanea dentata) declined from chestnut blight . It is
currently replacing American beech where that species is succumbing to
beech bark disease . Eastern hemlock is not successful in
regenerating in canopy gaps in areas such as the New York Botanical
Forest, where the occasional light arson fire, trampling, and other
urban stresses kill seedlings. In addition, the removal of fallen logs
in the forest decreases the amount of adequate substrate for germination
The slow invasion of oak-dominated sites by eastern hemlock appears to
be related to heavy leaf litter and the absence of favorable seedbed
Trees begin producing seed when they are 20 to 30 years old. Eastern
hemlocks older than 450 years still produce large seed crops. This
species bears cones every year, and large crops are frequent, usually
every 3 to 4 years. The small winged seeds are dispersed by gravity and
wind; most fall within one-tree-height distance from the source [20,54].
The seeds are partially dormant and germinate best when stratified for
about 10 weeks at or slightly above freezing. Germination occurs at a
range of temperatures; seeds from the northern portion of its range
germinate at lower temperatures than seeds from the southern portion
[20,54]. Seeds do not remain viable if they do not germinate the first
spring after seedfall .
Seeds germinate best on moist substrates, such as rotten wood, mineral
soil, mineral soil mixed with humus, well-decomposed litter, and moss
mats [14,62]. The number of seedlings established on rotten logs and
stumps increases as the wood decays and the moss cover increases.
Seedlings commonly establish on "tip-up mounds" formed by fallen trees
. Seedlings grow slowly and cannot tolerate full sunlight until
fully established, usually when they are 3 to 5 feet (0.9-1.5 m) tall
Eastern hemlock regeneration appears to be periodic and is influenced by
fire, windthrow, drought, and stand conditions. A young dense stand may
exclude regeneration for many years because of severe root competition
in the upper soil layers, dense low shade, and dry acidic litter
[27,56]. Hemlock regeneration is present in the understory of stands
with a parent overstory density of up to 140 square feet per acre (32 sq
m/ha) but is most abundant when eastern hemlock comprises 80 to 100
square feet per acre (18-23 sq m/ha) of the overstory .
Eastern hemlock does not sprout and layers only rarely .
Growth Form (according to Raunkiær Life-form classification)
Plant Response to Fire
Forest in southwestern New Hampshire, 80 percent of old-growth hemlock
germinated within the first 37 years after a major fire in 1665 .
Reaction to Competition
At all ages, however, eastern hemlock responds to release in both height and diameter growth. Growth rates in excess of 6.4 cm (2.5 in) per decade are possible following release either from side or overhead suppression. Excessive release often results in reduced growth and mortality and has been a contributing factor to partial uprooting or windthrow because of shallow rooting. Trees originating on logs or stumps often develop stilted root systems and also are susceptible to windthrow (19).
Even-aged or uneven-aged (selection) management systems can be successfully used to manage hemlock, but with certain limitations on the selection system. In the Lake States, the selection system has not always been successful and is not recommended for upland sites. In the East, the selection system has been used successfully on a limited basis, but the even-aged system is preferred and most frequently used.
A 2- or 3-cut shelterwood system is the best even-aged method for regenerating eastern hemlock. It is effective because it promotes seed germination and early seedling development by reducing moisture stress. However, the site must be properly scarified and all competing understory hardwoods removed to develop satisfactory seedbed conditions before or immediately after the first and sometimes the second cut.
In mixed stands of hardwoods and hemlock, where the proportion of hemlock is 15 percent or more, it is feasible to manage for hemlock, but at various residual stocking levels. Hemlock does not require as much growing space as hardwoods, so residual stocking is greater in stands where hemlock predominates. For example, a stand of trees averaging 25 cm (10 in) in diameter that contains 15 to 29 percent hemlock would be marked to favor hemlock at a residual stocking of about 22 m² (95 ft²) basal area of both hemlock and hardwoods. This same stand with 30 percent or more hemlock would be managed to 29 m² (125 ft²) of basal area. If less than 15 percent hemlock, the stand should be managed for the hardwood type represented.
Many fully stocked stands of eastern hemlock have basal areas in excess of 69 m²/ha (300 ft²/acre). When thinning heavily stocked stands- 46 m²/ha (200 ft²/acre)- no more than one-third of the total basal area should be removed at one time. Excessive cutting results in reduced growth and increased mortality and contributes to windthrow. In addition, hardwood encroachment interferes with the successful establishment of hemlock. Fully stocked stands with densities less than 46 m²/ha (200 ft²/acre) can be thinned to a minimum of 27 m²/ha (120 m²/acre) without jeopardizing the residual stand (22).
Acceptable standards for implementation of the uneven-aged system, based on field experience, include a residual stocking of 30 m²/ha (130 ft²/acre) in stands predominantly hemlock (50 percent or more); a stand structure (diameter distribution) of 35 percent poles 13 to 25 cm (5 to 10 in) d.b.h., and 65 percent sawtimber 30 cm (12 in) and larger. These guidelines will ensure a balanced growth between poletimber and sawtimber size classes. In addition, a continuous flow of ingrowth will occur and regeneration is assured if proper care is given to seedbed requirements.
Life History and Behavior
Eastern hemlock male strobili open and pollen is dispersed in late April
to early June, depending on locality. This is usually 2 weeks after the
leaf buds open. Fertilization is complete in about 6 weeks, and cones
reach full size in late August or early September. The cones open in
mid-October, but seed dispersal may extend into the winter . Cones
close in wet weather and open again in subsequent dry weather,
prolonging seed dispersal. Germination occurs in the spring .
Most of the stock used in planting, both under forest conditions and as ornamentals, is grown from seed. Nursery grown seedlings grow slowly; 3-0 stock ranges from 13 to 23 cm (5 to 9 in) tall. Survival and height growth of planted hemlock, unlike natural regeneration, tend to be good both in the open and under partial overstories. Trees in a study in the Alleghenies grew significantly faster on north slopes under overstories of intermediate densities.
Eastern hemlock seed is partially dormant at maturity and must be stratified about 10 weeks at or slightly above freezing temperatures for best germination. Unstratified seed must be exposed to light to break the partial dormancy. Under natural conditions the chilling requirements are met during the winter and the spring germination seldom is delayed because of seed dormancy (35). Germination is epigeal.
The temperature requirements for germination of eastern hemlock are more exacting than for other species in the genus. A constant temperature of 15° C (59° F) is about optimum for germination. High germination percentages usually occur at temperatures ranging from 7° to 18° C (44° to 64° F), depending on the seed source (29,35). These temperatures are nearly identical to those required for yellow birch (Betula alleghaniensis), the most common associated species in the northern region, and help to explain the association of two species differing so much in tolerance. Achieving desirable temperatures for germination under natural conditions is difficult because eastern hemlock seeds require from 45 to 60 days to reach their peak in germinative energy. Contrary to common belief, the species requires a warm, moist site for stand establishment rather than the cool, moist conditions that usually develop as stands mature.
Eastern hemlock seeds are easily damaged by drying. In one study 60 percent of the seeds were severely damaged after only 2 hours of drying, and 80 percent died or did not recover after 6 hours of drying (35). Drying of the seedling after germination caused heavy root mortality that could not be overcome once moisture conditions improved.
Natural stands of eastern hemlock nearly always contain a large component of relatively even-aged trees but consistently have a stocking of older age classes and larger diameter trees that provided shelter during the regeneration period (17,33,35,39). Consequently, new stands of eastern hemlock and yellow birch can be established under a high density overstory (from 70 to 80 percent crown cover) using the shelterwood regeneration system. The site must be prepared, however, by thorough mixing of organic and mineral soil or by prescribed fire to expose a partially decomposed layer (6,12,14,18,26,32,34, 35,38). Under this system, optimum conditions are created for germination and seedling establishment. Without these conditions most eastern hemlock regeneration is restricted to rotten logs, stumps, and mounds that normally have warmer surfaces and better moisture retention than the forest floor.
The rigid overstory and seedbed requirements for successful natural regeneration of eastern hemlock were evident in a direct seeding study in northwestern Pennsylvania. "No hemlock germinated on prepared spots in the open (hemlock rarely germinates and becomes established in open areas) and only a few germinated under a light overstory because of the moisture stress created under these conditions." Germination was good, however, on prepared sites under a pole-size stand, especially on north slopes (20).
Under ideal growing conditions, seedlings of eastern hemlock develop slowly. First-year seedlings may grow only 25 to 38 mm (1 to 1.5 in) in height and the roots extend less than 13 mm (0.5 in) into the soil. These conditions provide moisture in the upper soil horizon throughout the growing season. "Because of their stable moisture requirements, seedlings are very sensitive to high temperatures and drying of the surface soil during the establishment period. Once the root system has reached a soil depth not radically affected by surface drying, usually after the second year, the seedlings grow more rapidly without interference of overhead shade. Seedlings are fully established when they are 0.9 to 1.5 m (3 to 5 ft) tall and at that time, can be released completely from overhead competition without fear of mortality."
Eastern hemlock seedlings are subject to damping-off as well as root rot fungi (23,35). The fungi may be present in the soil or within the seed before it is dispersed. At least seven species of fungi are known to attack the seed, and several other species cause damping-off. Treatment of seeds with fungicides is frequently ineffective in controlling diseases and also delays or reduces germination. The high incidence of seedling disease combined with low seed viability suggest that supplemental seeding would enhance natural seeding under most conditions.
Seed Production and Dissemination
Seeds ripen about the time the cones change from yellowish green to purple brown. Dispersal of the seeds begins when the cones turn deeper brown indicating a reduction in moisture content. Most seeds fall within tree height because of the small wings. Additional distribution may occur from drifting on crusted snow. Some seeds may remain in the cones through the winter but usually they are sterile, having developed without an embryo (35). In healthy, vigorous seeds, the embryo extends the full length of the seed.
Eastern hemlock is one of the most frequent cone producers among the eastern conifers. Good or better cone crops occur 61 percent of the years, based on 32 years of observation in Wisconsin (13,29,37). Successive good or better cone crops did occur for one 5-year period and successive poor cone crops for a maximum of only 2 years. Excellent cone production has been reported for trees more than 450 years of age (35).
Flowering and Fruiting
Pollen usually is dispersed by the wind beginning about 2 weeks after leaf buds burst, when the bracts on the female conelet are partially open (28,29,35). At the close of pollination receptivity, the conelets are in a drooping position and the cone scales reclose. Fertilization is complete in about 6 weeks. During this period the pollen is extremely sensitive to drying, often the cause of seed failure (28). Cones reach full size in late August to early September, about the same time as the winter buds begin to form. Cones open fully in mid-October, and seed dispersal extends into the winter. Opened cones may persist on the trees for slightly more than 1 year.
Growth and Yield
Mature eastern hemlock trees attain relatively large diameters and height as well as retaining excellent stem form. The record age is reported to be 988 years, largest diameter 213 cm (84 in), and maximum height 49 m (160 ft) (34). In typical stands, however, ages approaching 400 years, diameters of 89 to 102 cm (35 to 40 in), and heights in excess of 30 m (100 ft) are most common (table 1). "Accurate site index curves are not available for hemlock because most dominant trees have been suppressed during their early years, a result of the species' rigid overstory requirements for successful natural regeneration."
Table 1- Average dimensions of dominant eastern hemlock trees at selected locations Southern Appalachians Michigan New York Age D.b.h. Height D.b.h. Height D.b.h. Height yr cm m cm m cm m 40 23 16 14 13 11 12 60 33 22 24 19 19 18 80 43 26 33 23 27 22 100 52 30 41 26 35 26 120 62 33 49 28 43 28 140 71 35 57 29 52 30 160 81 37 65 30 61 31 180 91 38 - - 70 - 200 100 39 - - 78 - yr in ft in ft in ft 40 9.0 53 5.7 42 4.4 39 60 13.1 71 9.4 62 7.4 58 80 16.9 86 12.8 76 10.5 73 100 20.6 98 16.1 85 13.8 84 120 24.3 107 19.4 91 17.1 91 140 28.0 114 22.6 96 20.4 97 160 31.9 120 25.7 100 23.9 102 180 35.7 125 - - 27.4 - 200 39.5 129 - - 30.9 - Yields of eastern hemlock tend to be higher than in most forest types except for white pine and red pine (Pinus resinosa). In New England, hemlock stands have about twice the volume of oak stands at 80 years of age but only from 50 to 60 percent of the volume of white pine stands at the same age (35). In northeastern Wisconsin on a typical loam podzol, well stocked hemlock and yellow birch stands attain volumes of 154 m³/ha (11,000 fbm) by age 110. On heavier soils, hemlock stands with a mixture of hardwoods reach volumes of 217 m³/ha (15,500 fbm) at 100 years. In pure, older stands gross volumes are reported in excess of 322 m³/ha (23,000 fbm) in Wisconsin and more than 560 m³/ha (40,000 fbm) in New England, but cull percent tends to increase rapidly in large diameter trees (31,35).
Molecular Biology and Genetics
Comparison of an outlier source with one from Wisconsin indicated that races of eastern hemlock differ in physiological and morphological characteristics associated with locality (7). No further studies have been reported on the genetics of eastern hemlock and no superior trees have been selected.
The primary effort in genetic research is propagation of variants for ornamental purposes. At least 280 clones are recorded as being variants, ranging from prostrate to weeping forms (11).
Barcode data: Tsuga canadensis
Statistics of barcoding coverage: Tsuga canadensis
Public Records: 2
Specimens with Barcodes: 7
Species With Barcodes: 1
IUCN Red List Assessment
Red List Category
Red List Criteria
National NatureServe Conservation Status
Rounded National Status Rank: N5 - Secure
Rounded National Status Rank: N4 - Apparently Secure
NatureServe Conservation Status
Rounded Global Status Rank: G5 - Secure
Reasons: Hemlock is widespread and still abundant in the northern portion of its range, but being killed by the hemlock woolly adelgid, an exotic insect pest, especially in the southeastern portion of its range. It is still unclear whether the adelgid will spread throughout the entire range of eastern hemlock, but in 2002 the insect had established in about one-half of its range and as far north as New Hampshire (USFS 2002). Isolated infestations have been discovered and treated in Maine and Michigan (Onken 2001). Eastern Hemlock is widespread and abundant in Canada.
Please consult the PLANTS Web site and your State Department of Natural Resources for this plant’s current status, such as, state noxious status and wetland indicator values.
Global Short Term Trend: Decline of 30 to >90%
Comments: Being killed by an exotic insect pest (hemlock wooly adelgid) in large portions of its range, particularly in the mid-Atlantic area and central Appalachians. In 2002, the adelgid is documented to be established in about half of eastern hemlock's range. The adelgid is established from New Hampshire to South Carolina and as far west as West Virginia (USFS 2002).
Comments: A serious threat appears to be the hemlock woolly adelgid, an insect that is believed to have originated in Asia. "During the last decade, it has become a major killer of Canadian and Carolina hemlocks in forests from Maine to Virginia" (Cohn, 1993). Once established the adelgid is a chronic problem and trees that are attacked can die in several years (Souto and Shields 1999). A related threat is "pre-emptive logging" which may be occurring well outside the insect's present range (Foster 1999).
Multiple removal cuttings are the best method for regenerating eastern
hemlock. Suddenly released seedlings often die, and a series of
removals releases hemlock more slowly . On moist sites, a two-cut
shelterwood system leaving about 50 percent cover may be adequate. On
drier sites, a three-cut system is appropriate, initially leaving 70 to
80 percent crown cover and 50 percent after the second cut . If too
few residual trees are left, they may die when exposed, and they are
subject to windthrow . Scarification of seedbeds and removal of
competing hardwoods may be necessary . Eastern hemlock regeneration
must be at least sapling size when released if it is to compete
successfully with uncontrolled hardwoods . Single tree selection is
also an effective method to harvest and regenerate eastern hemlock .
Effective reproduction may be absent in areas with high deer populations
[3,10]. Regeneration in the Porcupine Mountains in Michigan has
declined over the last several decades because of white-tailed deer
browsing in the winter . In the Allegheny National Forest in
Pennsylvania, the eastern hemlock-northern hardwoods forest type covered
83.4 percent of the land in 1800 and only 15.8 percent in 1986.
Extensive harvesting, fire, and overbrowsing are responsible for the
Numerous insects attack eastern hemlock, but only a few are of economic
importance cause sporadic or local mortality . Mortality
usually occurs following complete defoliation by insects [43,62].
Eastern hemlock seedlings are sensitive to damping-off fungi, root rots,
and stem and needle rusts .
Eastern hemlock appears to be resistant to ozone .
Cultivars, improved and selected materials (and area of origin)
These plant materials are readily available from commercial sources. Contact your local Natural Resources Conservation Service (formerly Soil Conservation Service) office for more information. Look in the phone book under ”United States Government.” The Natural Resources Conservation Service will be listed under the subheading “Department of Agriculture.”
Eastern hemlock generally does not tolerate nutrient-poor soils, wet soils or poorly drained sites, prolonged drought, prolonged heat, sun scorch, windy and exposed sites, aerial pollution, or winter salt spray. Drought is probably the most serious damaging agent to the species, especially during the seedling stage. Damping-off fungi and root rots also are seriously damaging to young plants. Containerized plants are best for transplanting – move into sites that are cool, well-drained, and wind-protected, in partial sun to partial shade. Good drainage is essential for transplant success -- the porosity of the soil should be improved with peat moss or sand, with the root ball elevated about 2" to 4" above the surrounding soil grade.
A shallow root system makes trees highly susceptible to wind-throw when exposed through timber cutting or planted in open sites. Plants should be staked for the first two or three years following transplant, to prevent wind-throw. Saplings and small trees are highly susceptible to damage from fire because of the thin bark, and root injury often occurs from high intensity fires because of heavy litter concentration.
The most severe insect pest is the Asian hemlock woolly adelgid, a phloem-feeding insect that causes branch dieback and tree decline. Trees typically die after several years of adelgid infestation.
Relevance to Humans and Ecosystems
Eastern hemlock provides cover to ruffed grouse, wild turkey, fishers,
and other wildlife [4,20]. It provides excellent thermal protection and
snowfall interception for moose and white-tailed deer in the winter
In the winter, eastern hemlock browse is moderately preferred by moose
and highly preferred by white-tailed deer [2,10]. In the summer,
white-tailed deer prefer hardwood sprouts and seedlings to eastern
hemlock . The seeds of eastern hemlock are not as preferred by
white-footed mice, red-backed voles, and meadow voles as red pine (Pinus
resinosa) and white pine seeds .
Importance to Livestock and Wildlife
Cove forests in the southern Appalachian Mountains provide nesting
habitat for many species of birds. The black-throated blue warbler,
black-throated green warbler, and blackburnian warbler are especially
abundant in virgin eastern hemlock cove forests .
Large eastern hemlocks can be climbed by small black bear cubs. In
northeastern Minnesota, black bear mothers and cubs spent more than 95
percent of the time in April and May within 600 feet (183 m) of either
an eastern hemlock or an eastern white pine (Pinus strobus) larger than
20 inches (51 cm) in d.b.h. .
Eastern hemlock has high cavity value for wildlife . Large hollow
trees are commonly used as dens by black bears .
The seeds are eaten by birds and mammals , and in the winter the
foliage is browsed by white-tailed deer, moose, and snowshoe hares
Wood Products Value
knots . It is used for pulp, light framing, sheathing, roofing,
subflooring, and boxes and crates .
Other uses and values
bark which is a source of tannin .
Eastern hemlock is planted as an ornamental .
Currently, eastern hemlock stands are considered essential for shelter and bedding of white-tailed deer during the winter. In regions of marked reductions in type area, many public agencies have restricted cutting until reliable methods of regenerating the stand become operational (6). The type also is considered important as cover for ruffed grouse, turkeys, and many other animals.
Eastern hemlock often is planted as an ornamental because of its relative freedom from insects and disease, good foliage color, and adaptability to shearing. Some effort is being made to plant the species under forest conditions because it is so important to wildlife.
Tannin from the bark of eastern hemlock formerly was extracted for use in processing leather. Now synthetic and important products are used and a once prosperous industry has been eliminated (19).
Industry: Eastern hemlock was early valued for its bark, an important source of tannin for the leather industry. Trees were felled and stripped of their bark, which was then milled for tannin extraction. To simplify stripping the bark and turning the logs, trees were often felled into lakes. Many of these logs were much later extracted from northern lakes and milled.
The wood has been used for light framing, roofing, boxes and crates, and pulping, but it tends to be brittle and eastern hemlock is not presently important as a timber tree. Commercial stands have been greatly reduced by prior harvesting and lack of restocking.
Ethnobotanic: American Indians used the cambium as the base for breads and soups or mixed it with dried fruit and animal fat for pemmican. Natives and white settlers also made tea from hemlock leaves, which have a high vitamin C content.
Conservation: Eastern hemlock can be used as a specimen, screen, or group planting, and it can be sheared over time into a formal evergreen hedge, which is densely leafy all the way to the ground (due to its full shade tolerance), although as a hedge it must be repeatedly pruned to keep it in size. It has a naturally open growth habit – if bought with a dense canopy effect, it may have been repeatedly sheared at the nursery (or Christmas tree farm).
Numerous cultivars of eastern hemlock have been developed, including compact shrubs, dwarfs, form mutants (weeping, fastigiate, prostrate, etc.), color mutants (variegated), and graceful trees. The wild type apparently also is common in cultivation. Eastern hemlock stands are considered important as shelter and cover for white-tailed deer and other wildlife species, such as turkey, ruffed grouse, and others.
Tsuga canadensis, also known as eastern hemlock or Canadian hemlock, and in the French-speaking regions of Canada as Pruche du Canada, is a coniferous tree native to eastern North America. It is the state tree of Pennsylvania.
The eastern hemlock grows well in shade and is very long lived, with the oldest recorded specimen, found in Tionesta, Pennsylvania, being at least 554 years old. The tree generally reaches heights of about 31 meters (100 feet), but exceptional trees have been recorded up to 53 metres (173 feet). The diameter of the trunk at breast height is often 1.5 metres (5 feet), but again, outstanding trees have been recorded up to 1.75 meters (6 feet). The trunk is usually straight and monopodial, but very rarely is forked. The crown is broadly conic, while the brownish bark is scaly and deeply fissured, especially with age. The twigs are a yellow-brown in colour with darker red-brown pulvini, and are densely pubescent. The buds are ovoid in shape and are very small, measuring only 1.5 to 2.5 mm (0.05 to 0.1 inches) in length. These are usually not resinous, but may be slightly so.
The leaves are typically 15 to 20 mm (0.6 to 0.9 inches) in length, but may be as short as 5 mm (0.2 inches) or as long as 25 mm (1 inch). They are flattened and are typically distichous, or two-ranked. The bottom of the leaf is glaucous with two broad and clearly visible stomatal bands, while the top is a shiny green to yellow-green in colour. The leaf margins are very slightly toothed, especially near the apex. The seed cones are ovoid in shape and typically measure 1.5 to 2.5 cm (0.6 to 1 inch) in length and 1 to 1.5 cm (0.4 to 0.6 inches) in width. The scales are ovate to cuneate in shape and measure 8 to 12 mm (0.3 to 0.5 inches) in length by 7 to 10 mm (0.3 to 0.4 inches) in width. The apex is more or less rounded and is often projected outward. Twenty-four diploid chromosomes are present within the trees' DNA.
The wood is soft, coarse-grained, and light buff in color. Air-dried, a cubic foot weighs 28 lbs. The lumber is used for general construction and crates. Because of its unusual power of holding spikes, it is also used for railroad ties. Untreated, the wood is not durable if exposed to the elements. As a fuel, it is low in value. The wood is also a source of pulp for paper manufacturing.
Distribution and habitat
T. canadensis occurs at sea level in the north of its distribution, but is found primarily at elevations of 600–1,800 metres (2,000–5,900 ft). It ranges from northeastern Minnesota eastward through southern Quebec and into Nova Scotia, and south in the Appalachian Mountains to northern Georgia and Alabama. Disjunct populations occur in the southeastern Piedmont, western Ohio and into Illinois, as well as eastern Minnesota. In Canada, it is present in Ontario and all provinces to the east except Newfoundland and Labrador. In the USA, it is found in all states east of and including Minnesota, Wisconsin, Indiana, Kentucky, Tennessee and Alabama, but excluding Florida. Its range completely overlaps that of the closely related Tsuga caroliniana.
It is found primarily on rocky ridges, ravines and hillsides with relatively high levels of moisture.
Eastern hemlock is generally confined to areas with cool and humid climates. Precipitation in the areas where it grows is typically 740 mm (29 inches) to more than 1270 mm (50 inches) per year. The lower number is more typical of northern forests that receive heavy snowfall; the higher number is common in southerly areas with high summer rainfall. Near the Atlantic coast and in the southern Appalachians where the trees often reach their greatest heights, annual rainfall often exceeds 1520 mm (60 inches). In the north of its range, the temperatures in January average -12 °C, while in July they average only 16 °C. In these areas, the frost-free season can last fewer than 80 days. In contrast, the southern end of the range experiences up to 200 days without frost and January temperatures as high as 6 °C.
Hemlock woolly adelgid
The species is currently threatened by the hemlock woolly adelgid (Adelges tsugae), a sap-sucking bug accidentally introduced from East Asia to the United States in 1924, and first found in the native range of eastern hemlock in the late 1960s. The adelgid has spread very rapidly in southern parts of the range once becoming established, while its expansion northward is much slower. Virtually all the hemlocks in the southern Appalachian Mountains have seen infestations of the insect within the last five to seven years, with thousands of hectares of stands dying within the last two to three years. Attempts to save representative examples on both public and private lands are on-going. A project named "Tsuga Search", funded by the Great Smoky Mountains National Park, is being conducted to save the largest and tallest remaining eastern hemlocks in the Park. It is through Tsuga Search that hemlocks have been found with trunk volumes of up to 44.8 m³ within the Park, making it the largest eastern evergreen conifer, eclipsing in volume both eastern white pine (Pinus strobus) and loblolly pine (Pinus taeda). The tree is currently listed as a least concern species in the IUCN Red List, but this is based largely on its wide distribution and because the adelgid populations have not reached the northern areas of its range.
A 2009 study conducted by scientists with the U.S. Forest Service Southern Research Station suggests the hemlock woolly adelgid is killing hemlock trees faster than expected in the southern Appalachians, and rapidly altering the carbon cycle of these forests. According to Science Daily, the pest could kill most of the region's hemlock trees within the next decade. According to the study, researchers found "hemlock woolly adelgid infestation is rapidly impacting the carbon cycle in [hemlock] tree stands," and "adelgid-infested hemlock trees in the South are declining much faster than the reported 9-year decline of some infested hemlock trees in the Northeast."
In a 2009 case study, entomologists from the U.S. Forest Service, Cornell University and the University of Massachusetts-Amherst released 900 Laricobius nigrinus beetles into a stand of adelgid-infested hemlocks near Lansing, New York. L. nigrinus, which is native to the Pacific Northwest, naturally preys on the hemlock wooly adelgid. The particular site near Lansing was chosen because its hemlocks are only lightly infested with the woolly adelgid, and there are enough trees to sustain a long-term study. The site will be left untreated with pesticides for 10 years to study how well the L. nigrinus beetles become established; if the experiment proves successful, researchers expect the population will take two to three years to build to levels where they can be readily detected.
The mid-Holocene decline of hemlock populations is a much-studied phenomenon. From its foundation in the early Holocene (c. 16,000 BP) in what is now the southeastern US, T. canadensis expanded rapidly and successfully into its potential range. However, palynological analyses show the hemlock population experienced a pronounced decline approximately 5,500 BP that lasted for about 1,000 years. Continued research points to other, though less dramatic, dips in Holocene hemlock populations. Pathogens, insects, and climatic change, and a combination of these, have all been proposed to explain these anomalies. The eastern hemlock increased again after the major decline, but did not recover its former place as a dominant species.
Due to its being a long-lived tree, several very large or otherwise impressive trees exist along the east coast of North America. One organization, the Eastern Native Tree Society (ENTS), has been particularly active in discovering and measuring these trees. In the southern Appalachians, many individuals reach 45 metres (148 ft) tall, and one tree has been measured in the Great Smoky Mountains National Park to 52.8 metres (173 ft) tall, though this tree is now dead from hemlock woolly adelgid; the tallest now surviving, the "Noland Mountain tree", is 51.8 metres (170 ft) tall. Altogether, ENTS has confirmed four trees to heights of 51 metres (167 ft) or more by climb and tape drop. In the Northeast, the tallest accurately measured tree is 44 metres (144 ft). This tree, named the Seneca hemlock, grows in Cook Forest State Park, PA. Above 43°N latitude, the maximum height of the species is less, under 39 metres (128 ft). In New England, ENTS has measured hemlocks to 42 metres (138 ft), although trees above 39 m are extremely rare in New England. By 44°N, the maximum height is probably not more than 35 metres (115 ft). Diameters of mature hemlocks range from 0.75–1.8 metres (2 ft 6 in–5 ft 11 in), with trees over 1.6 metres (5 ft 3 in) diameter being very rare. In New England, the maximum diameter is 1.4 metres (4 ft 7 in).
Trunk volume is the third dimension to receive attention by ENTS. Many eastern hemlocks have been modeled to over 30 m³ trunk volume, and the largest has been calculated to be 44.8 m³, making it the largest natural evergreen conifer in the eastern United States. The center of maximum size development for the species is the southern Appalachians, especially the Great Smoky Mountains.
Tsuga canadensis has long been a popular tree in cultivation. The tree's preference for partial shade and tolerance of full shade allows it to be planted in areas where other conifers would not easily grow. In addition, its very fine-textured foliage that droops to the ground, its pyramidal growth habit and its ability to withstand hard pruning make it a desirable ornamental tree. In cultivation, it prefers sites that are slightly acidic to neutral with nutrient-rich and moist but well-drained soil. It is most often used as a specimen, for a screen, or in small group plantings, though it can also be trained as a dense formal hedge. It should not be used on roadsides where salt is used in winter, as its foliage is sensitive to salt spray. It is also poorly adapted as a windbreak tree, as wind exposure causes dieback in winter. It has several drawbacks, such as a fairly low tolerance of urban stress, intolerance for very wet or very dry soils, and susceptibility to attack by the hemlock woolly adelgid, though this is treatable. Its tendency to shed needles rapidly after being cut down renders it unsuitable as a Christmas tree.
It was introduced to British gardens in 1736. In the UK, it is encountered frequently in gardens both large and small, as well as some parks, and is most common in the eastern areas of the country. It is sometimes employed as a hedge, but is considered inferior for this usage compared to Tsuga heterophylla (western hemlock); it is not well adapted to the UK climate and as a consequence often has a poorly developed, forked and sinuous trunk there. In Germany, it is the most frequently seen hemlock in cultivation, and is also used in forestry.
- 'Beehive' – a very small dwarf shrub typically growing to 1 m high and 1.5 m wide, resembling a spreading beehive in form
- 'Bennett' – a dwarf shrub reaching 1 m high and 1.5 m wide, with upper branchlets that first ascend and then arch upper, this selection prefers partial shade.
- 'Cole's Prostrate' – a groundcover form that can also be used in bonsai as an alternative to the prostrate junipers, it slowly grows to only 30 cm tall with a 1.3 m spread, with the central stems eventually becoming visible. It also prefers partial shade.
- 'Gentsch White' – a dwarf shrub growing to 1.3 m tall with an equal spread and new spring growth that turns creamy-white in autumn through winter, creating a dramatic contrast with the dark green old growth, it is easily scorched by the sun and requires partial shade. It is recommend to feather shear annually to keep it compact and create more tip growth.
- 'Jeddeloh' – a dwarf shrub reaching to 1 m high and 1.5 m wide, showing a small concavity in the centre, it is an alternative to the bird's nest spruce (Picea abies 'Nidiformis'). This cultivar has gained the Royal Horticultural Society's Award of Garden Merit.
- 'Pendula' – an upright weeping form whose height is dependent on how long it is staked, but is typically seen 0.6–1.5 m tall with a 1.5 m spread
- 'Sargentii' – a popular large weeping shrub that grows to 3 m tall with a wide spread up to 6 m, it features numerous pendulous branches and is most effectively employed near water, in rock gardens or on embankments.
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- Predator Beetle to Battle Hemlock Pest
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Eastern hemlock ( Tsuga canadensis ) is the state tree of Pennsylvania.
Names and Taxonomy
canadensis (L.) Carr. . Fernald  recognizes a dwarf form, T.
canadensis forma parvula Vict. and Rousseau, that grows in mats up to 3
feet (1 m) high in Quebec and New England.