Lanceleaf Ragweed

Hybrids between Ambrosia bidentata and A. trifida have been recorded.

Annuals, 10–30(–100+) cm. Stems erect. Leaves mostly opposite; petioles 0–0.5 mm; blades ± lanceolate to lance-linear, 15–40+ × 3–6(–10+) mm, bases rounded to cordate, margins entire or with (1–)2(–4) basal lobes, abaxial and adaxial faces ± piloso-hispid and gland-dotted. Pistillate heads clustered, proximal to staminates; florets 1. Staminate heads: peduncles 0–0.5 mm; involucres obliquely cup-shaped (lateral lobe longer than others), 2.5–4 mm diam., piloso-hispid; florets 6–8+. Burs: bodies pyramidal, 5–8 mm, piloso-hispid, spines 4(–5), ± distal, ± acerose, 0.5–1 mm, tips straight.

Ambrosia bidentata Michx. Fl. Bor. Am. 2: 182. 1803
An annual; stem 3-10 dm. high, rough-hirsute, branched; leaves mostly alternate, sessile, lanceolate, acuminate, 3-5 cm. long, often with one lanceolate lobe or tooth on each side, hirsute on both sides, the hairs with pustulate bases; staminate heads numerous, sessile, in dense terminal spikes; involucre very oblique, glandular-granuliferous, turbinate, 2 mm. broad, the upper margin produced into a lanceolate, hispid incurved lobe; corolla glabrous; pistillate heads 1 or 2 in the axils of the leaves; fruit elongate, obovoid, 4-angled, hirsute; body 6-7 mm. long; beak subulate, fully 2 mm. long; spines very sharp, subulate, directed forward, 1 mm. long or more.
Type locality: In the Illinois region.
Distribution: Prairies, Illinois and Kentucky to Louisiana, Texas, and Nebraska.

Ambrosia bidentata, the lanceleaf ragweed, is a North American species of plants in the family Asteraceae. It is native to the central and eastern parts of the United States, primarily the Mississippi Valley and the eastern Great Plains.

Ambrosia bidentata is an annual herb up to 100 cm (40 inches) tall. Leaves have only a few lobes compared to the complexly divided leaves of some related species, sometimes no lobes at all. Flower heads are small and inconspicuous, as the plant is wind-pollinated. The heads develop into spiny burs as the seeds ripen.