7. Parthenium hysterophorus Linnaeus, Sp. Pl. 2: 988. 1753.
Annuals, (10–)30–120+ cm. Leaf blades ovate to elliptic, 30–180+ × 10–50(–90+) mm, (1–)2-pinnately lobed (ultimate lobes lanceolate to linear, 3–50 × 2–15 mm), faces sparsely to densely scabrellous and gland-dotted (seldom with additional erect hairs 1–2 mm). Heads obscurely radiate, borne in open, paniculiform arrays. Peduncles 1–8(–15+) mm. Phyllaries: outer 5(–6), lance-elliptic, 2–4 mm, inner 5(–6) ovate to ± orbiculate, 2.5–4 mm. Pistillate florets 5(–6); corolla laminae reniform or orbiculate to oblong, 0.3–1 mm. Disc florets 12–30[–60]. Cypselae obovoid, 1.5–2(–3.5) mm; pappus-like enations erect, deltate to ovate, 0.5–1 mm (sometimes a third, subulate spur near apex adaxially). 2n = 34.
Flowering Mar–Nov. Disturbed sites; 0–300 m; Ala., Ark., Conn., Del., D.C., Fla., Ill., Kans., La., Md., Mass., Mich., Miss., Mo., N.J., N.Mex., N.Y., Ohio, Okla., Pa., Tex., Va.; Mexico; introduced in subtropical, tropical, and warm-temperate New World and Old World.
In some places, especially in Old World, Parthenium hysterophorus causes serious contact dermatitis in humans. The plants should be handled with caution.