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.