Parthenium hysterophorus

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.

Annual to 1 m, usually much branched, ±hairy, and often glandular above; lvs pinnatifid or usually bipinnatifid, to 2 נ1 dm; heads small, numerous in an often leafy infl, the disk 3-5 mm wide; achenes obovate, black, 2-2.5 mm; 2n=34. A weed of waste places; trop. Amer., n. to se. U.S. and occasionally to Mass., Mich., and Kans. July-Oct.

Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.

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