Perennials, 10-60(-100+) cm. Stems erect. Leaves proximally opposite, distally alternate; petioles 0-25 mm (often ± winged); blades deltate to lanceolate, 20-60(-140) × 8-35(-50+) mm, pinnately toothed to 1-pinnately lobed, bases cuneate to truncate, ultimate margins entire or toothed, abaxial and adaxial faces hirsutulous to strigose and gland-dotted. Pistillate heads clustered, proximal to staminates; florets 1. Staminate heads: peduncles 0.5-2 mm; involucres obliquely cup-shaped, 2-4(-5) mm diam., hirsutulous; florets 5-15(-30+). Burs: bodies ± obpyramidal to globose, 2-3 mm, hirsutulous, spines or tubercles 0 or 1-6, mostly distal, stoutly conic to acerose, (0.1-)0.5-1 mm, tips straight. 2n = 18, 27, 36, 45, 54, 63, 72, 100-104, 108, 144. Flowering Jul-Oct(-Dec). Disturbed sites, often wet, alkaline, clay soils; 0-2200 m; Alta., B.C., Man., N.S., Ont., P.E.I., Que., Sask.; Ala., Ariz., Ark., Calif., Colo., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., La., Maine, Mass., Mich., Minn., Miss., Mo., Mont., Nebr., Nev., N.H., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., S.C., S.Dak., Tenn., Tex., Utah, Vt., Wash., Wis., Wyo.; Mexico (Baja California, Chihuahua, Coahuila). Hybrids between Ambrosia psilostachya and A. artemisiifolia have been called A. ×intergradiens W. H. Wagner. Some botanists consider the type of A. cumanensis Kunth (1818) to be conspecific with that of A. psilostachya.