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. Flowering Jul-Oct. Dry, disturbed sites; 200-500 m; Ala., Ark., Conn., D.C., Ga., Ill., Ind., Iowa, Kans., Ky., La., Md., Minn., Miss., Mo., N.C., Ohio, Okla., Tenn., Tex., Va., W.Va. Hybrids between Ambrosia bidentata and A. trifida have been recorded.