Annuals, 10-60(-150+) cm. Stems erect. Leaves opposite (proximal) and alternate; petioles 25-35(-60+) mm; blades deltate to lanceolate or elliptic, 25-55(-90+) × 20-30(-50+) mm, 1-2-pinnately lobed, bases cuneate, ultimate margins entire or toothed, abaxial faces sparsely pilosulous to strigillose, adaxial faces strigillose, both gland-dotted. Pistillate heads clustered, proximal to staminates; florets 1. Staminate heads: peduncles 0.5-1.5 mm; involucres shallowly cup-shaped (usually without black nerves), 2-3+ mm diam., glabrous or hispid to pilosulous; florets 12-20+. Burs: bodies ± globose to pyriform, 2-3 mm, ± pilosulous, spines or tubercles 3-5+, near middles or distal, ± conic to acerose, 0.1-0.5+ mm, tips straight. 2n = 34, 36. Flowering Jul-Oct. Wet to dry soils, disturbed sites; 0-1000+ m; Alta., B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.W.T., N.S., Ont., P.E.I., Que., Sask.; Ala., Alaska, Ariz., Ark., Calif., Colo., Conn., Del., D.C., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo. Hybrids between Ambrosia artemisiifolia and A. psilostachya have been called A. ×intergradiens W. H. Wagner. The name Ambrosia ×helenae Rouleau applies to hybrids between A. artemisiifolia and A. trifida.

Plant: Annual < 7 dm, much-branched; Stem green, red- or black-marked, weakly hairy Leaves: opposite below; cauline leaves 3-12 cm, widely ovate, generally 2-3-pinnately parted, ± hairy INFLORESCENCE: primary inflorescence a head, each resembling a flower; staminate heads generally many in ± spikes or racemes, 2-5 mm diam, involucre asymmetric, lobes 5-10, green; pistillate heads 1-flowered, generally spiny, bur-like; receptacle chaffy; chaff scales spirally arrayed, fused below, tips generally becoming spiny Flowers: Staminate flowers ± many; corolla yellow or translucent; anthers free; style unbranched; Pistillate flowers 1-5; corolla 0; style branches long Fruit: Fruit: enclosed in bur 2-4 mm, widely obconic, green to brown, ± puberulent; spines 4-12, blunt, vestigial, ± in 1 whorl below beak; pappus 0 Misc: Disturbed sites; < 650 m.; Aug-Oct