Annuals, 30-150 cm. Stems erect. Leaves mostly opposite; petiole 10-30(-70) mm; blade orbicular-deltate to ovate or elliptic, 4-15(-25) × 3-7(-20) cm, usually some palmately 3(-5)-lobed, both surfaces scaberulose and gland-dotted, base truncate to cuneate, sometimes decurrent onto petiole, margin usually toothed, rarely entire. Female capitula clustered, proximal to male; floret 1. Male capitula: peduncles 1-3 mm; involucres saucer-shaped, 2-4 mm in diam., scaberulose; florets 3-25. Bur pyramidal, 3-5(-7) mm, glabrous or glabrate, spines 4 or 5, apical, acerose, 0.5-1 mm, tips straight. Fl. Jul-Aug, fr. Sep-Nov. 2n = 24, 48.
Annuals, 30–150(–400+) cm. Stems erect. Leaves mostly opposite; petioles 10–30(–70+) mm; blades rounded-deltate to ovate or elliptic, 40–150(–250+) × 30–70(–200+) mm, usually some blades palmately 3(–5)-lobed, bases truncate to cuneate (sometimes decurrent onto petioles), margins usually toothed, rarely entire, abaxial and adaxial faces ± scabrellous and gland-dotted. Pistillate heads clustered, proximal to staminates; florets 1. Staminate heads: peduncles 1–3+ mm; involucres ± saucer-shaped, 2–4 mm diam., scabrellous (often with 1–3 black nerves); florets 3–25+. Burs: bodies ± pyramidal, 3–5(–7+) mm, glabrous or glabrate, spines 4–5, ± distal, ± acerose, 0.5–1 mm, tips straight (bases ± decurrent as ribs). 2n = 24, 48.
"Coarse annual weed to 2(–5) m; stem spreading-hairy above, often glabrous below; lvs opposite, petiolate, ± scabrous, broadly elliptic to more often ovate or suborbicular, often 2 dm, serrate, palmately 3–5-lobed, or, especially in depauperate individuals, lobeless; staminate invols ca 1.5 mm, slightly oblique, unilaterally evidently 3-nerved; fruiting invols 5–10 mm, several- ribbed, each rib ending in a tubercle or spine; 2n=24. Moist soil and waste places throughout our range, s. to Fla., w. to B.C. and n. Mex. (July) Aug.–Oct. The common form in most of our range is var. trifida, with the petioles of at least the upper lvs ± wing-margined, and the ribs of the fr ending in short spines. (A. striata) From s. Ill. and Mo. to Miss., Ariz., and n. Mex. is found var. texana Scheele, with wingless petioles, and with the ribs of the mostly smaller fr ending in blunt or almost obsolete tubercles. (A. aptera)"