Herbs 30-160 cm tall, perennial, dioecious. Roots far creeping, bearing adventitious shoots. Stems erect, branched above, unwinged. Leaf surface smooth. Capitula often numerous, terminal, corymbose. Involucre ± narrowly ovoid, 1.5-2 cm in diam. Phyllaries imbricate, in 5-7 rows, lacking wings and scarious appendage; outer and middle phyllaries triangular to ovate, 3-8 × 1.2-2.5 mm, apex acute; inner phyllaries elliptic-lanceolate to broadly linear, 9-20 × 1-3 mm, apex acute to acuminate and scarious. Corolla reddish purple or rarely white; female florets 1.6-2.4 cm, tube 1.3-1.8 cm; male florets 1.5-1.8 cm, tube 0.9-1.2 cm. Achene yellowish, 3-4.5 mm. Pappus bristles dirty white, 2.5-3.5 cm.
Perennials, dioecious or nearly so, 30–120(–200) cm; colonial from deep-seated creeping roots producing adventitious buds. Stems 1–many, erect, glabrous to appressed gray-tomentose; branches 0–many, ascending. Leaves: blades oblong to elliptic, 3–30 × 1–6 cm, margins plane to revolute, entire and spinulose, dentate, or shallowly to deeply pinnatifid, lobes well separated, lance-oblong to triangular-ovate, spinulose to few-toothed or few-lobed near base, main spines 1–7 mm, abaxial faces glabrous to densely gray-tomentose, adaxial green, glabrous to thinly tomentose; basal absent at flowering, petioles narrowly winged, bases tapered; principal larger cauline proximally winged-petiolate, distally sessile, well distributed, gradually reduced, not decurrent; distal cauline becoming bractlike, entire, toothed, or lobed, spinulose or not. Heads 1–many, borne singly or in corymbiform or paniculiform arrays at tips of main stem and branches. Peduncles 0.2–7 cm. Involucres ovoid in flower, ± campanulate in fruit, 1–2 × 1–2 cm, arachnoid tomentose, ± glabrate. Phyllaries in 6–8 series, strongly imbricate, (usually purple-tinged), ovate (outer) to linear (inner), abaxial faces with narrow glutinous ridge, outer and middle appressed, entire, apices ascending to spreading, spines 0–1 mm (fine); apices of inner phyllaries flat, ± flexuous, margins entire to minutely erose or ciliolate. Corollas purple (white or pink); staminate 12–18 mm, (remaining longer than pappus when head is fully mature), tubes 8–11 mm, throats 1–1.5 mm, lobes 3–5 mm; pistillate 14–20 mm, (overtopped by pappi in fruit), tubes 10–15 mm, throats ca. 1 mm, lobes 2–3 mm; style tips 1–2 mm. Cypselae brown, 2–4 mm, apical collar not differentiated; pappi 13–32 mm, exceeding corollas. 2n = 34.
"Colonial perennial 3–15(–20) dm from deep-seated creeping roots, subglabrous, or the lvs ± white-tomentose beneath; heads ± numerous in an often flat- topped infl, unisexual or nearly so, the plants polygamo-dioecious; invol 1–2 cm high, its bracts all innocuous, or the outer with a weak spine-tip ca 1 mm; fls pink-purple (white); achenes 2.5–4 mm; pappus of the pistillate heads surpassing the corollas, that of the staminate heads surpassed by the cors; 2n=34. A noxious weed of fields and waste places; native of Eurasia, now widely intr. in n. U.S. and s. Can. July, Aug. The var. arvense has merely toothed or shallowly lobed, weakly spiny lvs. (C. setosum; C. a. var. mite) The more common var. horridum Wimmer & Graebner has more spiny, deeply pinnatifid lvs."