Annuals or biennials, 30–150 cm. Stems openly branching, villous with curled, septate hairs to nearly glabrous, spiny wings to 1.5 cm wide, wing spines to 3 mm. Leaves: basal tapering to winged petioles, blades 10–20 cm, margins spiny-toothed to ± shallowly pinnately lobed; cauline sessile, gradually smaller, margins often more deeply divided, marginal spines to 3 mm; abaxial leaf faces ± tomentose with long, one-celled hairs and/or long, curled, septate hairs along veins or glabrate; adaxial faces sparsely hairy or glabrate. Heads borne singly or in groups of 2–5, 15–18 mm. Peduncles spiny-winged to near apex or throughout, to 4 cm. Involucres ± spheric, 12–17 × 12–17 mm. Phyllaries narrowly lanceolate, outer and middle with appressed bases ca. 1 mm wide and appressed to spreading appendages 0.5–1 mm wide, spine tips 1–1.5 mm, inner with unarmed, straight tips. Corollas purple or ± white, 11–16 mm, lobes ca. 3.5 times length of throat. Cypselae light brown to gray-brown, 2.5–3.8 mm; pappus bristles 11–13 mm. 2n = 16 (Sweden).
Phenology: Flowering summer–fall (Jul–Sep).
Habitat: Weed of waste ground, pastures, roadsides, fields
Elevation: 0–500 m
Introduced; B.C., N.B., N.S., Ont., Que., N.J., Pa., Eurasia.
Canadian distributions above follow R. J. Moore and C. Frankton (1974); I have not seen those specimens. Carduus crispus has been reported also from Arkansas, Connecticut, Iowa, Illinois, Maine, Maryland, Massachusetts, Missouri, New York, North Dakota, Ohio, Rhode Island, Vermont, Virginia, and West Virginia; I have not seen specimens from those states.
Two subspecies of Carduus crispus have been recognized (S. M. A. Kazmi 1964); those are not differentiated here.
Carduus crispus closely resembles the much more common C. acanthoides. Some published records of C. crispus are probably C. acanthoides. Although the degree of spininess and tough versus brittle stems were used as key characters (A. Cronquist 1980; H. A. Gleason and A. Cronquist 1991) to differentiate the two taxa, both characters are subjective, and the second is impractical with dry material.