Biennial or perennial herb from a taproot; stems erect, thick, fleshy, ribbed, sparsely hairy, simple or with a few short upper branches, 0.2-1.0 m tall.
Basal leaves egg-shaped to elliptic, entire to nearly entire, stalked; lower stem leaves narrowly egg-shaped, tapering to the base, stalked, shallowly to pinnately lobed, marginally spiny with numerous, slender, yellow spines up to 10 mm long, lobed 1/2 or less the width of the blade, segments lanceolate to deltoid, green and nearly glabrous above, woolly-hairy below; the uppermost leaves narrower, ascending, extending beyond the compact group of heads.
Heads discoid, 5-15, in a terminal cluster or rarely at the ends of the few branches or in the leaf axils; involucres bell-shaped, 2-3 cm tall; involucral bracts without glands, essentially glabrous except for the sparse marginal hairs, outer ones egg-shaped to lanceolate, with a slender 2-4 mm long spine, inner ones unarmed, the tips membranous, often dilated, papery and irregularly-margined; disk flowers white to pink, 18-30 mm long, the tubes 9-13 mm long, the lobes 4-6 mm long.
Achenes 5.5-6.5 mm long, light or dark brown with a narrow, yellow apical band; pappus light yellow, 2-3 cm long, shorter than the corollas by 4-5 mm.
TAXONOMIC KEY TO CIRSIUM
1. Heads small; involucres 1-2 (rarely 2.5) cm tall; plants introduced.
2. Stems distinctly spiny-winged; plants with perfect flowers C. palustre
1. Heads large; involucres more than 2 cm tall; plants native (except C. vulgare).
3. Leaves bristly-spiny above, stems distinctly spiny-winged............ C. vulgare
4. Pappus of mature seeds exceeding the corollas by 1-10 mm.............. C. foliosum
5. Heads large; involucres 3-5 cm tall........... C. drummondii
6. Outer involucral bracts more than 2 mm wide at base, slightly if at all hairy and if so, then mainly marginal.
7. Outer involucral bracts strongly glandular; lowermost leaves deeply pinnately lobed more than 1/2 the width of the blade.............. C. undulatum
6. Outer involucral bracts less than 2 mm wide at base, densely cobwebby.
8. Corollas white or creamy-white, rarely pinkish; involucral bracts greenish............. C. hookerianum
9. Styles exceeding the corollas by at least 3 mm; achenes 5-6.5 mm long; leaves usually lobed more than 1/2 the width of the blade.............. C. edule
Source: Illustrated Flora of British Columbia