1. Heads small; involucres 1-2 (rarely 2.5) cm tall; plants introduced.

2. Stems distinctly spiny-winged; plants with perfect flowers C. palustre
2. Stems not conspicuously spiny-winged; plants partly of male and female flowers........ C. arvense

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
3. Leaves cobwebby to loosely woolly-hairy, woolly or nearly glabrous above, stems not spiny-winged.

4. Pappus of mature seeds exceeding the corollas by 1-10 mm.............. C. foliosum
4. Pappus of mature seeds shorter than the corollas.

5. Heads large; involucres 3-5 cm tall........... C. drummondii
5. Heads small; involucres less than 3 cm tall.

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
7. Outer involucral bracts without glands; lowermost leaves shallowly pinnately lobed to 1/2 or less the width of the blade............... C. scariosum

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
8. Corollas purplish-red to purplish-pink; outer involucral bracts purplish.

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
9. Styles nearly equal to or exceeding the corollas by only 1.5 mm; achenes 4-4.5 mm long; leaves usually lobed less than 1/2 the width of the blade................. C. brevistylum

Source: Illustrated Flora of British Columbia