Canada thistle is our only species of thistle with separate male and female plants (Moore 1979). Moore (1979) indicates that it was introduced to North America "very early in the colonial period, probably in the 17th century". A single seedling can produce a large patch through vegetative propagation (Moore 1979). This species is listed as one of the top fourteen species of concern by the Coastal Invasive Plant Committee, for more information visit their web site
Moore, R. J. Cirsium arvense (L.) Scop. In: Mulligan, Gerald A. 1979. The Biology of Canadian Weeds: Contributions 1-32. Biosystematics Research Institute, Ottawa.
Illustrated Flora of British Columbia 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
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