E-Flora BC: Electronic Atlas of the 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

Source: Illustrated Flora of British Columbia

There are several plant genera commonly named “thistles”, and the reason for this is they look very similar and bear prickles. The most common ones are the Cirsium and Carduus genera. When trying to identify a Canada thistle-like plant with prickles, one should first check if it is really a Cirsium, or not. The easiest way is to see if the pappus bristles have conspicuous feather-like hairs, which makes them species in the genus Cirsium, or if the bristles are missing or very tiny, which is a feature of the genus Carduus. Following that, the Canada thistle can be readily separated from the other species of Cirsium based on the small involucres that are less than 2 cm in height. However, there is another small-headed thistle, similarly introduced from Europe, that could cause confusion--the marsh thistle (Cirsium palustre). However, this species does not have creeping rhizomes, and mostly grows in moist meadows.

From a distance, one might mistake knapweed species (Acroptilon or Centaurea) for the Canada thistle because of their pink flower heads and branching stems. However, their flowers are of varying sizes--those at the edge of the disk are larger than those near the center--and their leaves do not develop prickles.

Note Author: Anna-Mária Csergo, February 2011.