General: Biennial herb from a fibrous root, 0.3-2.0 m tall; stems erect, branched, the branches spreading and ascending, stiff, ridged, sparsely to densely hairy, conspicuously spiny-winged at the base of the leaves.
Leaves: Basal and stem leaves broadly lanceolate, deeply lobed, 3.5-10 cm long, glabrous and strongly spiny-hairy above, nearly glabrous to sparsely white- or grey-woolly below, bases clasping, winged and descending down the stems, marginally spiny, the lobes and tips with stout spines 5-10 mm long; upper stem leaves reduced.
Flowers: Heads several to many, solitary at the end of the branches; disk flowers purple, rarely white; involucres egg-shaped to more or less rounded, 2.5-4 cm tall; involucral bracts lanceolate to linear-lanceolate, the outer and middle ones with a strong, yellow, 2-4 mm long spine, hairy on the margins, the midrib woody, the inner bracts longer, with a stiff point.
Fruits: Achenes glossy, light brown with parallel lines; pappus of white, feathery bristles.
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
Habitat / Range
Mesic to dry roadsides, fields, pastures and disturbed areas in the lowland, steppe and montane zone; common in S BC, infrequent in N BC; introduced from Eurasia.
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 bull thistle-like plant with prickles, one should first check to make sure it is really a Cirsium, or not. The easiest way is to see if the pappus bristles have conspicuous feather-like hairs, in which case it is a Cirsium, or if the bristles are missing or very tiny, which is a feature of the genus Carduus.
There are at least five large-headed, native thistles (inflorescences longer than 2 cm) which can be mistaken for the bull thistle. They can be separated from the exotic bull thistle based on the absence of, or only very short, wings on the stem.
Scotch thistle (Onopordum acanthium) is another spiny plant with pinkish flowerheads which grows in similar habitat conditions. However, its larger, fewer inflorescences, and the broader leaves and wings on the stem can readily separate it from bull thistle. Additionally, while bull thistle flowerheads and leaves can be woolly, the Scotch thistle is more pronouncedly woolly-hairy and the whole plant is rather silvery as compared with the greener bull thistle.
The exotic milk thistle (Silybum marianum) can also be mistaken for the bull thistle, however, its range is rather restricted to a few localities in southwestern British Columbia. The conspicuously white mottled leaves and the fewer involucral bracts with long spines in the milk thistle readily differentiates this species from the bull thistle.
The inflorescence of burdock species (Arctium) might be mistaken for bull thistle because of the involucral bracts with hooked tips and the purple flowers. They can be readily separated, however, based on the large basal leaves and the absence of the prickles from the leaves.
The table below shows the species-specific information calculated from original data (BEC database) provided by the BC Ministry of Forests and Range. (Updated August, 2013)
