Canada Thistle Cirsium arvense
Canada Thistle is an introduced perennial herb, found in Alaska, Canada, and Greenland. It grows to 2.1 ft and blooms Jun – Sep.
More about this plant
Cirsium arvense is a perennial species of flowering plant in the family Asteraceae, native throughout Europe and western Asia, northern Africa and widely introduced elsewhere. The standard English name in its native area is creeping thistle. It is also commonly misnomered as Canada thistle and sometimes as field thistle. Wikipedia →
Canada Thistle is flagged invasive in the U.S. These natives fill a similar niche — same growth habit, bloom season, height, and region — so you keep the look and feed local wildlife instead of spreading a problem.
Part of why it adapts so well: Canada Thistle is a polyploid complex — multiple chromosome races are on record (2n = 34, 68). Spare genome copies give a plant extra raw material to evolve fast, and polyploidy is a documented correlate of successful plant invasions (te Beest et al. 2012). A labelled association from open cytology (ChromoDB), not a prediction for your specific site.
Growing & care
USDA PLANTS · TRY- Hardiness
- ≥ zone 4 derived from its U.S. range
- Height
- 2.1 ft
- Lifespan
- Perennial
- Foliage
- Broadleaf
- Flower colour
- Purple AI AI image-analysis of community-science photos (~87% expert agreement) — not a botanical record
Wildlife & pollinators
How pollinator value is scored →❧ Caterpillar hosts ~29 caterpillar species
Cirsium supports ~29 caterpillar species.
Native butterfly & moth caterpillars are the base of the terrestrial food web — most songbirds rear their young almost entirely on them. As a host for native Lepidoptera this is a moderate genus.
Recorded feeding on Cirsium in North America, including:
+ 8 more species → ↑ show fewer
✦ Bees specialist-bee host · 109 bee visitors
Specialist native bees depend on it.
Some native bees are pollen specialists (oligolectic) — they raise young only on pollen from particular plant genera. Cirsium is a recorded specialist-bee host, so losing it can mean losing the bee that relies on it.
109 native & managed bee species are documented visiting Canada Thistle — the 12 most-recorded:
Wildlife & visitors 58 birds · 3 mammals · 267 nectaring
Open records of who else uses Canada Thistle — a generalist food-web signal, kept separate from the keystone Ecological Value.
Recorded eaten by 58 birds and 3 mammals species (fruit, seed, browse) — the most-recorded:
+ 3 more species → ↑ show fewer
267 adult butterfly & moth species are recorded nectaring at its flowers — the most-recorded:
Across 68 U.S. vegetation-survey plots that recorded Canada Thistle, these catalog species turned up most often in the same plot — a real field co-occurrence signal, not a planting prescription.
Related plants
More Cirsium →
How we know this (4) Methods & honest limits
We read each species’ measured specific leaf area (leaf area per unit dry weight) from the Global Spectrum dataset and band it: thin & fast (high SLA), medium, or thick & tough (low SLA). The global median is about 10 mm²/mg.
Honest limits: A species-mean from pooled measurements — individual plants vary with light and site. A broad strategy signal, not a precise per-plant figure.
A recorded categorical fact: each species is tagged C3 (standard), C4 (heat/water-efficient) or CAM (succulent, night-time CO₂ uptake) — or a facultative combination. We only show a trait card for the noteworthy C4/CAM cases; C3 is the unremarkable majority, kept in the data but not surfaced as a card.
We take the plant’s U.S. county range and look up each county’s long-term climate, then summarise: native rainfall = the median annual rainfall across its counties; heat tolerance = the warm end (90th percentile) of average temperature across its range (mirroring how our cold-hardiness floor uses an extreme). Counties + medians blunt the roadside/observer bias that makes point-level climate unreliable. Needs at least five placeable counties.
Honest limits: A realized, sampling-biased niche (where it has been recorded, not its physiological optimum), and county climate is coarse — large Western counties span deserts and mountains. Derived guide, never a measured fact.
McKenzie et al. assigned each species a flower colour with a GPT-4V vision model over iNaturalist photos. We use the confident tier plus a separately-labelled lower-confidence top-up (mostly inconspicuous green/brown flowers), and render it as a small tint — never as an asserted fact, and kept out of the written synopsis.
Honest limits: AI image inference (~87% expert agreement on the confident tier); a decorative, confidence-tiered indicator, not a measured trait.
Sources for this entry (29) Open & cited
Cite this page Open data, please attribute
PlantKey’s data is open under CC BY-SA 4.0 — free to reuse and adapt, with attribution and the same licence. Photos keep their own per-image licence + credit (see Sources above).
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