Canada thistle (Cirsium arvense L.)
Canada thistle is a creeping perennial that spreads aggressively by both seed and an extensive rhizomatous root system, making it particularly difficult to control. Infestations often begin in disturbed soils and can quickly expand without integrated management.
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Plants produce erect stems that typically reach 3–5 feet tall. Stems are smooth or slightly hairy and may branch toward the top, forming numerous flower heads.
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Leaves are arranged alternately along the stem. They are waxy and range from light to dark green (often described as kelly green). Leaf shape varies from oblong to lance-shaped and may be lobed or smooth. Margins are serrated and usually bear sharp spines.
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Flowering occurs from early to late summer. Flowers form in clusters at the ends of stems and branches. Each flower head consists of many small, tubular florets that range from light pink to deep purple. As flowers mature and go to seed, they may appear pale or whitish due to the developing pappus. The involucre (bulb-shaped base) is covered in overlapping bracts.
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Canada thistle has an extensive rhizomatous root system that may reach up to 15 feet deep and spread 15 feet or more horizontally. Seeds are small, light to dark brown, and topped with tufts of fine hairs that allow for wind dispersal.
Quick ID Guide
Life Cycle:
• Perennial
• Spreads by seed and rhizomes
Height:
• 3–5 feet tall
Stems:
• Erect
• Smooth to slightly hairy
• Branched near the top
Leaves:
• Alternate
• Waxy green
• Serrated margins with spines
• Variable shape (oblong to lance-shaped)
Flowers (Early–Late Summer):
• Light pink to deep purple
• Clustered at stem tips
• Turn pale/whitish at the seed stage
Roots:
• Extensive rhizomes
• Up to 15 feet deep and wide
Seeds:
• Small, brown
• Tufted pappus for wind dispersal
Management Options
Prevention and cultural control strategies should be emphasized whenever possible.
Hand pulling or digging is generally not recommended unless repeated every 2–3 weeks throughout the growing season, as rhizome fragments readily resprout. Mowing at regular 3–4 week intervals can help deplete root reserves and reduce seed production.
Larger infestations typically require herbicide applications. Early to mid-summer treatments before bloom can be effective. Fall applications of systemic herbicides are often especially effective because products are translocated to the root system. Always read and follow the herbicide label before application. Contact Sublette County Weed & Pest for assistance with product selection, rates, and timing.
