Bull thistle ( Cirsium vulgare (Savi) Ten.)
Native to Eurasia, bull thistle was introduced to North America multiple times as a seed contaminant and is now widely established. It commonly invades pastures, fields, roadsides, and other disturbed areas, where it can become highly competitive.
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Bull thistle is a biennial forb with a short, fleshy taproot. In its first year, it forms a basal rosette of deeply lobed leaves. In its second year, it produces one to many erect, branching stems that typically reach 2–6 feet tall and may occasionally grow up to 7 feet.
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Stems are green to brownish, sparsely hairy, and have spiny wings running vertically along them. Leaves are arranged alternately on the stem and decrease in size toward the top of the plant. Basal leaves are usually 3–12 inches long and deeply lobed, though some may be lance-shaped with toothed margins. Leaves are dark green, prickly, and hairy on the upper surface, and cottony underneath. Spines occur along the margins, upper surface, and midrib.
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Flowering occurs from early summer through fall (typically July through September). Flowers are large—about 1½ to 2 inches across—and are usually solitary at the ends of branches, though they may occasionally form small clusters. Flower heads are dark pink to purple and enclosed in long, narrow, spine-tipped bracts that form a bulb-shaped base.
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Seeds are oblong achenes (up to 5 mm long) topped with a circle of plume-like white hairs (pappus) that allow for wind dispersal.
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Similar Species
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Bull thistle can be distinguished from Cirsium arvense (Canada thistle) by examining the leaves. Bull thistle leaves are prickly and hairy above and cottony underneath, while Canada thistle leaves are generally smooth (glabrous) on the upper surface and may be smooth or lightly hairy below.
Quick ID Guide
Life Cycle:
• Biennial
• First year rosette
• Second-year flowering stems
Height:
• 2–6 feet tall
• Occasionally up to 7 feet
Stems:
• Spiny-winged
• Sparsely hairy
• Branched
Leaves:
• Deeply lobed
• Alternate arrangement
• Hairy and prickly above
• Cottony below
• Spines on margins and midrib
Flowers (July–September):
• 1½–2 inches wide
• Dark pink to purple
• Spine-tipped bracts
• Usually solitary
Seeds:
• Oblong achenes
• White plume-like pappus
Management Options
Prevention and cultural control strategies should be emphasized whenever possible.
Because bull thistle has a taproot, it can be effectively managed by hand pulling or digging, ensuring the root is severed below the soil surface. Tilling or hoeing is also effective when the taproot is cut beneath ground level.
Controlled grazing by goats may reduce populations, particularly when plants are in the flowering stage. Mowing or chopping will not eradicate established stands but can help prevent seed production if conducted before seeds mature. Do not mow once seeds are present.
Larger infestations may require herbicide applications. Spring and fall treatments targeting rosettes are recommended, though treatment of bolting plants can also be effective. Flowering plants may be chopped, and the remaining rosette or stem treated with herbicide to prevent reflowering. Always read and follow herbicide label instructions before application. Contact Sublette County Weed & Pest for assistance with product selection, rates, and timing.
