Scotch thistle (Onopordum acanthium L.)
Scotch thistle is a large, biennial forb known for its coarse, spiny leaves and distinctive gray-green appearance. The plant is heavily covered in woolly hairs, giving it a silvery or gray cast.
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In its first year, Scotch thistle forms a large basal rosette from a fibrous taproot. During the second year, it produces one to many upright, branching stems typically 2 to 6 feet tall, though plants may reach 10 to 12 feet under favorable conditions. Stems feature broad, spiny wings running vertically along their length.
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Leaves are very large — up to 2 feet long and 1 foot wide. They are arranged alternately along the stem, clasping it at the base and decreasing in size toward the top. Leaf margins are wavy and toothed with sharp spines. Spines occur at leaf tips, along stem wings, and sometimes along the underside midrib. The dense woolly hairs covering leaves and stems are a key identifying feature.
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Flowering occurs from late June through early September. Flowers are terminal and usually solitary or found in small clusters. Each flower head consists of white to purple florets forming a dense disk 1 to 3 inches across, enclosed by spiny bracts.
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Seeds are oblong to cone-shaped achenes with long pappi attached directly to the seed, aiding in wind dispersal.
Quick ID Guide
Life Cycle:
• Biennial
• First-year rosette
• Second-year flowering stems
Height:
• Typically 2–6 feet
• May reach 10–12 feet
Roots:
• Fibrous taproot
Leaves:
• Very large (up to 2 ft long)
• Gray to gray-green
• Woolly, hairy surfaces
• Wavy, spiny margins
• Broad spiny stem wings
• Alternate and clasping
Flowers (Late June–Early September):
• White to purple• 1–3 inches across
• Solitary or small clusters
• Surrounded by spiny bracts
Seeds:
•Oblong to cone-shaped achenes
• Long pappi attached
Management Options
Prevention and cultural control practices should be emphasized whenever possible. Clean equipment and gear before moving between sites, use certified weed-free forage, minimize soil disturbance during construction, and monitor disturbed areas for new infestations.
At this time, there are no approved biological control agents available for Scotch thistle.
Because Scotch thistle develops a taproot, small infestations can be effectively controlled by hand pulling or digging, ensuring the root is removed. Tillage or hoeing is also effective when the taproot is severed at least 2 inches below the soil surface.
Targeted grazing may provide suppression. Sheep can be effective before bolting, and goats may help reduce seed production when plants are in the bud stage. Mowing or chopping will not eliminate established stands but can help reduce seed production if properly timed. Avoid mowing when seeds are present to prevent spread.
Larger infestations may require herbicide treatment. Spring and fall applications targeting rosettes are recommended, though bolting plants can also be treated. Flowering plants may be chopped, and the remaining rosette and stem treated with herbicide to prevent reflowering. Always read and follow herbicide label directions. Contact Sublette County Weed & Pest for assistance with product selection, rates, and timing.
