Scotch Thistle
Onopordum acanthium
Washington State Classification
Class B — Control Required (Designated)
Scotch thistle is a problem in rangelands and other open areas.
Required control in Region 1 (all western Washington counties)
Quick Reference
Identification
Growth Habit
Scotch thistle is a branched, biennial or annual with a broadly winged stem that can grow up to 8 feet or more in height and 6 feet in width.
Leaves
Leaves are up to 2 feet long and 1 foot wide, are covered with sharp yellow spines and have a gray-green appearance from being covered with a thick mat of cotton-like or woolly hairs.
Flowers
Plants flower in mid-summer. The globe-shaped flowerheads solitary or in groups of 2-7 on branch tips. Flowerheads are up to 1 to 3 inches in diameter, rounded, with long, stiff, needle-like bracts at the base. Flowerheads contain many disk flowers that range from dark pink to lavender in color, though occasionally white.
Fruit & Seeds
Seeds are smooth, slender, and plumed.
Impact
Scotch thistle is a problem in rangelands and other open areas. Infestations of Scotch thistle reduce forage production and virtually prohibit land utilization for livestock and block access for people and wildlife. Dense stands of the large, spiny plants exclude animals from grazing and access to water.
Ecology & Spread
Habitat
Scotch thistle will grow in wet meadows and pastures as well as dry pastures and rangelands. It may also be found alongside streams and rivers. Please click here to see a county level distribution map of Scotch thistle in Washington.
Spread Mechanisms
Reproduction
Scotch thistle reproduces by seed. Each plant can produce 8,400 to 40,000 seeds.
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