War on Weeds—weeds are everybody’s problem
Photo courtesy of Steve Dewey, Utah State University Bugwood.org
THE ISSUE: Scotch thistle
Scotch thistle (Onopordum acanthium) is a biennial native to Eurasia. It was introduced into the U.S. as an ornamental and as a contaminate in feed. It invades rangeland, pastures, cultivated fields and other disturbed sites. Its sharp spines deter livestock and wildlife, giving it an advantage over more palatable species.
Scotch thistle stems can grow up to 8 feet tall, have a bluish-green appearance, and are covered in wooly hairs. Stems also have vertical ridges giving them a ‘winged’ appearance. Leaves are toothed to slightly lobed and can be up to 3 feet long when mature. Flower heads can be up to 2 inches in diameter, with sharp spiny bracts, and are purple to pink in color. Unlike musk thistle, scotch thistle flower heads remain upright instead of nodding. Scotch thistle spreads by seed, which is easily transported by animals (on fur) and humans (on clothing and equipment).
Integrated Pest Management (IPM) Options:
• Prevention— Learn to identify this plant. Never transport unknown plant material. Clean clothing, pets, and vehicles of debris when leaving infested areas.
• Mechanical— Hoeing, pulling, and tillage are all effective forms of mechanical control that can be used. Mowing needs to be done early and often to be effective.
• Cultural— Plant beneficial perennial plants that will compete with weed seedlings. Minimize soil disturbance and use proper grazing techniques to benefit beneficial plants.
• Chemical— There are several herbicide options available to use on this weed species. For more information on products and rates visit: https://pnwhandbooks.org/weed. Then search “thistle”. Always read and follow herbicide label directions!
Justin Hatch, University of Idaho Extension Agriculture Educator in Caribou County. 208-547-3205 [email protected]
