Description:
obust biennial (or sometimes annual) that often grows 8 feet or more in height and 6 feet in width. Main stems may be up to 4 inches wide at the base. Stems have vertical rows of prominent, spiny, ribbon-like leaf material or "wings" that extend to the base of the flower heads. Leaves, which are armed with sharp, yellow spines, are up to 2 feet long and 1 foot wide. Upper and lower leaf surfaces are covered with a thick mat of cotton-like or woolly hairs, which give the foliage a gray-green appearance. Plants flower in mid-summer. The globe-shaped flower heads are borne in groups of 2 or 3 on branch tips. Flower heads are up to 2 inches in diameter, with long, stiff, needle-like bracts at the base. Flowers range from dark pink to lavender. Seeds are smooth, slender, and plumed (Dewey 1991; Callihan and Miller 1998).

• Mechanical: Small areas can be eradicated by digging. Plants must be cut off below the soil, leaving no leaves attached (Julian and Rife). Mowing has limited effectiveness for controlling Scotch thistle. It usually only prevents seed production if done either immediately prior to flowering or when plants are just starting to flower. When mowing is conducted too early, it may only delay flowering. However, when plants are cut too late in the flowering process, viable seed may still develop in the capitula following cutting. Because there can be a wide variety in the maturity of plants, a single mowing is unlikely to provide satisfactory control (Sindel 1991).
• Biological: No biological controls are currently available in the United States. Australia has released several biocontrol insects, but some of them have failed host specificity tests in the U.S. Additional insects are being evaluated by USDA for release in the U.S. (Joley et al. 1998).
• Herbicide: Picloram, dicamba, 2,4-D, dicamba + 2,4,-D, and metsulfuron are effective for controlling Scotch thistle (Beck 1991; Cargill et al. 1998). Application rates vary depending on stand density and environmental conditions. Herbicides should be applied in the spring before Scotch thistle bolts, or in the fall to rosettes (Beck 1991). . For chemical control recommendations, refer to the Pacific Northwest Weed Control Handbook, an annually revised publication available from Washington State University Cooperative Extension. Refer to the State Noxious Weed Control Board site

For More Information:
Detailed information about Scotch Thistle is available at the Washington State Noxious Weed Control Board Web Site.