Idaho OnePlan recommends the website of the Idaho Weed Awareness Campaign as the best resource for up-to-date information about Idaho's noxious weeds, and their control.
Source for this page: Idaho's Noxious Weeds by Robert H. Callihan & Timothy W. Miller

BACKGROUND

Scotch thistle (Onopordum acanthium) is a native of Europe and eastern Asia and is probably an escaped ornamental. Scotch thistle stands are dense and practically impenetrable due to the weed's spiny nature and large size. It spreads by seed and generally inhabits moist sites or drainages in dry locations.

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DESCRIPTION

Scotch thistle is a biennialPlant that germinates in one growing season, then flowers, seeds, and dies during a second that can grow up to 8 feet tall. The plant forms a rosetteA circular, normally basal, clump of leaves of leaves the first year and produces a seed stalk the second year. Leaves are large(up to 2 feet long and 1 foot wide) and strongly armed with spines, and the blades form conspicuous fringelike extensions down the side of the stem. The entire plant is finely hairy to woolly, giving it a silvery-gray color. Flower heads are borne in midsummer in groups of 2 or 3 on branch tips. Flower headsA group of flowers borne tightly together are globe shaped, upright, intensely spiny, and up to 2 inches in diameter; flowers are purple. Seeds are slender, smooth, and plumed.

DISTRIBUTION

Scotch thistle is widely but sparsely distributed in the United States and throughout Idaho.

CONTROL

Biological control agents are not available, but herbicides can provide excellent control of this weed if applied to very small plants.