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Scotch Thistle (cotton
thistle)
Asteraceae (= Compositae), the aster family
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Background
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 Scotch
thistle 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
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 Scotch
thistle is a biennial that grows up to 8 feet
tall. The plant forms a rosette 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 along the stem. The entire plant is finely hairy to
woolly, giving it a silvery-gray color. Flower heads develop in midsummer in
groups of 2 or 3 on branch tips. Flower heads are globe shaped, upright,
intensely spiny, and up to 2 inches in diameter; flowers are purple. Seeds
are slender, smooth, and plumed.
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Distribution
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 Scotch
thistle is widely but sparsely distributed in the U.S. and throughout Idaho.
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Control
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Biological control agents are not available, but herbicides can provide
excellent control of this weed if applied to very small plants.
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© 1999 University of Idaho:
Text and photographs for these pages from Idaho's Noxious Weeds, by
Robert H. Callihan and Timothy W. Miller (revised by Don W. Morishita
and Larry W. Lass).
Please contact: Ag Publishing, University of Idaho, Moscow,
Idaho 83844-2240; 208 882-7982 visit the Resources for Idaho website at
http://info.ag.uidaho.edu, for more information about this or other
publications.
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