Adams County
CO Noxious Weeds

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Scotch Thistle

(Onopordum acanthium)

Scotch Thistle Flower

Scotch thistle was introduced into the United States from Europe. This plant is considered a noxious weed and is included on the Colorado and the Adams County Noxious Weed Lists. Scotch thistle is a biennial, which means its first year of growth is a rosette from a stout, corky taproot. Rosettes can appear in either spring or fall. During the second year, the stem bolts and can grow up to 8 feet tall. This plant has large, hairy, spiny leaves which have a velvety-grey appearance.

Scotch thistle produces many flowers which are violet to reddish in color and have spine-tipped bracts. Flowering occurs between mid-June and early July. Flower heads remain upright, rather than nodding as musk thistle flowers do.

 

Scotch thistle can create an impenetrable barrier to man and to wildlife and nearly eliminates forage use by livestock and big game species such as deer and elk.

Scotch Thistle Early Growth

Scotch thistle is found in many county locations such as rangeland, riparian areas, and ditch bank sites.


 

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