Plumeless thistle is a member of the Aster family, Thistle tribe. It is an introduced winter annual or biennial. This plant can be distinguished from musk thistle by its smaller flowers - ½ to 1 inch in diameter.
The leaves of plumeless thistle lack the prominent white margin present on musk thistle leaves. The plant may grow to a height of 5 feet or more. Flowers are reddish-purple and are either solitary or clustered. Taproots are large and fleshy.
Plumeless thistle is an extremely prolific seed producer. It is found in pastures, river valleys, and along roadsides. It is rapidly increasing in this state, particularly in northwestern Colorado (Pitkin, Garfield, Gunnison, and Eagle Counties).