Musk thistle is a member of the Aster family. Introduced from Eurasia, it is a winter annual or biennial which reproduces by seed. The first year’s growth is a large, compact rosette from a large, fleshy, corky taproot. The second year stem is erect, spiny, 2 to 6 feet tall and branched at the top. The waxy leaves are dark green with a light green midrib and mostly white margins; flowers are purple or occasionally white. Musk thistle is also known as "nodding thistle" and is commonly found in pastures, roadsides, and waste places. It prefers moist bottomland soil, but also can be found on drier uplands. It is found primarily in Old Snowmass and along the Crystal River.

Comments: Populations of musk thistle have been significantly reduced in Snowmass Canyon and along River Road. Most of the musk found in Pitkin County is along State Highway 133 and other sites along the Crystal River drainage.

Biological controls: The musk thistle seed head weevil Rhinocyllus conicus is widespread in Colorado. Larvae of this insect destroy developing seeds but are not 100 percent effective by themselves. The weevil normally impacts seed production by about 50 percent. Herbicides can be combined with weevils if the insects are allowed to complete their life cycles. Another weevil Trichosirocalus horridus attacks the crown area of musk thistle rosettes and weakens the plant before it bolts. This weevil has reduced stand density in areas where it has become well established. A leaf feeding beetle Cassidia rubiiginosa causes considerable damage by skeletonizing leaves. It is recommended to release more than one type of insect on a weed since each type may work on different parts of the plant.

Chemical controls: For additional information contact your local city, county or town natural resource office. (See section VII for phone numbers.)

Cultural controls: Musk thistle, like other biennial thistles, thrives on disturbance. The best management therefore is to minimize disturbance and if it does occur be certain to revegetate with competitive native perennial species. Please refer to the revegetation guidelines of section 4.05 of the Pitkin County Weed Management Plan or contact Pitkin County Land Management for more information.