Plumeless Thistle (Carduus acanthoides)

Plumeless thistle is a member of the Aster family. Introduced from Eurasia, it is a winter annual or biennial. This plant can be distinguished from musk thistle by its smaller flowers from 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 a major problem in and around the Aspen area.

Comments: The most common noxious weed in Pitkin County, it is especially prevalent in Aspen and Snowmass Village

Biological control: The same seed head weevil, Rhinocyllus conicus that attacks musk thistle also feeds on plumeless thistle seeds. Another musk thistle weevil, Trichosirocalus horridus has been released on plumeless thistle in Pitkin County. This weevil appears to be ineffective on plumeless.

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

Cultural control: Cultural management actions should aim to minimize disturbance and maximize the use of native grasses and forbs for revegetation of any disturbed soil.

Mechanical control: Mowing is generally not effective on plumeless due to the plant’s capacity for rapid re-growth. Hand cutting is not effective unless there are repeated follow-up treatments. Hand cutting should only be conducted if there is a commitment to follow-up efforts. Plumeless tends to branch out where it is cut and then it re-flowers. Work at North Star Nature Preserve has indicated that pulling plumeless can be very effective, especially if done after a light rain. Hand pulling, with a good set of gloves, is preferable to shoveling. Shoveling disturbs the ground thus creating a potential seedbed for future infestations. If shovel treatment is used be certain to cut the root several inches below the soil surface to reduce the chance of re-growth of the cut root.

Locations of Infestations of Plumeless Thistle on Roads in Pitkin County:

Locations of Infestations of Plumeless Thistle on Open Space in Pitkin County: