Musk Thistle

Carduus nutans L.
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Status:
State Designated
•
Priority 4
Synonyms:
nodding thistle, giant plumeless thistle

Description

Musk thistle is a biennial forb in the Asteraceae (daisy) family. It grows from a fibrous taproot to form a basal rosette in its first year of growth; in the second year of growth, it produces one to many stems that can reach up to 7 feet tall. Basal leaves may reach 6 inches long and are usually deeply lobed. Occasionally basal leaves are not lobed but are rather lanceolate with toothed margins. Leaves occur alternately and clasp the stems; they decrease in size up the plant. Leaves are green and may have silver or pink margins. Spines occur on the tips of leaves, on wings that run vertically on the stems, and sometimes on the rib on the bottom of leaves. Musk thistle flowers from early summer to fall. Flowers are large, reaching up to 3 inches across, and terminal. Florets may be pink to deep purple and form a dense disk enclosed in large, spiny bracts. Flower heads droop when mature giving them a nodding appearance. Seeds are oblong achenes (about 5mm long and 1mm diameter) with dark longitudinal spots and fine pappi.

Origin and Spread

Musk thistle is native to Europe, Asia, and Northern Africa and was accidentally introduced to North America in the mid-1800s possibly as a contaminant in ship ballasts. It spreads solely by seeds which are readily dispersed by the wind. Additionally, seeds may be spread by animals, water, humans, and equipment. Musk thistle is frequently found in rangeland and sites disturbed by humans, animals, and other natural causes, but it may also be found in lawns and maintained landscapes and forested areas.

Management Options

Prevention and cultural control strategies should be utilized as much as possible.

There are no biological control agents currently available for release in Teton County. Two biological control agents, thistle seed head weevil (Rhinocyllus conicus) and thistle crown weevil (Trichosirocalus horridus), were released prior to 2000 when USDA APHIS PPQ revoked the permission for interstate distribution of these agents due to lack of host specificity. These agents are present throughout the county, but they do not substantially reduce musk thistle density or reproduction.

Because musk thistle has a taproot, it can be readily managed by hand pulling or digging out the root. Tilling or hoeing is also effective for management when the taproot is severed 2 or more inches below ground. Controlled grazing by sheep, goats, or cattle may also be effective until the bolting stage. Mowing or chopping will not eradicate stands of musk thistle, but it can be beneficial in preventing seed production. Do not mow while seeds are present.

Larger infestations can be controlled with herbicides. Spring and fall treatments of rosettes are recommended, but treatment of bolting plants can be effective. Flowering plants may be chopped and the remaining rosette and stem treated with herbicide to prevent reflowering. Read the label before using any herbicide. Contact TCWP if you have any questions about application rates or how to use an herbicide.

Treatment AreaRecommended Herbicides
Range, Pasture, Natural AreasAminopyralid or metsulfuron-methyl
Pasture where manure or hay will be used for compost2,4-D or dicamba
Lawn2,4-D or dicamba
Riparianglyphosate (aquatic label)

Additional Resources