Musk Thistle
Carduus nutans
Washington State Classification
Class B — Control Required (Designated)
It invades pastures and fields, crowding out desirable forage plants.
Required control in Region 1 (all western Washington counties)
Quick Reference
Identification
Growth Habit
Musk thistle is typically a biennial though may be a winter annual. It can grow up to 7 feet tall.
Leaves
Leaves are alternate, deeply lobed, spiny and may have some hairs on the underside leaf veins.
Flowers
Flowerheads mostly solitary and large, up to 3.1 inches across and nodding. Bracts under flowerheads are broad, some reflexed with a spiny tip. Each plant can have 50 to 100 flower heads.
Fruit & Seeds
Seeds (achenes) are smooth and hairless.
Impact
It invades pastures and fields, crowding out desirable forage plants. Livestock will not graze in areas heavily infested with must thistle, thus decreasing available pasture. It hinders stream bank access, and is problematic in grain fields.
Ecology & Spread
Habitat
Musk thistle grows in pastures, rangeland, native meadows, roadsides, forests, stream banks, and occasionally grain fields. Please click here to see a county level distribution map of musk thistle in Washington.
Spread Mechanisms
Reproduction
Musk thistle reproduces by seed.
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Control Methods
Mechanical
- Musk thistle can be dug or grubbed out.
Biological Control
- A seed eating weevil, Rhinocyllus conicus, is quite effective in reducing seed output.