Musk Thistle

Musk Thistle

Carduus nutans

Information Sheet (PDF)

Colorado List B - Control required in Jefferson County

General

• Biennial (short-lived perennial)
• Family: Sunflower (Asteraceae)
• Herbaceous
• Rosettes formed the first year and bolt early in May to June of the next year

Habitat

• Grows in all soil textures, but soils must be well drained
• Musk thistle out-competes desirable plants for resources
• Pastures, rangeland, forests, disturbed areas, right-of-ways, ditch banks and grain fields from sea level to 8,000 feet

Plant

Vegetation

• Bolts in late March through May
• Leaves are hairless, dark green with light green mid-rib, deeply lobed; 1  to 1 1/2 inches wide and 4 to 5 inches long; yellow to white spine near tip
• Leaves extend onto the stem, giving it a winged appearance
• Mature plants grow up to 6 feet tall
• Multi-branched

Roots

• Tap root

Flower

• Buds 1 inch or less wide; mature flowers are 1 1/2 to 3 inches wide
• Color: Purple
• Droop from stem, giving it the name "nodding thistle"
• Each head produces an average of 1,200 to 1,500 seeds 45 to 55 days after bolting
• Season: May through August
• Seed is dispersed 7 to 10 days after flowering

Seed

• Can germinate 6 to 8 weeks after falling to ground
• Dispersed by wind
• May remain dormant in soil for more than 10 years

Seedling

• Rosettes can grow up to 4 feet in diameter
• Seedlings emerge in mid to late July
• Spends winter as rosette

Reproduction

• Seed

Control

Biological

• Rhinocyllus conicus, seed head weevil (may infest native thistles)

Chemical

• View Information Sheet (PDF)

Cultural

• Flowers must be bagged and disposed of
• Prevention – maintain health of site
• Removal – hand pulling before seed set; remove at least the top 2 to 4 inches of the root

Mechanical

• Burning - Fire has not been and effective control because it doesn’t get hot enough to kill the plant and its roots. Burning may improve grass growth, encouraging competition
• Grazing - Ineffective because livestock only eat a few flowers. Heavy grazing and associated disturbances near water, salt and loafing areas increase establishment of seedlings
• Mowing - Between the first appearance of pink on the earliest buds and brown on the pappus. Plants may resprout

Use all chemicals according to the manufacturer's label. No specific recommendation or endorsement is made or implied by listing methods or products.