Musk Thistle

musk thistle purple flower
musk thistle multiple flowers
musk thistle leaves
musk thistle habit

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


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.