Scotch Thistle

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Scotch Thistle

Onopordum acanthium

Information Sheet (PDF)


Colorado List B - Control required in Jefferson County

General

  • Biennial
  • Family: Sunflower (Asteraceae)
  • Native to Europe and eastern Asia


Habitat

  • Found in rangelands, disturbed areas and right-of-ways
  • Thrives in moist areas


Plant


Vegetation

  • Covered with dense hair
  • Leaves are up to 2 feet long and 1 foot wide
  • Mature plants are 12 feet tall
  • Stems have spiny wings


Roots

  • Tap root


Flower

  • Color: violet
  • Season: mid-June through September
  • Size: 1 to 2 inches wide
  • Spiny bracts


Seed

  • May remain viable in soil for more than 30 years
  • One plant can produce 20,000 to 40,000 seeds
  • Seeds germinate in the fall and spring


Seedling

  • Forms dense patches
  • Rosettes can grow 6 feet wide
  • Rosettes form the first year and bolt early May to June of the next season


Reproduction

  • Seed


Control


Biological

  • None


Chemical


Cultural

  • Prevention – maintain health of site using good grazing management and keeping good cover
  • Removal
  • Re-vegetation of highly disturbed sites


Mechanical

  • Burning - May cause seeds to germinate
  • Grazing - N/A
  • Mowing - When used with other methods


More Information


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

Treatment Timeline


JanFebMarAprMayJuneJulyAugSeptOctNovDec
Control











Removal











Herbicide











Growth











Rosette











Vegetative











Flowering











Seed