Canada Thistle

Canada thistle seedlings.
Canada thistle flowers next to logs.
Canada thistle rust fungus.

Canada Thistle

Cirsium arvense

Information Sheet (PDF)


Colorado List B - Control required in Jefferson County


General

  • Colony-forming unisexual plants
  • Elevation: 4,000 to 9,500 feet
  • Family: Sunflower (Asteraceae)
  • Herbaceous
  • Perennial


Habitat

  • Infestations are found in cultivated fields, riparian areas, pastures,  forests, lawns and gardens, roadsides and in waste areas; due to its  seeding habits, vigorous growth and extensive underground root system,  control or eradication is difficult; Canada thistle is one of the most  widespread and economically damaging noxious weeds in Colorado


Plant


Vegetation

  • Leaves are 4 to 8 inches long, usually dark green with spiny, serrated edges
  • Mature plants are 2 to 5 feet tall


Roots

  • Horizontal growth is quick, roots can regenerate from small pieces
  • Horizontal roots may extend 15 feet or more, vertical roots may grow 6 to 15 feet deep


Flower

  • Buds are 3/4 inch or less in diameter
  • Color: purple, pink or white
  • Season: June to August


Seed

  • Can remain dormant in soil up to 20 years
  • Seed can be viable 8 to 10 days after flowering
  • Seeds germinate from late May through early fall


Seedling

  • Seeds germinate in spring or fall


Reproduction

  • Root propagation is main source for new plants
  • Seed and horizontal roots


Control


Biological

  • Not effective in our area


Chemical


Cultural

  • Prevention - maintain health of site


Mechanical

  • Burning - N/A
  • Grazing - N/A
  • Mowing - Mowing is best before seed set; mow every 3 to 4 weeks June through  September to weaken plants; works best when combined with fall herbicide  treatments. 


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











RemovalNot recommended - plants regrow from root fragments




Herbicide











Herbicide + Mow




MowMowMowHerbHerb

Biological











Growth











Vegetative











Flowering











Seed