Canada Thistle

canada thistle rosette
canada thistle flower
canada thistle plant

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


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