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Squarrose knapweed

Squarrose knapweed

Squarrose knapweed

(Centaurea virgata)

Priority: -  Prevent

Tags: Agricultural | Biocontrol

Identification and Reproduction

Identification:

  • Squarrose knapweed is a taprooted perennial that orginates from Western Asia. 
  • This plant is grows upto 50 cm tall and the stems are rough and covered in fine hairs with resin dots. Stems are very branched. 

  • Lower leaves can reach 15 cm long, alternate, lobed and will decrease in size moving up the stem. 
  • Their flowers are slender and urn-shaped, pink to purple and flowerheads are curved downwards. Bracts are pale green or straw coloured, hairy and curved outwards. 
  • It flowers from June through til August. 

Reproduction: 

  • Primarily reproduces through seeds. 
  • Seeds are often dispersed in soil, hay, animal fur or vehicle traffic. 
  • Seeds have a hard exterior and can remain viable for 5-10 years. 
  • Squarrose knapweed introduction has been linked to sheep ranching. 

Habitat & Ecology

  • Well adapted to harsh climates, with cold winters and dry summers. 
  • It has become an aggressive invader to dry, rocky steep sites. 
  • This plant also has the capability to remain a rosette for several years when conditions are unfavorable. 
  • Cannot withstand cultivation or irrigation. 
  • Currently this plant has been listed as a high risk species in Canada and has not yet established. 

Impacts

Ecological: 

  • Knapweeds are very competitive and vigorous, displacing native vegetation. 
  • They are known as pioneer species and are quick to take over recently disturbed sites.
  • Has an allelopathic property that will suppress the growth of other plants. 

Management

Prevention is a high priority for this plant.

  • Maintain healthy plant communities to prevent the introduction of knapweed.
  • Ensure hay and seed mixtures are certified and invasive-free.
  • Replant recently disturbed areas with native species. 
  • Map, record or report suspected knapweed infestations.  
  • If squarrose knapweed does establish, remove as soon as possible. Small infestations are easily hand-pulled or dug up. 

Resources

Downlad the Alberta Invasive Species Coucil's factsheet on Squarrose Knapweed here

For more information on how to identify this Weed Seed, check out the Canadian Food Inspection Agency's datasheet on Centaurea virgata subsp. squarrosa (Squarrose knapweed). 

View header photo here