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Yellow starthistle

Yellow starthistle

Yellow starthistle

(Centaurea solstitialis)

Priority: -  Prevent

Tags: Agricultural | Toxic | EDRR

Identification and Reproduction

Identification:

  • Yellow starthistle is in the Asteraceae family and is a long-lived winter annual
  • Stems are erect, rough, branching and can grow up to 1 m in height. Sometimes it can grow in clusters that appear shrubby. 

  • It is much like other knapweeds and will form a rosette in its first year of growth. After bolting the lower stem will appear wooly and grey. 
  • Lower leaves are deeply lobed while upper leaves are toothless and sharply pointed. 
  • This plant produces bright yellow flowerheads that are found at the end of stems. They are distinct with sharp straw-coloured thorns that can reach 2 cm long.

Reproduction: 

  • Yellow starthistle reproduces by seeds which are easily dispersed. 
  • Seeds are known to spread through hay and vehicles. 

Habitat & Ecology

  • They prefer dry sites that are fully exposed or have a south facing aspect. 
  • It is also commonly found on rocky, shallow soils. 
  • Taproots extend well below the ground to tap into undergrown water resources. 
  • Considered a pioneer species, taking over recently disturbed sites. 
  • Yellow starthistle has been observed in Alberta, British Columbia, Ontario and Saskatchewan but populations have been eradicated. 

Impacts

Social: 

  • This aggressive plant is out-competing foraging plants and reducing rangeland values. 
  • It is toxic to horses and if consumed can cause "chewing disease" in horses. It contains a toxin that can disrupts the nervous system and brain, restricting eating and drinking of horses. 
  • Can contaminate alfalfa and grain crops. 

Ecological: 

  • Invades environments and can reduce biodiversity and wildlive habitat. 
  • Can exhibit allelopathic effects on native species. 

Management

Prevention is a high priority for this plant. 

  • Use clean and certified grain, hay, straw and seed mixtures. 
  • Clean equipment, machines and vehicles of any soil or plant parts before leaving a work site. 
  • Contact the Canadian Food Inspection Agency (CFIA) if you think you have seen this plant. 

Mechanical/Manual Control: 

  • Plants can be pulled, hoed, tilled or mowed prior to flowering. 
  • Mowing is only effective if plants have bolted and are tall. 
  • Cultivation of the soil can also help control yellow starthistle, helping to bury seeds deep in the soil. 

Chemical Control: 

  • Herbicides are most effective when applied during its transition from rossette to bolt (prior to flowering). 
  • Currently picloram, dicamba and 2,4-D are registered for use on yellow starthistle in BC. 
  • Please carefully read chemical labels prior to application. 

Biological Control: 

  • There are 6 natural predators of the yellow starthistle. They have been imported into the USA from Greece and have been used as a biological agent. 
  • These insects: weevils and flies, attach the flowerhead and halt seed production. 

Resources

Download A Guide to Weeds in British Columbia on Yellow Starthistle here

For more details check out the Invasive Species Compendium datasheet on Centaurea solstitialis (yellow starthistle). 

Header photo (Matt Lavin).