Yellow starthistle

Identification & Biology
Alias' : Yellow starthistle, Golden Cockspur, St. Barnaby's thistle, Barnaby Thistle
Latin Name : Centaurea solstitialis
Category : Terrestrial Plants
Description :
  • Tap-rooted winter annual that grows from 0.6 m to over 1 m tall  
  • Pineapple shaped flower heads that have long, yellow spines and bright yellow flowers 
  • Grayish-green stems are winged and covered with fine hairs  
  • Stem leaves are linear and basal leaves are deeply lobed, often with a large end lobe 


Yellow starthistle is a member of the knapweed family and reproduces entirely through the production of seed. A single large plant can produce up to 150,000 seeds. Most seeds germinate within a year, but some can last 10 years or more in the soil. Seeds move over long distances with livestock, vehicles, equipment and contaminated seed, grain and hay. Seeds have bristles that may also stick to clothing. 

Yellow starthistle is commonly confused with blazing-star (Mentzelia laevicaulis), a native species that occurs on dry slopes in south-central British Columbia. Blazing-star has five bright yellow petals and the yellow filament stalks of the stamen have become widened and enlarged into petal-like structures. These unique flowers close in the afternoon. Blazing-star lacks the distinctive long spines which are characteristic of yellow starthistle. 

Look Alikes

Yellow starthistle is native to Eurasia. It is established in many US states, including Washington, but no populations have persisted to date in Canada. Yellow starthistle prefers dry grassland or shrub-steppe habitat with well-drained soils. It readily invades rangelands, pastures, roadsides, cropland and disturbed areas. The optimum environment is cool, wet winters followed by hot, dry summers. 

Impact & Risks
  • Yellow starthistle forms dense infestations that reduce available forage for livestock and wildlife, and decrease biodiversity in grasslands by displacing native vegetation.  
  • The plant is also known to cause a neurological disorder in horses called “chewing disease” that can lead to permanent brain damage and even death.  
  • Similar to its relatives, diffuse knapweed and spotted knapweed, yellow starthistle may suppress the growth of other species due to the release of toxic substances into the soil (i.e. allelopathic effect).  
  • It can reduce land value and access to recreational areas. 
Prevention & Mitigation

Yellow starthistle is a federally regulated plant. Contact your local Canadian Food Inspection Agency (CFIA) office if you suspect you have found this invasive plant. The  CFIA  will follow up and determine if further action is needed.

The most effective way to ensure that your lands do not become infested with yellow starthistle is by prevention. Here are some recommendations to prevent yellow starthistle from invading your property: 

  • Use clean grain, hay and straw. 
  • Use clean, high-quality seed that is certified if possible. 
  • If you are in an area that has been infested, ensure machinery, vehicles and tools are free of soil and plant parts before moving them from one area to another. 
  • Maintain your crops and natural lands in a healthy, vigorous condition to ensure a competitive plant community; competitive perennial grasses and forbs utilize water and nutrients that would otherwise be readily available to yellow starthistle. 
  • Regularly patrol your property for yellow starthistle plants. 
  • Immediately re-vegetate disturbed, bare soils with a suitable seed mixture that provides dense, early colonization to prevent weed invasion.  
  • Do not move contaminated soils to a new area.  
Treatment & Disposal
  • Heavy grazing by goats, sheep and cattle can reduce seed production and biomass. Grazing should be timed to the bolting stage in late May or June, before spines are on the plant.  
  • Hand-pulling, hoeing or weed whipping can be effective for isolated plants or small populations. The best time to remove plants is after they have bolted and before they have produced seeds. 
  • Mowing can be effective when combined with revegetation.  
  • Herbicides are best applied at the early rosette stage in the first or second year of a long-term management plan. If applying herbicides, read the label for full use and precautionary instructions.  
  • For further information on the selection and application of chemicals to protect your crop, contact AgriService BC at 1-888-221-7141 or email  
  • Six biocontrol agents (natural insect enemies) have been imported from Greece and are established as biological control agents in the USA. There are currently no biocontrol agents available in BC. 
Okanagan Distribution

Priority Level Definitions

Watch For - Poses a significant threat (very high risk) and does NOT presently occur in the region OR is relatively new to the region and is very limited in extent.
High - High risk/impact; limited population with significant potential to spread in the region.
Medium - Medium risk/impact; limited distribution – broader population distribution with potential to spread further in a region.
Low - Low risk/impact; may be widespread or not, may be of concern in specific situations with certain high values – e.g. specific agriculture crops. Some species may be treated primarily with biological control agents.