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Invasive Plants and Agricultural Pest Management

Welcome to Alaska's Invasive Plant Program. Our program coordinates prevention, outreach, and management strategies for invasive plant issues through collaboration with land managers, agencies, organizations, and policy makers across Alaska. These efforts are guided by the implementation of our Strategic Plan and relevant noxious weed regulation and policy. Our goal is to help keep Alaska's pristine landscapes and natural resources free from impacts of noxious and invasive plants.

Canada thistle (Cirsium arvense)

AK Natural Heritage Program Invasiveness Ranking: 76

Identified as a prohibited noxious weed in Alaska (11 AAC 34.020), and in 34 other states.
The Alaska State Department of Natural Resources (DNR), Department of Agriculture classifies Canada thistle as a Top Priority invasive species for control, eradication and prevention.


  • Biology
  • Impact
  • Distribution
  • Taking Action
  • Resources

• Perennial
• Grows 1-4 feet tall
• Flowers are typically purple, but can be white or pink; bracts are tipped with prickles,
• Leaves are distinctively spiny, lance-shaped, wooly hairs on the underside and hairless on top
• Deep, wide-spreading roots
• The only thistle that is dioecious (separate male and female plants)


canada thistle collage

Canada thistle is not native to Alaska and it threatens natural plant communities by directly competing for resources and displacing native vegetation. Canada thistle plants produce allelopathic chemicals which can inhibit the growth of other plants in close proximity. It's dense, rhizomatous growth has the potential to impact natural and agricultural resources statewide.

Canada thistle:

• Displaces native vegetation through competition for nutrients
• Decreases agricultural production
• Destroys yards, gardens, and parks
• Degrades natural habitats
• Expensive to manage

Dense stand of C. thistle

Habitat: Fields, pastures, meadows, roadsides,  and disturbed areas. Common at low to middle elevations, mostly in agricultural areas.



To help prevent the spread of Canada thistle:

• Report any sightings to 1-877-INVASIV
• Organize neighborhood weed pulls
• Pass the word along about C. thistle to neighbors
• Use Weed Free Hay and Straw
• Wash your vehicle before re-entering the state
• Check pet fur for seeds before leaving an area
• Check potted plants for invasives growing in their pots before you purchase



1-877-INVASIV (468-2748)
or contact Dan Coleman
Invasive Weeds and Agricultural Pest Coordinator