Invasive Plants and Agricultural Pest Management

Invasive species are typically recognized as non-native species that once introduced accidentally or on purpose, spread beyond control to affect natural and agricultural resources or human health. Not all non-native species are invasive, and many are highly beneficial for agricultural or ornamental purposes.

Canada Thistle

Cirsium arvense, also known as Canada thistle or creeping thistle, has been declared noxious by 35 states and is considered a prohibited noxious weed in Alaska (11 AAC 34.020). Once established, it is extremely difficult to remove and often requires years of control and monitoring. The Alaska State Department of Natural Resources (DNR), Division of Agriculture has identified C. thistle as a priority species for control, eradication and prevention where possible in Alaska focusing efforts in Anchorage. Identifying and managing C. thistle early is key to prevent infestations in natural areas and agricultural lands.


  • Biology
  • Impact
  • Distribution
  • Taking Action
  • Resources

C. thistle is a dioecious (separate male and female) plant with purple flowers or rarely white flowers. It is a perennial with deep roots that spread horizontally forming new shoots, growing between 1 and 4 feet tall. C. thistle reproduces by seed and vegetatively.  C. thistle produces abundant amounts of seed, with pappus for wind dispersal.  Seeds usually germinate in the first year but some may remain viable for up to 20 years. Vegetative spread is through creeping lateral roots and root fragments that break off from the parent plant. Occurring in a large range of habitats including croplands, ditch banks and riparian areas, gardens and pastures, this noxious perennial weed is particularly hard to control because of its reproductive durability.

Canada Thistle Spreads Easily By:

• Contaminated nursery stock
• Hiker's clothing
• Car and bike tires
• Animal fur
• Imported hay brought into the backcountry
• Wind dispersal

Identifying Characteristics:

Canada thistle forms dense stands of large, spiny-leaved plants with many flowerheads in clusters at the tips of branched stems. Plants begin as a rosette in the spring and grow upright stems that flower between June and August. The majority of flowers are pink or purple, but white flowers can also occur. All 3 flower colors occur in Alaska. After flowering, the seedheads are characterized by whitish-brown papus or tufts of hair.

• 1-4 foot tall, branching plant
• Purple or white flowers ½-¾ inch in diameter
• Lack have conspicuously spiny/thorny margins and soft/wooly hairs on the underside
• Leaves alternate along the stem and are moderately to deeply lobed
• Blooms June-August

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

Canada Thistle In Alaska:

Canada thistle is found throughout Canada and the northern half of the United States and has been found in many locations throughout Alaska. Most Canada thistle infestations are currently found in either Southeast or Southcentral Alaska with few infestations occurring north of the Anchorage area.  Because of the concentration of Canada thistle, Anchorage infestations have the potential to serve as a source for new populations in adjacent agricultural and wildlands such as the Mat-Su and Kenai Peninsula areas. 

In Anchorage, Canada thistle is fairly widespread, however most populations are a manageable size.  Efforts are focused on containing existing infestations, identifying new infestations, and working with the public to understand the issue and prevent further spread.


Canada Thistle Around The World:

In South Dakota, over 1.7 million acres were infested at a cost of $47.8 million in lost agriculture production.


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

For more information or to report a sighting contact:

1-877-INVASIV (2748) or contact Brianne Blackburn
Invasive Weeds and Agricultural Pest Coordinator

View the 2013 Canada thistle report

Download a Canada thistle fact-sheet from the US Forest Service to learn more about Canada thistle.

View our Canada thistle rack card by clicking on the image below:

CT rack card




1-877-INVASIV (468-2748) or contact:
Brianne Blackburn
Invasive Weeds and Agricultural Pest Coordinator


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