Canada Thistle

Canada Thistle

Cirsium arvense
IMG_1312-600x400
Status in Squamish:
STRATEGIC-CONTROL
Status in Whistler:
STRATEGIC-CONTROL
Status in Pemberton:
STRATEGIC-CONTROL
Vectors of spread:
Overview

Don’t be fooled by its name, Canada Thistle is invasive to British Columbia. Interestingly, it was one of the first weeds brought over to North America by early European settlers. Canada Thistle can be identified by its small purple flowers and prickly leaves

ID Characteristics

Flowers: Small purple-pink to white spineless flower heads (1 – 2 cm tall). Flower heads are borne on clusters of 1 – 5 at branch tips, and have a sweet vanilla scent. Flower bracts are spineless.

Stems: Mature plants are 0.3—2 m tall. Shoots begin as a rosette and grow upright stems after 2 – 4 weeks.

Leaves: Dull, glossy, prickly, dark green with white hairs underneath, narrow and alternate on the stem with wrinkled, deeply lobed, and spiny edges. Base leaves are stalkless and clasping, or extended down the stem.

Fruits: One-seeded, pale yellow (straw) or light brown in colour; straight or slightly curved. Seeds have a pappus (cluster of fluffy hairs).

Roots: Wide-spreading roots and creeping rhizomes

Similar Species
Native
  • Wavy Leaf Thistle (Cirsium undulatum) is sometimes mistaken for Canada Thistle, however it is usually smaller, and has more prickles covering the plant. Wavy Leaf Thistle also tends to have narrower flowers than Canada Thistle.
Cirsiumundulatum
Photo credit: JW Stockert, Wikipedia
Invasive
Bull_thistle003_RMueller
Photo credit: R Mueller
  • Bull Thistle (Cirsium vulgare): flowers are pinkish-dark purple, 4-5 cm wide and clustered at the ends of branches. The leaves are deeply lobed and have stout spines at the lobes and tips. The base of the leaves clasp the stem with distinctive spiny wings.
Scotch_thistle015_JLeekie
Photo credit: J Leekie
  • Scotch Thistle (Onopordum acanthium): has woody stems that may grow up to 3 m tall with spine-edged wings that run up the sides. It is identifiable mainly by its large bright violet to reddish flowers that are supported by large, spine-tipped bracts. The leaves have sharp, yellow spikes and covered in woolly hairs.
Milk_Thistle005_JLeekie
Photo credit: J Leekie
  • Milk Thistle (Silybum marianum) has distinctive marbling patterns on its leaves.
Exotic
  • Plumeless Thistle (Carduus acanthoides) has smaller flowers than Canada Thistle (about 2.5 cm in diameter). Canada Thistles leaves are much less spiny than those of Plumeless Thistle.
5374305-PPT
Photo credit: Joseph M. DiTomaso, University of California - Davis, Bugwood.org
Report

Please report any sighting of Canada Thistle by clicking here.

Habitat and Origin

Eastern Mediterranean in origin, Canada Thistle is not from Canada but was one of the first weeds brought to North America by early European settlers.

Canada Thistle is a versatile plant, capable of growing in a wide range of elevations and environments. It can be found in disturbed soil or where bare ground is exposed. Often seen in manmade plant communities (gardens, fields, roadsides), as well as in the margins of native plant communities (forests, meadows, wetlands).

Propagation and Vectors of Spread

Canada Thistle spreads by seed and vegetative methods. One plant produces 1000 -1500 seeds per flower shoot; the seeds germinate mid-spring or can lay dormant for up to 20 years. Roots can form new genetically-identical plants by creeping (via horizontal roots growing outwards from the main plant and sending up clones). New plants can also form from root fragments.

Canada Thistle can spread locally by vegetative reproduction, but long-distance dispersal occurs through the transport of seeds. The abundant pappus (clusters of hairs) helps wind carry seeds, but water, animals, and humans (via clothing, equipment, vehicles) may also spread the seeds.

Distribution

Economic and Ecological Impacts

Economic

  • Reduces crop yields due to dense colonies, crowding out forage fields and decreasing soil moisture and nutrients.

Ecological

  • Outcompetes native plants due to rapid spread and dense patches (up to 5.5m per season from a single plant).
  • Especially problematic in riparian areas.
  • While decaying, Canada Thistle may release toxic chemicals to inhibit the germination of other plant species.
  • Considered noxious under the Weed Control Act.
What Can I Do?

Canada Thistle is found in the Sea to Sky region and its distribution is beyond landscape-level control. When Canada Thistle is present at high-priority locations and negatively impacts them, their control is considered a high priority.

Otherwise, the goal is to prevent it from spreading to new (uninfested) areas, and to control it where possible to limit its impact on biodiversity.

 

Learn to identify Canada Thistle: Use the images on this profile page.

What to do if you spot it: You can report any Canada Thistle sighting by clicking here.

 

DO:
  • Regularly monitor properties for weed infestations.
  • Remove plant material from any equipment, vehicles or clothing used in infested areas and wash equipment and vehicles at designated cleaning sites before leaving infested areas.
  • Ensure soil and gravel are uncontaminated before transport.
  • Minimize soil disturbances (e.g., use grazing plants that prevent soil exposure from overgrazing), and use seed mixes with dense, early colonization (e.g., alfalfa or barley) to re-vegetate exposed soil and resist invasion.
  • Ensure invasive plant (particularly flowering heads or root fragments) are bagged or covered to prevent spread during transport to designated disposal sites (e.g., landfill).
DO NOT:
  • Plant Canada Thistle in a garden, no matter how well-contained its enclosure may seem.
  • Move soil that has been contaminated with Canada Thistle.
  • Unload, park, or store equipment or vehicles in infested areas; remove plant material from any equipment, vehicles, or clothing used in such areas and wash equipment and vehicles at designated cleaning sites before leaving infested areas.
  • Compost Canada Thistle!

 

Control

Mechanical:

  • Aggressive, long-term, repeated mowing can deplete the root reserves.
  • Plant material can be left on site to decompose, but in the flowering stage, all plant parts must be bagged and deeply buried at a landfill.
  • This is most effective when the plant is undergoing abiotic stress.

Chemical

  • Numerous herbicides are registered for control or suppression of Canada Thistle.
  • In pastures and idle areas, spring and autumn applications of clopyralid or a dicamba/2,4-D mix have been effective when roots are actively growing. Spring applications should coincide with the rosette to bud stages.
  • Spring applications of picloram have also been effective when Canada Thistle is in the pre-bud to early bud growth stages. However, picloram is not suitable for wet coastal soils.
  • We recommend that any herbicide application is carried out by a person holding a valid BC Pesticide Applicator Certificate. Before selecting and applying herbicides, you must review and follow herbicide labels and application rates; municipal, regional, provincial and federal laws and regulations; species-specific treatment recommendations, and site-specific goals and objectives.

Biological

Some biological agents are present in BC, including seed weevils (Larinus planus), stem gall flies (Urophora cardui), leaf-eating beetles (Altica carduorum), stem and root mining weevils (Hardoplontus litura), thistle-head weevils (Rhinocyllus conicus), and lettuce fly (Terellia ruficauda). Success of any of these agents is limited.

References

Coastal Invasive Species Committee, Canada Thistle, https://www.coastalisc.com/canada-thistle/

Government of BC, Invasive plants with biocontrol, https://www2.gov.bc.ca/gov/content/environment/plants-animals-ecosystems/invasive-species/management/plants/biological-control/invasive-plants-with-biocontrol

Government of Saskatchewan, Canada Thistle, https://www.saskatchewan.ca/business/agriculture-natural-resources-and-industry/agribusiness-farmers-and-ranchers/crops-and-irrigation/weeds/canada-thistle

Invasive Species Council of BC, Canada Thistle, https://bcinvasives.ca/invasives/canada-thistle/

Invasive Species Council of BC, Canada Thistle Factsheet, https://bcinvasives.ca/wp-content/uploads/2021/01/Canada-Thistle_Factsheet_26032019.pdf

Invasive Species Council of Metro Vancouver, Canada Thistle, https://iscmv.ca/invasives/detail/canada-thistle

Nature Conservancy Canada, Canada Thistle, https://www.natureconservancy.ca/en/what-we-do/resource-centre/invasive-species/canada-thistle.html

Okanagan Invasive Species Online, Canada Thistle, https://www.oiso.ca/species/canada-thistle/

Ontario Ministry of Agriculture, Food and Rural Affairs, Noxious Weeds Profile – Canada Thistle, http://www.omafra.gov.on.ca/english/crops/facts/info_canadathistle.htm

Oregon State University, Canada Thistle, https://horticulture.oregonstate.edu/weed/canada-thistle

Pacific Northwest Pest Management Handbooks, Canada Thistle Control, https://pnwhandbooks.org/weed/agronomic/corn/sweet-corn/canada-thistle-control

Washington State Noxious Weed Control Board, Canada Thistle, https://www.nwcb.wa.gov/weeds/canada-thistle