Vectors of Spread:
General: One of the first weeds brought over to North America by early European settlers, Canada Thistle can be identified by its small purple-pink flowers and its prickly leaves.
Flowers: Flowers are purple-pink or white with spineless flower heads that grow in clusters of 1 to 5 at the tops of the branches. They also posses a sweet, vanilla scent.
Stem: Mature plants can range from 0.3-2.0 meters in height and lack prickles on the main stem.
Leaves: Leaves are dull, prickly and dark green with white hairs underneath. Leaves are between 0.5-17 cm long, narrow and alternate on the stem with wrinkled, deeply lobed and spiny edges. The base leaves are stalk-less and clasping, or extended down to the stem.
Seeds: One-seeded, pale-yellow (straw) or light brown in colour; straight or slightly curved.
Canada Thistle has two lookalikes, which are also invasive.
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 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.
Habitat and Origin
Canada Thistle originates from the Eastern Mediterranean and was introduced to North America by early European settlers.
It is a versatile plant that can grow in a multitude of different environments and elevations. It can be found in disturbed soil or where bare ground is exposed. Canada Thistle is often seen in man-made plant communities, such as gardens, fields and roadsides, as well as at the margins of native plant communities like forests, meadows and wetlands.
Propagation and Vectors of Spread
Canada Thistle can reproduce both through seeds and vegetative methods. A single flower can produce between 1,000 to 1,500 seeds. They may either germinate in mid-spring or lay dormant for up to three years. The seeds of Canada Thistle are responsible for long-distance dispersal. The abundant pappus, clusters of hair that look like white tufts, aids the wind in carrying the seeds. Seeds are also spread by water, animals and humans via clothing, equipment and vehicles.
Vegetative reproduction takes place through the root system, where genetically identical plants form via horizontal roots that grow outwards, away from the main plant, and shoot up clones. New plants may also form from root fragments.
Economic and Ecological Impacts
- Out-competes native plants due to rapid spread and dense patches (up to 5.5m per season from a single plant).
- Problematic in riparian areas; may release toxic chemicals while decaying that inhibit germination of other plant species.
- Reduces crop yields due to dense colonies by crowding out forage yields and decreasing soil moisture and nutrient.
What Can I Do?
Since Canada Thistle is found throughout the Sea to Sky Corridor, the best approach to controlling the spread is by STRATEGIC CONTROL.
Learn to identify Canada Thistle: use the images presented in this profile page to learn how to identify Canada Thistle
What to do if you spot it: You can report any Canada Thistle sighting by clicking here.
- Regularly monitor properties for weed infestations.
- Ensure soil and gravel is free of Canada Thistle before transporting.
- Minimize disturbances that cause exposed soil (e.g. use grazing plans that prevent soil exposure from overgrazing), and use seed mixes with dense, early colonization (e.g. alfalfa or barley) to re-vegetate soils and resist invasion.
- Ensure plants (particularly flowering heads or root fragments) are bagged or covered to prevent spread during transport to designated disposal sites (e.g. landfill).
- 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.
- Plant Canada Thistle in a garden, no matter how well-contained its enclosure may seem.
- Move soil that has been contaminated with Canada Thistle.
- Regularly mow/till to reduce weed progression. Material can be left on site to decompose, but in post-flowering stage, all plant parts must be bagged and deeply buried at landfill.
- When tilling, be sure to prevent machinery from spreading root fragments to new sites.
- 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.
Some biological agents exist, including seed weevils (Larinus planus), stem gall flies (Urophora cardui), leaf-eating beetles (Altica carduorum), and stem and root mining weevils (Hardoplontus litura).
Government of Saskatchewan Ministry of Agriculture, 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 British Columbia, Canada Thistle, https://bcinvasives.ca/invasive-species/identify/invasive-plants/canada-thistle
Invasive Species Council of British Columbia, Canada Thistle TIPS Factsheet, https://bcinvasives.ca/documents/Canada_Thistle_TIPS_Final_08_06_2014.pdf
Okanagan and Similkameen Invasive Species Society, Weed of the Week Series, http://www.rdosmaps.bc.ca/min_bylaws/NewAndEvents/Press_Releases/2007/weed_of_the_week_thistle_August15.pdf