Common Name: Canada thistle
Scientific Name: Cirsium arvense
Habitat: disturbed areas, roadsides, farmland, forests, open areas, wet grassland, dry grassland, ornamental beds
Provincial Designation: Noxious
Noxious weeds are plant species designated in the Alberta Weed Control Act. Noxious weeds must be controlled, meaning their growth or spread needs to be prevented.
Despite its common name, Canada thistle is native to Europe. It was most likely introduced to North America as a contaminant in seeds.
This highly competitive weed invades crop fields and grasslands where it lowers crop yields and forage productivity. Its ability to quickly establish on disturbed and open soil allows it to spread across the urban environment in green areas along roadways and in flower and vegetable beds. Canada thistle is one of the most problematic weed species. Roots readily withstand freezing, thawing and drying - root fragments may produce new plants with pieces as small as 3 mm. One plant can produce over 5,000 seeds.
Among thistle species, Canada thistle has a characteristically small flower head that grows to a maximum of 1-2 cm wide. The flowers are typically purple and occasionally white. There are two types of flowers - male and female. Male flowers slightly smaller and rounder than the larger, vase-shaped female flowers.
Although this species is quite prickly to the touch, the stem is actually smooth and has no spines or wings.
One reason for its “success” as a weed is its capability to reproduce by seeds and by its creeping roots (rhizomes). Plants can regenerate from very small root pieces in the soil.
Sheep and goats will readily graze Canada thistle and are not deterred by the spines. To find out more, check out Edmonton’s GoatWorks Project.
Can Be Confused with....
Native Thistles - Cirsium species - When in doubt, please contact firstname.lastname@example.org
Marsh Thistle - Cirsium palustre - but they’re both noxious weeds.
Did You Know?
Canada Thistle is the only species of today's Weed Control Act that was already regulated in the first Alberta weed regulation in 1907.
Learn more about this species:
See Fact Sheet - Alberta Invasive Species Council