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Canada ThistlePhoto credit: Daniel Laubhann

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 that are designated as "noxious weeds" in the Alberta weed regulation. Noxious weeds must be controlled, meaning their growth or spread needs to be prevented.

Origin

Despite its common name, Canada thistle is native to Europe. It was most likely introduced to North America in the 1600s as a contaminant in seeds.

Legislated Because

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.

Description

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.

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.

It is probably the oldest regulated species in Alberta; the first legal document addressing weeds in our region was the 1865 Canada Thistle Act of Upper Canada.

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 Sheets - Alberta Invasive Species Council

For More Information

Weed Management

Telephone

In Edmonton: 311

Outside Edmonton: 780-442-5311

Email invasiveweeds@edmonton.ca

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