Identification and Reproduction

Identification:

• This plant is a root-creeping herbaceous perennial that can grow up to 2 m tall.
• The main stem is hairy but spineless.
• Canada thistle leaves are lance-shaped with irregular lobes and sharp prickly margins.  

• Flowers can be white or purple, blooming in June to August.
• Seeds will look like white tufts and rante from 2-4mm long. 

Reproduction:

• Canada thistle spreads quickly through horizontal roots
• Each individual plant can produce 1000 to 1500 seeds. 
• Seeds can disperse through pets, wild animals, clothing, vehicles, and wind.

Habitat & Ecology

Mainly grows disturbed areas and can be found on forests, farmland, riparian, and roadsides. 

Impacts

Social: 

Canada thistly is a common weed in agricultural areas and is known to reduce crop yields and forage plants for grazers. 

Ecological: 

Outcompetes native vegetation and can establish dense stands. 

Management

Mechanical/Manual Control:

• Mowing is effective prior to flowering stages. 
• Consistent cutting of the plant will reduce plant growth and infestations.
• Hand pulling may be difficult as taproots are extensive and very deep. 
• Treatments over several years must take place to effect plant invasions. 

Biological Control:

Currently several weevils and a gall fly have been released to target Canada thistle. 

Resources

• Factsheet for Canada Thistle: Invasive Species Council of BC
• Video on Canada Thistle Identifcation: Univeristy of Wisconsin
• Native and Nonnative Thistle Identification: Clackamas Soil and Water Conservation District

Header photo (Matt Lavin). 

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