Description: Canada thistle is a perennial forb introduced into North America from south-eastern Europe or the eastern Mediterranean and is the most widely distributed thistle in North America (Morishita 1999). This plant has an extensive root system spreading laterally and quickly, up to 6 metres in a single growing season, and penetrating to 6.75 metres deep in the soil (Morishita 1999). Canada thistle may grow to 2.0 m tall and can form dense monocultures.
Habitat & Impacts: Dense infestations of Canada thistle may exclude most or all native species from heavily infested sites. Commonly occurs adjacent to riparian areas and after logging. The spiny leaves and stems make Canada thistle unpalatable to most livestock and limits recreational use of infested areas. Canada thistle can dramatically reduce forage production and use of pastures by livestock.
Method of Spread: A single plant can produce up to 6400 seeds per flower stalk (Morishita 1999) but the main method of spread is by roots. Seeds are spread by wind and water and in crop seed, hay or livestock manure. Canada thistle seeds may remain viable in the soil for over 20 years.
Location: Canada thistle is widespread in the Cariboo Chilcotin Coast. The number of sites and the distribution of this species have been increasing steadily over the past several decades. This species is likely more common than available data indicates.
Mechanical Control: Cattle and possibly other livestock may be trained to eat Canada thistle and other weeds (http://www.livestockforlandscapes.com/ ). Canada thistle can be controlled mechanically if plants are pulled or cut when young. Large or long-established infestations cannot be effectively controlled mechanically. Repeated cutting has been cited as an effective method for controlling Canada thistle, and prevents flower production and perhaps lateral spread of the plant but is not effective at eliminating the plant. Plants will re-sprout once mowing is ceased. The extensive creeping root system allows Canada thistle to survive cutting and cultivation and to grow on a wide range of sites.
Chemical Control: Chemical control of Canada thistle is possible, but repeated applications are required. A range of chemicals can be used to treat burdock including 2,4-D and Glyphosate. Chemical control is limited due to riparian values.
Biological control: A number of biocontrol agents have been released in British Columbia but to date none have proved effective at controlling Canada thistle in the region. They are not yet established due to the colder climate. Agents released here are seed weevils Larinus planus and Rhynocillus conuicus. As biocontrol agents become available, these should be released and monitored for effectiveness.
CCCIPC Priority & Treatment Strategy: Priority 3 (established)
Local Level – chemical or repeated mowing can prevent spread and seed production, however this must be diligently repeated before seed production due to intensive root spread.
Landscape Level – no viable option until biocontrol agents are established. Localized chemical control for private land.
John M. Randall, The Nature Conservancy, Bugwood.org
Canada Thistle flower head
Steve Dewey, Utah State University, Bugwood.org
Canada Thistle Infestation
Canada Thistle mature plant
Alec McClay, McClay Ecoscience, Bugwood.org
Larinus planus, Canada Thistle bud weevil.
Invasive species profile taken from the Cariboo Chilcotin Coast Invasive Plant Committee Invasive Plant Regional Strategic Plan
Page last modified: March 15, 2017 09:36:29 PDT