Canada thistle is a persistent weed that has threatened the yields of farmers far longer than Canada has been a country. A member of the sunflower family, this perennial broadleaf weed is a strong competitor for nutrients, sunlight and water, and can cause greater crop losses than almost any other weed in western Canada.
Colonies of the weed form root buds that can spread up to five metres from the parent plant. These deep roots can reach up to 3 metres in length, making the weed incredibly hardy and drought-resistant. Its seeds spread by wind, and though they typically germinate within a year, they can lay dormant for years until they reach shallow soil. Control is difficult, and often requires a combination of cultural and chemical methods.
Plants are from 12 inches to 5 feet tall with several branches. Leaves are
a shiny, deep green colour and arranged alternately on the stem and branches. Some biotypes have thin, flat leaves with short, fine spines, arranged sparsely around a slightly toothed edge. Most plants have leaves that are thick and leathery with deep and irregular serrated margins and long sturdy spines. Stems have few spines, if any.
- Canada thistle is a persistent weed that requires a multi-year combination of chemical and cultural control practices. Root starvation is an important method for control. In addition, prevention of plant establishment and spread is essential.
- Use effective crop rotations with strong competitors for Canada thistle.
- Top growth can be suppressed in a number of crops by herbicides; however, roots and shoots can grow into new plants.
Please read each label to determine which herbicide is appropriate for the crop affected.
REFERENCES AND ADDITIONAL READING
“Canada Thistle.” Province of Manitoba Agriculture. Online. https://bit.ly/2BL73Zu
Raine, Michael. “Weed of the Week: Canada Thistle.” Western Producer. 2 July 2015. Online. https://bit.ly/2BHbl4j
“Tips for Controlling Canada Thistle and Avoiding Crop Losses.” ADAMA Canada. Online. https://bit.ly/2MtdF3L