Canada thistle [Cirsium arvense (L.) Scop.]
Word Alert: allelopathy - The growth suppression of one plant species by another due to the release of toxic substances (Webster 1983)
by: Nicole Vincent and Charles Geddes
Canada thistle (Cirsium arvense) is an aggressive perennial weed classified as noxious in Alberta. According to the 2017 Alberta Weed Survey, Canada thistle is the ninth most abundant weed present in annual field crops after post-emergence herbicide application; occurring in 22% of surveyed fields. It came to Canada in the 1600's from Europe where it is called creeping thistle.
Plants are often 30 to 150 cm tall with several branches. The stems are hollow and woody with little to no spines. Most leaves are shiny dark green with deeply serrated edges and long spines, but some plants have thin flat leaves with soft spines. Flowers are urn-shaped and range in color from white to purple. Canada thistle can grow in a variety of areas, particularly in disturbed soils and overgrazed pastures, but doesn't tolerate waterlogged soils or complete shade.
Canada thistle often reproduces by sprouting clones from its extensive root system. Vertical roots are often 2 to 3 m deep. Horizontal roots are 15 to 30 cm beneath the soil surface and can grow up to 6 m per year. Individual plants live about 2 years, but typical infestations can produce 13 to 18 new shoots per square meter to replace old plants.
Seed production requires male and female plants to grow in the same area because Canada thistle is dioecious. Plant produces viable seeds 8 to 10 days after flower emergence with an average plant producing 1500 seeds. Wind and moving water can distribute seeds, but seeds often stay within a few meters of the parent plant. Ninety percent of seeds germinate within one year with remaining seeds staying dormant for up to 20 years creating a small, but persistent seed bank.
Growing a competitive crop prevents Canada thistle establishment and suppresses existing infestations because seeds require full sun to germinate, and seedlings are not shade tolerant. Alfalfa, sweet clover, tall fescue, switchgrass, and smooth brome can outcompete Canada thistle, especially when fertilizer is used to increase crop fitness. Late fall tilling can delay Canada thistle emergence up to one month to give the crop a competitive advantage. Use tilling cautiously, however, because new plants can develop from root fragments as small as 1.25 cm that are distributed through soil disturbance.
Once an infestation establishes, control must target belowground tissue because the root provides an abundant food reserve for regrowth and can negatively impact crop yield through allelopathy. Broadleaf herbicides, like 2,4-D and dicamba, are somewhat ineffective at in-crop rates because they only control aboveground tissue. For in-crop control, clopyralid followed by pre-harvest glyphosate can be very effective at controlling Canada thistle, giving about 85% control.
An integrated approach to weed management is recommended for Canada thistle. The Bud method targets Canada thistle when it is vulnerable and drawing reserves from the roots. This method recommends application of a systemic herbicide just before budding.
The Rosette method uses a lower rate of herbicide but requires tillage at the bud stage. Tillage weakens the root because reserves are utilized aboveground for flowering then a systemic herbicide is applied, when there is less than 15 hours of light per day and Canada thistle forms a rosette. At this stage the weed is transporting sugars to storage, so the herbicide will provide deep root kill. This method can be an important form of Canada thistle management implemented in the fall.
Leeson, J.Y., Hall, L.M., Neeser, C., Tidemann, B., and Harker, K.N. 2017. Alberta weed survey of annual crops. Weed Survey Series Publ. 19-1. Agriculture and Agri-Food Canada, Saskatoon, SK. 237 p.
Saskatchewan Ministry of Agriculture. 2008. Canada thistle and its control [Online]. Available: [20 Apr. 2020].