canada thistle

Canada thistle. Photo: Caleb Slemmons, National Ecological Observatory Network,
Bugwood.org

Updated: June 28, 2024

About Canada thistle

Canada thistle (Cirsium arvense) is an invasive broadleaf weed originating from southeastern Europe and Asia. It is now widespread in the northern and western United States where it is a significant threat to agricultural systems and natural ecosystems. It is listed as one of the noxious weeds in Maryland. Its deep, creeping root system makes it difficult to control.

Growth habit

Identification

  • Broad-leaf perennial with spreading habit; grows to 2 - 5 feet high.
  • Leaves are long and narrow, alternate, irregularly lobed with sharp bristly hairs on the edges; spineless stems.
  • Pink-purple disk flowers bloom in clusters at the ends of stems from May to August. Unlike bull thistle and musk thistle, Canada thistle flowers generally do not have spines or prickles.

Reproduction

  • Spreads by seeds and rhizomes. 
  • Each shoot can produce 1,000-1,500 seeds. White fluffy material (“pappus”) aids in the dispersal of the seeds, which can remain viable in the soil for up to 20 years.
  • The roots can go up to 36 inches deep into the soil, spreading vertically and horizontally and forming distinct patches or colonies.

Flowers

Canada thistle flowers. Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Flowers and Seed Development

Seed development on Canada thistle. The white material (pappus) aids in wind dispersal of the seeds. Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Stem and Leaves

Canada thistle stems are usually smooth but can be grooved or slightly hairy. Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Seedlings

Young seedlings of Canada thistle (Cirsium arvense). Photo: Robert Vidéki, Doronicum Kft., Bugwood.org

Management

An established Canada thistle population may take several years to control. Focus on preventing seed dispersal and depleting the root system.

Physical control

  • Pull or dig out young plants as soon as they are observed. Hand pulling is only possible with the youngest seedlings in moist, pliable soil. 
  • Mow or cut down the top growth of plants to limit photosynthesis, “starve” the root system of its stored energy, and prevent seed development. Late spring, before flower bud formation, is the optimal time to cut plants down since they have expended stored energy to produce the first shoots. Continue to cut down new growth every 2-3 weeks.
  • Maintain dense groundcovers or turf; avoid excessive nitrogen fertilization.

Chemical control

  • The extensive root system makes this plant difficult to control with an herbicide, and repeat applications are likely to be needed.
  • Use a systemic herbicide (glyphosate) to spot-treat foliage/stems in late summer or early autumn to get the most effective results. This is the time of year when sugars and nutrients are moving down into the roots for winter energy storage. The herbicide will move into the roots with this process.
  • Read and follow product label instructions when selecting and using an herbicide.

Additional resources

(PDF) Canada Thistle Fact Sheet | Maryland Department of Agriculture & University of Maryland Extension

Canada Thistle | iNaturalist 

Plant Invaders of Mid-Atlantic Natural Areas, Field Guide

By Christa Carignan, Coordinator, University of Maryland Extension Home and Garden Information Center, June 2024

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