How to Get Rid of Canadian Thistle


Canadian thistle is a deep-rooted perennial. A single plant will give birth to thousands of seeds in late summer. The 2- to 4-foot high plants have roots that extend 10 feet deep. Cultivating the soil for control only propagates the Canadian thistle, as the broken root segments will generate a new plant. You still have a few options to control and get rid of Canadian thistle.

Blocking Out Light

Step 1

Cover young Canadian thistle plants with a large tarp.

Step 2

Weigh the tarp down with rocks to keep the material in contact with the soil.

Step 3

Leave the tarp in place for a full growing season. This action will kill the green thistles and starve the root system.

Using Weed Killer

Step 1

Apply the herbicide glyphosate to the plants in early spring before a flower head forms. Mix the herbicide according to label directions. Spray the chemical on each individual Canadian thistle plant.

Step 2

Inspect the area in early fall as the plant will attempt to emerge another green portion of the plant before winter. Apply another round of the herbicide.

Step 3

Check the area the following spring for any plants you may have missed and re-spray the herbicide. It may take several seasons for complete eradication of the Canadian thistle.

Tips and Warnings

  • Keep people and animals from any area treated with a chemical.

Things You'll Need

  • Large tarp
  • Rocks
  • Glyphosate
  • Sprayer


  • North Dakota Extension Service: Questions on Thistle
  • Montana State University: Canada Thistle in Lawns
  • Purdue University: Control Practices for Canada Thistle
Keywords: poison Canadian thistle, starve Canadian thistle, eradicate thistle plants, kill Canadian thistle

About this Author

G. K. Bayne is a freelance writer, currently writing for Demand Studios where her expertise in back-to-basics, computers and electrical equipment are the basis of her body of work. Bayne began her writing career in 1975 and has written for Demand since 2007.