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
Cover young Canadian thistle plants with a large tarp.
Weigh the tarp down with rocks to keep the material in contact with the soil.
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
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.
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.
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.
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.