Weed Control

Canada Thistle

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Cirsium arvense
Creeping perennial

Canada thistle has small purple flowers covered in spiny thorns.
Plants are male or female (dioecious) and grow in circular patches that often are one clone and sex.
It is difficult to control because its extensive root system allows it to recover from control attempts, and the seed can remain viable in soil up to 20 years.
For most effective control, persistence is imperative so the weed is continually stressed, forcing it to exhaust root nutrient stores and eventually die.
Canada thistle is not tolerant of shade, thus the first step is to invigorate competition from the desirable forage species planted in the pasture.
Grasses and alfalfa can compete effectively with Canada thistle if their growth is favored by good management.
Mowing and/ or tillage can be an effective tool if combined with herbicide treatments.
Research at Colorado State University shows that Tordon 22K (picloram), Curtail (clopyralid plus 2,4-D), Transline (clopyralid), Banvel/Vanquish/Clarity (dicamba), 2,4-D and Telar (chlorsulfuron) are effective against Canada thistle when applied pre-bud in the spring and/ or in the fall. These herbicides are most effective when combined with cultural and/or mechanical control.

Prevention (Prev) Monitor and destroy new plants before seed production.

Mechanical (Mech) Hand pulling, digging, cutting, mowing and tilling.

Cultural (Cult) Biological control agents, livestock grazing, and revegetation practices.

Chemical (Chem) Selective herbicides based on the plant and the specific location. Check our weed fact sheets for specific control information.