Canada Thistle

Canada Thistle (Cirsium arvense) is a creeping perennial that can spread by either seed or rhizomes (underground, horizontal stems). Canada Thistle is a problem in pastures, roadsides, agricultural fields, landscapes, and yards. It is an aggressive weed, often forming large “patches” which can be difficult to control. Canada Thistle problem can result in increased competition with desirable plants, unsightliness, and nuisance problems associated with spiny vegetation.

Seedlings begin as a low growing rosette. This rosette will eventually bolt a long stem as it becomes time for flowering and seed set. Bud stage will precede flowering and is evident by purplish buds formed at the tips of the stems. These buds will then bloom to form lavender colored flowers. Flowers then forms plumed seeds that are easily carried by wind. Seed can be carried by wind for long distances and can remain viable in the ground for long periods of time.

Rhizomes can produce numerous new seedlings along their lengths, making control especially important, as there is potential for even greater spread if the initial problem is not addressed. Rhizomes from one plant can spread horizontally as much as 20 feet in one year and roots can go as deep as 20 feet.

Control can be difficult due to the fact that Canada thistle spreads by both seeds and rhizomes. Goals of control should be to prevent the production of new seed as well as deplete root reserves or kill roots outright.

Several tactics may be used to eliminate Canada thistle. Mowing can be an effective method of controlling thistle, but only if it is done persistently. If mowing is done several times, it will help to deplete underground root and rhizome reserves as the plants will have to use up their stored energy to re-grow. 1 mowing will not solve the problem, as it will not damage the roots and rhizomes enough to prevent re-growth.

Tillage can also be used in non-crop areas or to control small patches. However, like mowing, tillage must be done persistently in order to deplete root reserves. 1 tillage treatment will not be adequate, and in many cases may perpetuate the problem by spreading rhizome pieces to non-infested areas where they can grow and start new patches.

Herbicides can be greatly effective and can be used either by themselves or in conjunction with mowing or tillage.

Recommended herbicides for Canada thistle include Glyphosate (Roundup), and 2,4-D (sold under several names and commonly in a mixture with another active ingredient, for example: Weedmaster is 2,4-D in combination with Dicamba). Remember when choosing herbicide that Glyphosate is non-selective and will kill all plants, including grasses.

When applying Glyphosate or 2,4-D as the sole control method, the most effective time for treatment is during the bud stage prior to flowering. Plants may also be somewhat suppressed if herbicides are applied at any time during active growth prior to seed set. If using tillage or mowing in conjunction with herbicide, conduct mowing or tillage 1st, then apply herbicide when plants have re-grown 4-8 inches.

Note: These recommendations are intended as a general guide. They are not meant to take the place of label directions and are not applicable to all herbicides. Always read label carefully before using any herbicide. Follow label directions exactly when applying herbicides. Never apply herbicides during rain, high winds, while ground is frozen, to mature plants (herbicide will be ineffective) or in any way contrary to label directions. Following label directions will help to prevent groundwater contamination, surface water degradation, drift from the target site, health hazards, and waste of materials.

With any treatment method, additional treatments may be necessary as seed may remain viable in soil for long periods of time and may not sprout right away. Also, underground root and rhizome reserves must be completely depleted in order to prevent re-growth. Thus, there is no “quick fix” to an extensive Canada Thistle infestation.

It is important to prevent new infestations from occurring. Make sure that any pasture or crop seed you use is not contaminated. Make sure that any equipment that may have been used in an infested area is thoroughly cleaned before using it in a non-infested area. Keep pastures in good condition. Do not allow pastures to become over-grazed, as this will reduce the competition provided by grasses and allow weeds such as thistles to take over. By following these simple guidelines, new infestations may be prevented.