Ada County Weed Pest and Mosquito Abatement
Ada County's 5 Worst Weeds - Canada Thistle
According to Idaho Code 22-2407, Idaho property owners carry the primary burden of controlling noxious weeds on their land. County Weed Superintendents may only treat noxious weeds on private property if the landowner fails to fully mitigate the situation. In such instances, the County will treat the noxious weeds and bill the property owner for the weed control efforts.
New Herbicides Tame Canada Thistle!
Note: Before applying herbicide products, always read the posted labels and follow all application directions. Make sure you completely understand the instructions for use of a product in your specific application. Get a recommendation from a licensed consultant, before using products you are not familiar with.
Canada thistle is a noxious weed commonly referred to as a “creeping perennial” that uses its extensive root system to spread to new areas. Canada thistle chokes other plants by forming dense colonies and absorbing moisture nutrients and moisture from desirable vegetation planted nearby. Canada thistle is a problem noxious weed due to its long stems, fast growth, and roots that have been found as deep as 27 feet below the surface! The weed can be very difficult to control with mechanical cutting, hoeing, or through field tillage. Fortunately, some excellent herbicides have been registered for use against this noxious weed. Several products marketed by home and garden stores contain the following herbicides, and are packaged and labeled for use by the general public.
Chlopyralid, the active ingredient found in some specialty herbicides, is extremely effective against Canada thistle. Consult with a crop protection specialist to find the product that fits your specific situation. Products differ, and some might not be appropriate for the intended use of your property. Glyphosate, the active ingredient in Round-Up®, can also be effective but should ONLY be used where no desirable plants are growing, since it kills both broadleaves and grasses.
A group of herbicides called Sulfonyl-Ureas, sometimes mixed with 2,4-D, and/or related herbicides, provides excellent control of Canada thistle. Using two or more herbicides together, with two or more modes of application, often increases effectiveness and can avoid the growth of herbicide-resistant weeds. These combinations can usually be used in pastures or other situations where desirable grasses are growing. Again, consult a licensed crop consultant when creating a weed control plan for your property.
Effective control of Canada thistle requires the knowledgeable application of herbicides. Some products should be applied while the plants are still small, where others should be applied when Canada thistles are in the “bud” stage, or fully mature. Carefully read the herbicide label to get the most out of your weed control efforts.
A common problem affecting Canada thistle control comes from applying too much herbicide on the targeted plant. Spot spraying a systemic herbicide until it drips off the plant's leaves will likely kill the above-ground vegetation, but it won't soak into the root system. This leaves the root system alive, and the Canada thistle will grow back the next season.
For help controlling a weed control problem, or for clarification about using herbicides, feel free to call Ada County Noxious Weed Control at (208) 577-4646, or visit our office at 975 E. Pine in Meridian. Our office hours are from 7:30 a.m. to 5 p.m. Monday through Friday except holidays.
All photos this page: University of Wyoming, Ext. Service, Weeds of the West.