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Canada Thistle Control
submitted by Waukon NRCS
If we are going to have any Canada thistle control this spring you need to begin right now! According to Meaghan Anderson, a field agronomist in central Iowa and an extension field specialist at Iowa State University Extension and Outreach, “Canada thistle has been a consistent threat to crop production and perennial habitats since its introduction to the state in the late 1800s. Canada thistle is commonly found in crop fields, pastures, hayfields, CRP, and other full sun habitats. While fall is typically the best time to manage this weed species, late spring, when Canada thistle plants are in the bud or early bloom stage, is a close second for providing consistent herbicidal control. Now is time to be treating Canada thistle in pastures, hayfields, and other non-crop areas if you don’t want to wait for fall.”
Canada thistle is a perennial that spreads by lateral roots and seed. New plants commonly develop from lateral roots in early spring and infestations can spread quickly. Aboveground, Canada thistle plants usually grow two to three feet tall. It can be distinguished from common biennial thistles using several characteristics, depending on whether plants are in vegetative or reproductive stages. Bob Hartzler has several videos on identifying three biennial thistles (bull, musk, field thistle) and Canada thistle. Canada thistle flowers are vase-shaped and smaller than other thistles, about ½ inch in diameter. Flowers vary from purple to pink to pale lavender, almost white.
According to Anderson Herbicide treatments during the bud to flower stage using products containing clopyralid (herbicide group (HG) 4), aminopyralid (HG 4), or glyphosate (HG 9) can be effective at reducing population size. “Glyphosate should only be used for spot treatments. HG 4 products like clopyralid and aminopyralid are effective options for areas with desirable grass species but will affect most broadleaf species they contact.”
Anderson says that regular mowing, possibly six or more times per growing season for several years, may help control Canada thistle in pastures, hayfields, or non-crop areas that can tolerate it. She adds that management with fewer mowing events per season is less likely to control Canada thistle but may contain or slow spread of infestations.
Purdue University has a thorough document titled Control of Canada Thistle in CRP and Other Noncrop Acreage that describes mowing and other non-chemical strategies for managing Canada thistle. The six mowings is an option for CRP but any mowing within the nesting season of May 15 to August 1 requires permission from the Farm Service Agency. Requests must be in writing to the Allamakee FSA office and must identify the areas requiring mowing. Blanket mowing entire fields after establishment is not allowed.
“Canada thistle is perhaps the poster child for a weed that requires a long-term commitment for effective control,” says Anderson. “In areas with established populations, the aboveground growth represents only a small percentage of the actual plant mass. The majority of biomass will be below ground, thus requiring significant fortitude to eradicate the infestation. Follow-up efforts are required to control plants that survive earlier treatments and plants that continue to emerge from rootstocks.”
