Weed Profile: Canada thistle (Cirsium arvense)

June 24, 2020 Jen Toews , Plant Recorder

Summer is officially underway. The cast of spring-blooming flowers we so eagerly awaited has been replaced by a rowdy and colorful mid-season crowd, who are having their day in the sun. And, while the initial excitement of the gardening season has not faded for many of us, reality has set in: weeds have proliferated alongside our beloved landscaping plants. Perhaps we were out of town for a couple of days, shifted our focus to a project other than our yards or needed a respite from the soaring 90°F temperatures. Meanwhile, weeds jumped at the opportunity, marched up and down sidewalk cracks and fence lines, colonized the darkest and most clay-heavy corners of our yards, and scrambled brazenly up desirable plants with their tendrils. 

Some of these weeds are annuals or biennials that are easily addressed in an après-dinner weed-pulling session, accompanied with a beverage and snippets of conversations with friendly passersby. Pleasant enough. However, there’s that suite of more problematic weeds, perennials that reproduce vegetatively, creating clones of themselves until their radius expands. These are the weeds we pull repeatedly, only to become overwhelmed when they return with an exponential vengeance. One such weed is Canada thistle (Cirsium arvense). 

Identifying Canada Thistle
A colony-forming species, Canada thistle emerges in the spring, punctuating yards (and other disturbed sites) with rosettes of leaves. Leaves are spiny lobed and from 4 to 8 inches long, mostly smooth and green above and white-woolly below. Around the time spring transitions to summer, a flower head emerges with one to five small pink to purple (rarely white) flowers. Fun fact: This species is dioecious, meaning individual plants have either female or male flowers. At maturity, Canada thistle stands 3 to 5 feet tall. What you don’t see is most of the plant—an extensive creeping root system that penetrates the soil to depths of 6 to 15 feet. 

Since its arrival to North America several hundred years ago, Canada thistle has spread throughout much of Canada and the United States and is considered a noxious weed (list B) in Colorado. It aggressively competes with native plants, reduces crop productivity by vying for essential resources such as space, nutrients, light and water, is unpalatable to livestock and, if left to its own devices, can form monocultures in our yards. These are all compelling reasons to manage this species, but where to begin?  

Research suggests that a combination of approaches works best for managing Canada thistle. Below are several suggestions: 

Mow frequently or cut back the rosettes with clippers from when the plant emerges in the spring through summertime. This will prevent flower formation and will also stress the plant by reducing carbohydrate storage in roots. 

Glyphosate: From late summer up until the first hard freeze, spot treat by applying glyphosate to Canada thistle leaves using a paintbrush. The herbicide will be translocated to the underground parts of the plant, ultimately bringing about its demise. 

Plant regionally appropriate plants: Once your yard’s thistle population is under control, plant regionally appropriate plants including a diversity of natives and Plant Select® plants. These require fewer inputs, including less fertilizer and water. Where bare ground remains, apply mulch. Covering bare ground with plants and mulch will make your yard less prone to future weed infestations.  

Puccinia punctiformis, a rust fungus that is host-specific and pathogenic to Cirsium arvense, shows great promise in managing Canada thistle populations. This biocontrol is currently being evaluated in Colorado for appropriate application timing and best methods. To learn more or to order, visit the Colorado Department of Agriculture’s Request-A-Bug page.

Although Canada thistle is one of the more challenging weeds to manage, the good news is that with a multi-pronged approach over the course of several seasons, it is entirely possible to eradicate this weed from our yards. 

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