Canada Thistle

This article by Laura Schwarz originally appeared in the March/April 2020 issue of Northern Gardener.

Invasive species removal is the most difficult challenge I encounter in my gardening work, and Canada thistle (Cirsium arvense) is my biggest adversary. Most northern gardeners are familiar with this prickly aggressor, but I’ll introduce you just in case you’re not.

Canada thistle is a perennial plant with spiny lobed leaves that grow in an alternating pattern along grooved stems. It reaches 5 feet in height at maturity, growing horizontally via rhizome. From June to September, the plant produces compact clusters of flowers at the end of each stem. Flowers are less than an inch long, and they are usually light purple, though they can also be pink or white. By mid-July, the earliest flowers have matured into fluffy white seedheads that are easily airborne. Rosettelike seedlings emerge in fall or the following spring.

patch of canada thistle in MinnesotaEach flowering shoot can produce up to 1,500 seeds, and these seeds can remain viable for more than 20 years. However, Canada thistle’s main staying power is in its root system. Both vertical and horizontal roots can spread several feet in a single growing season, and new shoots can emerge from almost any of the root segments. Attempting to till or dig an established thistle patch simply breaks roots into countless new plants. The tiny root segments can survive in the soil for up to 100 days without being replenished through photosynthesis.

Canada thistle grows in a wide variety of conditions and habitats. It commonly infests disturbed landscapes, arriving via seed and colonizing with both seed and rhizome. Roadsides, trails, pastures, new development sites and field margins are all common growth sites. An established thistle population will quickly outcompete other plants, displacing natives and diminishing a site’s vegetative diversity. Minnesota prairies, wetlands, savannas, forest glades and home gardens are all vulnerable to Canada thistle.

Prohibited Noxious Weed

Canada thistle is native to Europe, but it hitchhiked to North America as early as the 1600s. The species is well established throughout the northern United States, including most of Minnesota. Since 1872, Minnesota has legally qualified Canada thistle as a “prohibited noxious weed,” a plant designated as “injurious of public health, the environment, public roads, crops, livestock, or other property.”

The state has additional labels for its prohibited noxious weeds: “eradicate” or “control.” Canada thistle falls into the “control” category, which requires landowners to make efforts to stop the plant’s seeds from maturing and dispersing. Under this same law, transportation, propagation and sale of Canada thistle is not legal in Minnesota.

To control Canada thistle, you’ll need a management plan, and you’ll also need to commit to that plan for several years. A successful management plan is multi-faceted, limiting seed production while also weakening the plants’ extensive root systems. Most research supports a combination of mechanical and chemical control, with each approach carefully timed.

Spring and fall are the best times to apply herbicides to Canada thistle. In the spring, thistles use up much of their stored reserves to produce their initial flush of growth, and an herbicide application will force the plant to deplete its resources even further. In fall, thistle plants are actively storing sugars, so a systemic herbicide will move most effectively into their root systems.

There are several products that can be effective against thistles, including both selective and broad-spectrum herbicides—2,4-D, dicamba, glyphosate, clopyralid and metsulfuron are a few active ingredients that can, with repeated applications over one to three years, help control Canada thistle. Always read the pesticide label and follow it exactly.

Starting in late June, just before Canada thistle begins flowering, mowing or cutting can prevent seed production and dispersal. The plants will continue to send up new shoots, so you’ll have to keep up with them by mowing or cutting numerous times. If possible, allow your cut thistle stems to decompose on site rather than taking them somewhere else, where they might start a new infestation. If you must move them, take them to a disposal site that is qualified to accept noxious weeds. Minnesota law prohibits disposal of noxious weeds into solid waste trash bins.

As the thistles begin to weaken, densely replant the infested areas with desirable plants, including groundcovers that will shade and discourage germination of remaining thistle seeds. Continue to monitor regularly for thistles, and spot-spray if necessary, being careful not to drift herbicide on your new plantings. Again, it might take several years of careful management before the thistles retreat entirely.

Horticulturist and writer Laura Schwarz lives in Minneapolis, where she works in garden design, installation and maintenance.

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