Canada Thistle Control

Image of Invasive Species Control Canada Thisle Control Prevention

2020 Information

Some exciting nuggets from recent research:

  • There is no single best planting method for all situations: after 10 years, cover of planted, native non-planted, and exotic species varied little among the three planting methods (growing season broadcast, growing season drilled, dormant season broadcast) used in this study.
  • Patience can pay off: as the reconstructions matured, Canada thistle (Cirsium arvense) cover declined without herbicide application.
  • Exotic cool-season grasses may be the biggest threat to these reconstructions: none of the planting methods or seed mix richness levels slowed their increase. Going forward, timing and intensity of prescribed fire will be priorities to address this problem.

Using prairie restoration to curtail invasion of Canada thistle: the importance of limiting similarity and seed mix richness (pdf)

Persistence of native and exotic plants 10 years after prairie restoration (pdf)

Remember two important things:

  1. There is no substitute for diversity—ever. If a functional group is not filled, the number and severity of functional problems, including invasive species (or temporary problems like foxtail), in the restoration or remnant will be large. As long as you fulfilled the guilds (Cool season grass, warm-season grass, sedges/rushes, legume forb, and non-legume forb) you’re good.
  2. "A climax prairie, just like a climax forest, consists of thousands of different organisms, plants, animals, inverts, bacteria and soil fungi that rely on complex interactions of nutrient, moisture and energy flow to create and provide them with the food, water and shelter that they need to survive."—Chris Helzer. Megan Benage: “You wouldn’t expect to plant a tree seedling and have an old-growth forest the next year. The same is true for prairie reconstructions. They take time. LOTS of time. And patience.”                                                        (Megan Benage, Regional Ecologist MNDNR)

2018 Information

Conservation Easement Management: The window is short for spot spraying and it can be hampered by the large areas of dead vegetation that blocks ones view of the small plants at their most vulnerable point.  It would have been best to burn off the property corner to corner each year to help achieve control, but burning can really spur a resurgence of thistle growth. Spot Spraying mechanically looked like a solution but ended up to be a serious pain in the back. I used Milestone and felt that I was making progress, even though I would run into a time issue dictated by the budding and flowering of thistle.  Thistle normally is in full flower on July 4th.  Outbreaks would occur and I would eventually park my sprayer.  I then took to the field with a 1974 John Deere 4430 and a John Deere Model 27 Flail Shredder. Later, I purchased a Woods BT180 Series 3 Bat Wing Mower that would allow me not only to combat thistle but I could now mow down trees up to 2".  It clips down 4 ft+ vegetation down to 3 inches.  This was nearly effortless. The outside 15 feet of my service path that I clipped last year is for some reason free of thistle.  I did go selectively along edge with 6' ATV boom and sprayed Milestone on the thistle patches I saw. I mowed down a large patch on the inside edge of a berm and then followed up with broadcasting milestone.  No thistle survived this practice. Milestone really works great when applied in the fall on clipped plants.  The thistle plants get brown and do not re appear in the spring.   This technique works well on large patches. Mowing and Milestone treatments in dry conditions, during the time between bud and flower, also seem to work well. Trying to manage thistle can seems futile.  The key is to realize that the objective is to reduce population, not eliminate it. (Dan Ruby, Ruby's Wetland Bank, LLC)

2016 Information

Prevention:  Staying ahead of thistle can be difficult; the best plan of action involves having good timing, and using the combined methods of mechanical and chemical control to treat the plant before the flowers go to seed. Mature thistle plants can contain up to 10,000 seeds per plant, and has an aggressive root system that continually produces new shoots (Farm Bill Partnership Staff).

Clipping:  Mechanical treatment will be the most effective in areas where thistle is cohabitating with the desired flower species in a native prairie, as spraying will be harmful to the broadleaf forb varieties. For thistle intermixed with desired natives, clipping multiple times in the growing season to approximately 1’ with a mower attachment that does not create windrows, will allow for  the growth of desired natives, while also not allowing the quick growing thistles to go to seed (Farm Bill Partnership Staff).

Spraying: Areas that are dense with thistle and not with native flowers can be broadcast sprayed, as the grasses will not be affected by the broadleaf herbicide. Backpack or handheld spray works best for small areas. (Farm Bill Partnership Staff)

2013-2014 Information

Timing of Seeding:  If you have a Canada thistle problem (or other perennial noxious weeds), and want to establish wildflowers in your project area, don’t seed them at the beginning of the project. You may well wind up killing them all with herbicide to get the thistle under control. Seed only grass at project start, then invest 2-3 years in concentrated herbicide work to get the thistle under control. Once thistle is controlled, do a spring or fall burn and inter-seed forbs (Dennis Pederson, Habitat Forever).

2008-2012 Information

Competition:  For Canada thistle, if can be done, very aggressive, competitive plant communities that maintain cover will eventually relegate C thistle to fragmented patches or none at all as in the case of diverse, aggressive prairie communities or sod-bound brome on productive soils, or aggressive rotational grazing systems.  Sites with continual disturbance such as periodic flooding, traffic, low fertility, non-productive soils with perpetually open niches for invasion are problematic (Roger Becker, University of Minnesota Extension).

Herbicide:  If herbicides are needed my picks are: 1)Milestone (aminopyrchlor) 5 oz. followed by below labeled rate 3 oz the following year if needed to clean-up. 2) Transline/Stinger (clorpyralid) 10.67 oz. followed by below labeled rate 8 oz. the following year if needed to clean-up. -aminocyclopyrachlor (Perspective, Streamline from DuPont) is very effective but limited to localized spot treatments or to industrial areas, guard-rails, etc. - areas where vegetation impacts not that critical. -Tordon (picloram) same issues and less effective than aminocyclopyrachlor products.-2,4-D repeated over a few years will work, more non-target dicot damage than 1 and 2 above.  Tank mixes with products above work very well but relegated to grasslands, not mixed plant communities with dicots.-Garlon/triclopyr and Banvel/dicamba including Overdrive not that good on C thistle-Sulfonylureas like Glean/Telar (chorimuron-ethyl), Oust (sulfometuron-methyl) good at high enough rates but lots of nontarget issues, and Ally/Escort (metsulfuron-methyl) not that good (Roger Becker, University of Minnesota Extension).

Competition:  In plantings where primarily warm grass species were planted and early succession forbs are absent, thistles have no competition and often reach maturity before warm season grasses like big bluestem and Indian grass even start to green up. Having a strong Canada wild rye component as well as composite forbs, like black eyed Susan and Yellow coneflower seems to put pressure on thistle populations (Martin SWCD).

Spot Treatment:  Push Canada thistle control with spot spraying rather than whole field spraying when forbs are present (Redwood SWCD).

Outreach:  Renville SWCD has developed a landowner handout giving recommendations regarding chemical control for Canada thistle as they have nearly 400 RIM/PWP/CREP easements requiring management. The SWCD strongly recommends Milestone application in both spring and fall and has found that to be the only effective way to control thistles. The handout works for not only landowners, but is also given to other interested parties and to Township Boards as they try to administer the Noxious Weed Law (Renville SWCD).


Dan Shaw
Senior Ecologist/Vegetation Specialist