By Marissa Schuh, IPM extension educator
As I’ve made some early season visits to farmers around the state, one weed has come up again and again, putting a damper on early season optimism - Canada thistle.
Canada thistle as rosettes early in the season. Photo: Howard F. Schwartz, Colorado State University, Bugwood.org.
Vegetable farmers are used to dealing with all kinds of tricky weeds, but the brute biology of Canada thistle makes it one of the most frustrating weeds to deal with. Canada thistle isn’t just perennial, but it has storage roots that help the plant regrow after mowing and spread into new areas. These roots are also able to produce new plants if broken up, for example by tillage.
How does Canada thistle get on the farm?
Canada thistle is common in ditches and field borders.
Each flower produces up to 45 seeds, which can be blown by the wind or even carried by water.
Canada thistle is a common weed in forage crops, so seeds can enter the farm in contaminated straw, including that used as mulches and as straw bedding for animals. This means that Canada thistle seeds can also be found in manure that hasn’t been properly composted.
How does Canada thistle stay on the farm?
The extensive root system stores lots of energy, allowing Canada thistle to regrow after mowing, flaming, or contact herbicide applications.
The roots can also become new plants, either as branching roots send up new shoots on their own or after being chopped up by tillage.
Chopped up roots over 0.1 inches in size are likely to become new plants. Smaller segments can become plants if they are close to the soil surface.
With this biology, management that includes both prevention and active management will be needed. In managing Canada thistle, there are two goals:
Preventing seeds from entering the farm system
Exhausting the storage roots that allow the weed to return year after year
This is not a one year or one tactic project, multiple strategies over a few years will be needed.
Preventing seed from entering the farm
Be thoughtful about straw mulches and compost entering the farm. Don’t be afraid to ask questions about handling and processing around this type of amendments.
Work to manage canadian thistle populations around field edges, as these can make their way into production areas either by seeds or roots.
Exhausting established plant Canada thistle colonies
Destruction of new shoots
The goal here is the removal of young shoots before they establish leaves and are able to generate their own energy. Remove new shoots before they have many leaves and are less than a foot tall. A key time to do this is in late spring, as the storage roots sending up new shoots but not yet getting significant energy from these shoots. Another time when storage roots are kind of depleted are when the plant produces flower buds, meaning this is another key time to take a chunk out of the above-ground growth. Studies have shown that weeding every 21 days is a good balance of preserving your own energy while still depleting the thistles.
Mowing doesn’t disturb the soil and can be used on thistle along field edges and in walkways.
Flaming can be used on very small Canada thistles, but the needed frequency can make flaming prohibitively expensive.
Contact herbicides exist for both conventional and organic growers, but are often broad-spectrum, so it will kill the aboveground parts of the thistle and plants you care about. Contact herbicides only affect the plant parts they touch (though conventional growers have some systemic herbicides that will be moved into the storage roots if applied correctly). The Midwest Vegetable Production Guide has the most up-to-date herbicides info.
Prevention of new shoots
Thick, light blocking mulches like landscape fabric can prevent new shoots from getting enough light to emerge. Mulches that still allow light through (for example, straw) won't do much to stop new shoots.
Solarizing and occultation for 4-6 weeks in the spring can clean up problem areas and allow for planting later in the season. This is not a one-and-done practice – the results in the covered areas are often dramatic, but remember how big the root system of this plant is.
Dense cover crops, like a solid, even stand of Sorghum Sudangrass or a mixture of sorghum sudangrass and another cover crop, can keep enough light from reaching new shoots of Canada thistle. Cover crops have to be carefully chosen and well-established, as gaps in cover crop coverage will allow Canada thistle to emerge and survive. The addition of mowing one or two times during the season can help deal with escapes.
For more information on Canada thistle and other weeds, I cannot recommend the recent SARE publication Manage Weeds On Your Farm enough. The entire thing is available as a PDF, including discrete chapters that go deep on the relevant biology of many weeds common on vegetable farm.
Sources and further reading
Battling Thistles Organically at Cornercopia Student Organic Farm
SUPPRESSION OF CANADA THISTLE WITH SUMMER ANNUAL COVER CROPS AND MOWING – YEAR 2. Abram Bicksler and John B. Masiunas, Graduate Research Assistant and Associate Professor, Department of Natural Resources and Environmental Sciences, University of Illinois, 1201 West Gregory Drive, Urbana, IL 61801.
Mohler, C. L., Teasdale, J. R., & DiTommaso, A. (2021). Manage weeds on your farm: A guide to ecological strategies. Sustainable Agriculture Research & Education (SARE).
Anderson, R. 2009. Managing Canada thistle in organic cropping systems. Small Farmer’s Journal 33: 79–82.
Graglia, E.B. Melander and R.K. Jensen. 2006. Mechanical and cultural strategies to control Cirsium arvense in organic arable cropping systems. Weed Research 46: 304–312.
Sciegienka, J.K., E.N. Keren and F.D. Menalled. 2011. Impact of root fragment dimension, weight, burial depth, and water regime on Cirsium arvense emergence and growth. Canadian Journal of Plant Science 91: 1027–1036.
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