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Cirsium arvense--Canada Thistle
Aster Family

      About Canada Thistle: Unlike other thistles the male and female flowers appear on separate plants in the Canada thistle. (Male and female parts may appear together, but only one sex is fertile.)

      Honeybees are the main pollinators of Canada thistle. Individual plants produce an average of 1500 seeds, but there must be both male and female plants in the vicinity for successful pollination. About 90% of the seeds will germinate within one year, but other seeds can remain viable for about 20 years. Seeds can be blown a half mile in the wind.

      The seedlings require full sun for normal development. Growth is reduced if full sunlight is not available, and the seedlings die when shade reduces light intensity to 20%.

      Also unlike other thistles, the Canada thistle has a deep and wide-spreading root system. Dense Canada thistle patches are formed where a single male or female plant has spread by its roots. Individual roots only live for about two years, and older patches will occasionally die out and disappear. Small fragments of the roots can sprout new thistle plants, so the plant is especially troublesome in farm fields where it is spread by cultivation.

      Canada thistle is misnamed, since it was brought to this continent from Europe in the early 1600's. Due to competition with crop plants, weed control legislation for Canada thistle was passed as early as 1795 in Vermont and 1831 in New York. Canada thistle is a problem even in Europe where natural enemies abound.

      Canada thistle flourishes where over-grazing, cultivation or mulching has created open habitat for it. The plant does not do well in tight and healthy ecosystems.

      Edibility: Young Canada thistle leaves are edible and remarkably tasty. Individual leaves can be rolled up to smash the spines and eaten like that, or placed whole on sandwiches. Young thistles or thistle tops can also be boiled as a potherb. The stalks, while still flexible, can be peeled and eaten raw. The plants seem highly nutritious and might make a healthy green drink with the aid of a blender.

      Pam Sherman of Boulder, Colorado shared this recipe for Canada thistle with the Plants for a Future database: "I pick them in quantity when the plant is under 1 foot tall, then rinse them to remove dirt and bugs. I find that rinsing (under a stream of water) tends to disable the prickers to a noticeable extent. I cut the plant's stem with scissors via gloved hands, but rinse the leaves with bare hands. Then I cook them in water or stock (the latter is tastier) and the prickers are fully disabled. I then puree the leaves with a good quantity of milk (or milk substitute), season to taste with onion, garlic, salt, etc. and have a cream soup. I also put it in quiche or savory pancakes. I tell my friends I'm serving them something special, but don't say what until after they have tasted it and have exclaimed how good it is. You do NOT have to remove the prickers by hand from the raw plant, so it is not a fiddly process. It's a great green vegetable source."

      Fire: Late spring burns have been an effective tool to control Canada thistle (May-June), while early spring burns stimulate the plants to increase sprouting and reproduction. Late spring burns should be conducted three years in a row to be most effective. These studies were done in non-brittle prairies of Illinois. Repetitive burning in more brittle habitats could have serious negative consequences.

      Mechanical Controls: Hand-pulling or cutting Canada thistle can stimulate the plants to send up more sprouts from the roots, however repetitive treatments will reportedly starve underground stems, probably only where there is dense vegetation, like alfalfa, to compete with it. Mowing in pastures can be an effective control, if the thistles are cut at least once during the early bud stage. There must be dense vegetation present to compete with and shade the thistles. An alfalfa field in Montana mowed two times per year eliminated virtually all thistles after four years. Please note that cultivating chops the roots into little pieces that sprout new thistles. Cultivating is only successful if it is repeated every 10-15 days through the growing season for up to two years.

      Biological Controls: World-wide there are at least 84 species of insects that feed on Canada thistle, and about half of them feed inside various parts of the plants. Five European insects (Ceutorhynchus litura, Rhinocyllus conicus, Altica carduorum, Lema cyanella and Urophora cardui) have been introduced into this continent to help control Canada thistle. Only the first of those has become established and begun to suppress the plant. A type of fungus (Puccinia spp.) weakens the thistles enough to make them more susceptible to 2,4-D and more susceptible to other insects. Thistles treated with this fungus and a weevil (Ceutorhynchus litura) had a 50% increase in damage over untreated plants.

      Grazing: Research has shown that Canada thistle is equal or greater than alfalfa in nutrition, but livestock usually avoid the plant because of the spines. Sheep and goats eat the tender young thistles in the spring. Later in the year the plants can be treated with salt to encourage grazing and trampling. Even horses eat the thistles if they are cut first, apparently so they can graze without being poked in the nose. Grazing or trampling does not kill the plants, but it does reduce energy reserves in the roots and well-timed grazing can prevent seed production.

      Animal Impact: Focused animal impact may effectively trample concentrations of the thistle while stimulating more desirable plant species to compete. Controlled tests with livestock need to be conducted with this weed.

      Seeding: Canada thistle can be partly controlled with the aid of "smother crops" which develop earlier in the season, forming a dense cover to shade thistle seedlings. Smother crops are most effective if combined with regular cuttings, such as alfalfa, or forage grasses. Cutting Canada thistle on a regular basis depletes carbohydrate food reserves in the roots. Seeding intermediate wheat grass and tall fescue reduced Canada thistle density by 60 percent when combined with mowing. Sugar beets, buckwheat and small grains have also been planted as smother crops.

      Chemical Controls: For small infestations, spot applications of glufonsinate-ammonium (Finale®) or glyphosate (Roundup®) are effective means of control, especially early in the season. These are non-selective herbicides, so care should be taken to avoid hitting non-target plants. The herbicides can be dribbled on the plants for wiped on with a sponge, but be sure to wear rubber gloves and goggles.

      Large infestations of Canada thistle require selective herbicides to avoid killing desirable plant species. The herbicide 2,4-D amine is effective during the bud to flower stage when root carbohydrate reserves are low. Triclopyr and dicamba herbicides may be more effective, but also more persistent in the soil. These herbicides can migrate into the groundwater, especially in sandy soils.

      Spraying should ideally be done when there is plenty of moisture for growth, otherwise the herbicides may not translocate well through the plants. Also note that there are many varieties of Canada thistle, some with resistance to certain herbicides. If one herbicide fails to get results then consider trying another. Nitrogen fertilizer may be added to stimulate better translocation of the chemicals.

      Important: Most "weed problems" are really "people problems" from poor land management and a lack of ecological insight. It is easy to reach for a tool like fire, mowing, or herbicides to attack an out-of-control weed, but often those tools do little to get to the root cause of the weed infestation, and sometimes make the problems worse. Please read more about range ecology, desertification, and invasive weeds on this website before applying any tool of weed control Go to: Desertification and Invasive Weeds.

Notes

  • Thomas J. Elpel. Botany in a Day. HOPS Press: Pony, MT. January 2000.
  • ______. "Cirsium arvense." Plants for a Future: http://www.pfaf.org/database/plants.php?Cirsium+arvense. June 8, 2009.
  • ______. "Cirsium arvense." Species Abstracts of Highly Disruptive Exotic Plants at Effigy Mounds National Monument. Northern Prairie Wildlife Research Center: http://www.npwrc.usgs.gov/resource/othrdata/explant/cirsarve.htm
  • Erich Haber. "Canada Thistle Fact Sheet." Invasive Exotic Plants of Canada Fact Sheet No. 8: http://infoweb.magi.com/~ehaber/facthstl.html
  • Max Hutchinson. "Canada Thistle." Vegetation Management Guideline. Vol. 1, No. 7. Approved 02/06/90. Illinois Nature Preserves Commission. http://www.inhs.uiuc.edu/edu/VMG/cthistle.html

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