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Canada Thistle (Cirsium arvense)
Aster Family / Thistle Subfamily

Cirsium arvense: Canada Thistle

      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.

      Thistle seed are a favorite with several seed-eating birds, like goldfinches.

      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, as noted in Foraging the Mountain West. 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.

Eating Canada thistle leaves on a sandwich.       Using the greens, stem, root, and flowers, Katrina Blair of Durango, Colorado serves raw thistle-based food and drink at her wild food cafe, farmers market stand, and wild food csa: green drink lemonade, Mexican stem snacks, flower heads, root kraut, root chai, tea, and liqueur. [Retired USDA Ethnobotanist Jim Duke likes the cooked young leaves and stalks. Others like just-emerged, few-day-old baby thistles in stir fries, quiches, and anywhere else spinach might be used. When the very young thistles are well cooked (or strained out, as in a green drink), the spines are not an issue for most people and do not need to be removed. Sensitive individuals, as everyone, should take all necessary precautions when trying a new food.

      Medicine: Delaware Indians used Canada thistle for cancer and hemorrhagic hemorrhoids. Navaho used it to induce vomiting. Ojibwa used it for stomach cramps. Canada thistle has been used to coagulate milk. Mohegan Indians used it in a mouthwash for infants and in a tuberculosis remedy for adults. Montagnais also use it for tuberculosis. Ojibwa used it as a bowel tonic. A number of different thistle species are listed in Hartwell' s Plants Used Against Cancer, for cancer of the breast and nose, edematous tumors, and scirrhus.

      Other Uses: Thistle down can be used as tinder to start camp or woodstove fires in woodstove. Leaves and stalk, but not heads and roots, can be put in the compost, as it is a perennial and spreads by seed and root.       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. Clip the above-ground portions before seed set. The idea is to force it to use up its root energy reserves while not reproducing by seed either.

      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. Note that cultivating chops the roots into little pieces that sprout new thistles. Cultivating is only successful if it is repeated every 10 to15 days through the growing season for up to two years.

      According to Heather Roads from the website www.gardeningknowhow.com, "Controlling Canada thistle organically is done with a sharp eye and an even sharper pair of scissors. Find the base of the Canada thistle plant and simply snip it off at the base. Do not pull Canada thistle out, as this can split the root, which causes two Canada thistles to grow back. Check the location weekly and snip off any new growth that you may see. The idea is to force the weed to use up its energy reserves by regrowing but removing the new leaves before the Canada thistle has a chance to build its energy reserves back up."

      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. Several insects have been introduced into this continent to help control thistles, including the thistle defoliating beetle, Cassida rubiginosa, the thistle-stem gall fly, Urophora cardui, a stem-mining weevil, Ceutorhynchus litura, the thistle-head weevil, Rhinocyllus conicus and the thistle rosette weevil, Trichosirocalus horridus.

      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.

      The National Sustainable Agriculture Information Service (ATTRA), funded mostly by a cooperative agreement with the USDA, notes that the biocontrols listed are often slow to get established and would not be of much help in freeing fields of thistle in the first year. On the other hand, once they are established, and if they are not killed off by pesticides, they can provide long-term biological control of thistle...In general it seems apparent that some thistle plants will have to remain in order for these beneficial insects to retain their populations.

      Grazing: Research has shown that Canada thistle is equal or greater than alfalfa in nutrition. Livestock usually avoid the plant because of the spines, but they often eat the flowers. Sheep and goats eat the tender young thistles in the spring. Later in the year the plants can be treated with salt or molasses to encourage grazing and trampling. Kathy Voth has a small business teaching cows all over the West to eat weeds, including Canada thistle. 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. A dense stand of preferred plant seeded or transplanted and supported with good fertility soon after thistle is controlled...goes a long way toward good thistle control. 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.

      Vinegar: U.S. Department of Agriculture Researchers in Maryland tested various strengths of vinegar on Canada thistle. They found that either a 5% or 10% solution of vinegar burned off the top growth of Canada thistle. The plants, however regrew from the roots. Acetic acid in vinegar kills plant tissue by dissolving the cell membrane, which causes the plant to dry out. Vinegar works best when used in the sun. A word of caution, however: vinegar in concentrations greater than 5% acetic acid may be hazardous-burning the skin or damaging the eyes-and should be handled with care.

      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.

      According to ATTRA, The National Sustainable Agriculture Information Service, spraying thistles with herbicides kills the adult plants but doesn't stop seeds in the soil from sprouting and growing new plants. Biological controls (mentioned above) are slow to become established, but can provide long-term control to keep thistle populations in check, provided some plants remain as hosts for the beneficial insects.

      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

  • ______. "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
  • Blair, Katrina. The Wild Wisdom of Weeds Chelsea Green Publishing: White River Junction, VT. 2014.
  • Duke, James. Handbook of Edible Weeds. CRC Press: Boca Raton. l992.
  • Elpel, Thomas J. Botany in a Day. HOPS Press: Pony, MT. January 2000.
  • Haber, Erich. "Canada Thistle Fact Sheet." Invasive Exotic Plants of Canada Fact Sheet No. 8: http://infoweb.magi.com/~ehaber/facthstl.html
  • Hutchinson, Max. "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
  • Moerman, Daniel.Native American Ethnobotany. Timber Press: Portland, OR. 2008.
  • Sullivan, Preston. "Thistle Control Alternatives." ATTRA Publication CT156. August 2004. http://www.agrisk.umn.edu/cache/ARL02967.htm.
  • Rhoades, Heather. "Controlling Canada Thistle - Canada Thistle Identification and Control." Gardening Know How. http://www.gardeningknowhow.com/plant-problems/weeds/canada-thistle-control.htm

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