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IVM Technical Bulletin
Canada Thistle
Gather Background Information

The first step in an IVM program is to gather information on the life cycle and habits of the noxious weed.

Canada thistle, Cirsium arvense, is a perennial plant with horizontal, branching roots and usually occurs as a clump of stems. Seedlings can survive only on cultivated or disturbed ground where there is little competition and plenty of light (Hodgson 1968a).

A mature plant measures from 2 to 4 feet tall. Stems are usually green but can be brownish to reddish purple and they branch toward the top. The leaves are usually dark green, deeply lobed, ruffled, and spiny. Occasionally plants without these leaf characteristics can be found.

The flowers are borne at the end of the stem branches and are 3/4 inch or less in diameter. Each plant normally produces either all male or all female flowers, which means that not all plants produce seed. The flowers are mostly purple to rose but occasionally white. The seeds are 1/16 to 1/8 inch long and each is attached to a small tuft of hairs. When mature, the seeds are brownish.

Canada thistle reduces crop yields by competing for light, moisture, and nutrients. In pastures and on rangeland Canada thistle reduces forage yields, and its prickly leaves irritate the mouths of most grazing animals. If small buds are harvested with canning crops, special precautions must be taken at the cannery to remove them. Canada thistle can also be an annoying weed in landscaped areas and backyards.

In North America, Canada thistle is distributed widely over the northern half of the United States and in Canada. It grows best where summer temperatures are moderate and rainfall is between 15 and 30 inches per year (Moore 1975; Hodgson 1968b).

Look for Canada thistle in disturbed areas such as roadsides, waste areas, cultivated fields, pastureland, rangeland, riparian areas, and even in lawns and gardens.

Life Cycle
In the spring when soil and air temperatures have warmed sufficiently, seeds sprout and roots send up new shoots. Both seedlings and new shoots begin as a rosette of leaves. Plants begin to flower when the day length is 14 hours or more, around mid-June to September (Haderlie et al. 1991).

Flowers are insect-pollinated, chiefly by honeybees, and seeds are produced in the summer. Some of these seeds germinate immediately, overwinter as rosettes, and flower the following season, but most seeds germinate the following spring (Moore 1975).

After about July, plants no longer send up flower stalks and begin the process of sending energy down to the root system to prepare for the winter. In areas with hard winter frosts, the above-ground portions of the plant die back, although in some places the plants can overwinter as a rosette of leaves under the snow. The roots remain viable, even when they are fractured by freezing and thawing, and send up new shoots in the spring.

Canada thistle mainly occurs in clumps, and as the patch ages, the center declines and the more vigorous plants are found nearer the edges. It spreads mainly by its horizontal, branching roots. People also spread Canada thistle in contaminated crop seed and forage and on vehicles and equipment (Hodgson 1968a).

Special Challenges to Management
The roots of Canada thistle act as a huge underground food storage system for the plant and can produce new shoots for about 1 1/2 growing seasons despite repeated mowing or cultivation, which prevents new food from reaching the roots (Hodgson 1968a).

The roots can withstand adverse conditions (such as freezing, thawing and drying) and can generate new plants from fragments only 1/8 inch long. Because Canada thistle roots are very brittle, freezing and thawing can break up the root system, which then hinders translocating herbicides from reaching all the roots. The roots continue to grow each year and can produce vigorous new shoots that become mature plants in 7 to 8 weeks (Hayden 1934; Donald 1996; Hodgson 1968b).

Stem fragments that are partially or completely buried (as might happen after tillage) also have the potential to survive and produce roots that can overwinter and produce new shoots in the spring (Magnusson et al. 1987).

Because new seedlings are quite sensitive to shading and competition, they become established in ploughed or disturbed soil where there is a substantial amount of light and, for a time, little competition from other plants. Within 3 weeks after germination, seedlings become perennial, meaning that their roots produce buds from which new plants can sprout. Seeds germinate best at shallow depths and do not accumulate in the soil to form a persistent seed bank (Haderlie et al. 1991).

Site-Specific Questions
Some questions, such as those below, can only be answered on site.

  • Who are the people and the agencies that are concerned about this weed?
  • What is the natural history of the site you are trying to manage (the soil type, amount of rainfall, species of animals and competitive vegetation present)?
  • How is the land being used (present and future plans) and what is the history of land use?
  • What is the history of weed control?
  • Is this a recent invasion or an old problem?
  • How did the weed get there and by what means is it being dispersed?
  • Set Management Objectives

    Set Realistic Goals for Your IVM Program
    The answers to the following questions can help you set realistic objectives and goals.

  • What are the requirements of the law?
  • What resources (money, people, time) are available?
  • Who are the people or agencies I can collaborate with?
  • Which control strategies (see Apply Management Methods) are available and best suit the weed and location?
  • What environmental considerations will affect my program?
  • What is the desired level of control (see below)? Can my resources sustain this level?
  • Levels of Control
    Containment - keeping an established population of the weed from spreading to non-infested areas. This strategy is especially useful when time and money are in short supply or when the infestation is very large. For example, a barrier strip between infested and non-infested areas can be maintained and monitored so that adjacent lands remain weed free.

    Reduction - reducing the area covered by Canada thistle, or reducing its dominance. This strategy can also be used against new or established weeds, but it requires more resources and more time than containment.

    The "Bradley Method" (see Appendix 2), developed in Australia, is a simple yet innovative strategy for natural areas that combines containment and reduction.

    Eradication - completely eliminating the weed from the management area. This strategy usually consumes the greatest amount of time and resources and is applicable mainly to newly-invading weeds that are confined to a limited number of small areas.

    Establish Monitoring Programs

    When planning a monitoring program, keep in mind the context of your target weed: is it invading or has it already invaded?

    Locate and record Canada thistle infestations on a map. (Chapter 2 of the University of Northern Iowa IVRM Technical Manual contains a detailed discussion on how to map and inventory vegetation - see Bibliography). Note particularly sensitive areas on the map, such as critical habitat for threatened or endangered species, agricultural production areas, or areas subject to frequent disturbance and thus prone to invasion. Update maps at regular intervals.

    Focus monitoring efforts on sites where Canada thistle problems are most likely to occur (see Distribution). Encourage public sighting and reporting through an education or incentive program (see Educate Vegetation Management Personnel and the Public).

    Prioritize the sites you will work on. Make a realistic assessment of your weed management resources, keeping in mind the goals of your project and the cost of a follow-up program after any treatments. Without follow-up, your control efforts will be wasted. It is better to thoroughly control a weed at one or two sites than to use up resources to incompletely control the weed at many sites. If the weed is very widespread, try to determine where it poses the most serious economic, social, or environmental problem and concentrate on those areas.

    Plan monitoring and treatment efforts to coincide with critical life stages of the weed. To use your resources efficiently, try to include monitoring with other planned activities in the area.

    Maintain records of your monitoring activities. Creating standardized forms will make data collection easier and help remind you to gather all the information you need. Forms work best if they include labeled blanks for all pertinent information and allow the user to check or circle rather than having to write words or numbers (See Appendix 3 for some examples of forms).

    Include information such as the name(s) of the person(s) collecting the data, the location, and date of monitoring; a qualitative description of the vegetation, such as the names of the plants or types of plants (native vegetation, annual/perennial weeds, trees, etc.) and stage of growth (germinating, flowering, setting seed, etc.); a quantitative description, such as percent cover, plant density, size of the patch, or if possible, the number of plants.

    Note special conditions such as unusual weather events and record treatment history, including information on treatment applications (who, when, where, how, cost, difficulties, and successes). This will allow you to evaluate and fine-tune treatments.

    Set Treatment Thresholds

    Setting treatment thresholds includes prioritizing and balancing treatments with resources. Weeds are treated when populations increase beyond a pre-determined level. This level will largely depend on the characteristics of the site and weed. In some cases the level may be no weeds at all, and in other cases the number of weeds you can tolerate may be much greater.

    Considerations for Setting Priorities
    What is the size of the weed population? The opportunity for control is related to the infested area. Small patches can be more easily controlled than large infestations.

    What is the level of the threat? Is the Canada thistle population changing? Is it in an area where soils are frequently disturbed? Does it threaten agriculture, pastures, or rangeland? Is it encroaching on critical habitat for a rare, threatened, or endangered species? Is it displacing the best examples of native communities?

    What resources are available? Do you have the resources required for carrying out your goal?


    With the advent of herbicides, prevention as a weed management technique has often been neglected; however, it is a practical, cost-effective, and extremely important part of noxious weed control.

    General Weed Prevention Measures

    Revegetation - Follow-up Weed Prevention

    Establishing dense, competitive vegetation can help permanently replace weeds. Revegetation is critical in preventing weed infestations in areas where the soil has been disturbed or the vegetation removed. In some situations you may be able to encourage desirable vegetation that is already in place; but because of the aggressive nature of Canada thistle, it is more likely that you will need to thickly sow seeds of desirable, competitive plants.

    Apply Management Methods

    No individual method will control Canada thistle in a single treatment; diligence and persistence will be required over a number of years to subdue this weed. The treatment methods described in this section will help you to design an integrated program that will suit the circumstances of your particular situation.

    Biological Controls
    Biological control does not aim to eradicate weeds, but to keep them at low, manageable levels. After their introduction, biocontrol agents can take 5 to 10 years to become established and increase to numbers large enough to reduce the density of the target weed. Once established, effective biological controls provide an inexpensive, long-term, and non-toxic means to control weed populations. Since insects have specific requirements for growing and thriving, it is important to match the insect to the weed management site. Understanding these requirements will help you integrate the insects into other weed control efforts. When you release biocontrols, continue using other control methods on the perimeter of the release site, but avoid using them where they might adversely impact the insect population.

    The information provided below is only a summary. For more information consult Biological Control of Weeds in the West (see Bibliography) or contact commercial weed biocontrol insectaries (see Insectaries).


    There are 3 major pathogens under investigation as possible biological controls for Canada thistle: Pseudomonas syringae tagetis, a bacterium; Puccinia punctiformis, a rust fungus; and Sclerotinia sclerotiorum, another common soil fungus. None of these pathogens are commercially available yet, and unfortunately, will not be for a number of years.

    P. syringae tagetis looks very promising as an eventual control. Infected plants are stunted, cannot flower, and have lower root carbohydrate levels so that roots freeze and die during the winter. Although this bacterium can infect a few other plants, it is not economically important in any of them, and researchers have devised a method to keep bacterial treatments confined to only the target plants (Johnson 1996).

    Goats and sheep can be very useful in controlling Canada thistle. Sheep will eat the soft, young growth, but goats will eat any stage; the thorniness of the plant does not bother them. The animals must be restricted so that they will be forced to feed on the weedy species, which can easily be done with electric fencing or with trained dogs. Divide the field or paddock into manageable sections and confine the animals to each in turn. Sheep should be brought in when the thistle is very young, but goats can graze any time before the plant flowers. Because Canada thistle will resprout many times before it dies, the animals will have to be brought back each time you find sprouts and before the new sprouts have a chance to flower.

    As soon as you see Canada thistle rosettes, bring in sheep or goats to eat the rosettes and any shoots. When the animals have removed the weeds, take them out of the area. Monitor the area carefully and when the shoots begin to resprout, bring the animals back again. This process of moving the animals in and out should be repeated until the end of the summer. The next season the Canada thistle infestation should be greatly reduced, but you may still need to use the sheep or goats for another season or two.

    Many people balk at the idea of buying goats for weed management because there is little or no market for goat meat or milk; however, this problem can be ameliorated by employing angora goats. In addition to their value as weed control agents, they are highly prized for their wool.

    Physical Controls

    Manual Removal
    Hand cutting or hoeing is practical only in very small or very sensitive areas. For effective control, these treatments must be started when the root reserves are low in the spring and repeated every 3 to 4 weeks until the plants stop putting up new sprouts.

    Mechanical Removal
    Mowing and cultivating can be used to control Canada thistle; however in the literature there is a great deal of different, and sometimes conflicting, information. There are two points on which everyone seems to agree: these methods cannot control Canada thistle in one season, and the timing and frequency of these treatments has considerable impact on their effectiveness. There is no agreement on how long these methods take to control Canada thistle, but this is partly because the particulars of the site and the infestation (as well as the determination of the weed manager) will influence the time factor.

    There is also no agreement on the timing and frequency of mowing or cultivation. The purpose of these treatments is to prevent seed formation and to eventually deplete the thistle’s root reserves so that it can no longer send up shoots.

    Hodgson (1968a, b) reports that the thistle’s root reserves are lowest 4 to 6 weeks after the first plants emerge in the spring. When the plants are cultivated, new shoots emerge in about 10 days. Ten to 15 days after they sprout, these shoots are capable of furnishing food to the roots once again. This information led him to recommend cultivating 3 or 4 inches deep with overlapping sweeps of a duckfoot cultivator every 3 weeks throughout the growing season. He noted that when the weather is hot and dry, the shoots sprout more slowly and the interval between cultivations can be lengthened to 4 weeks.

    There seems to be agreement that there is no advantage to more frequent tillage. Although this kind of intensive cultivation can be very effective, it takes land out of production and exposes the soil to wind and water erosion (Donald 1990).

    Mowing and cultivation will be most effective when used to destroy newly emerged shoots before they are capable of replenishing the root reserves. The recommendations for frequency and timing of mowing are less precise and not as well documented; however, it would seem in theory that beginning mowing when root reserves are at their lowest and continuing every 3 to 4 weeks would eventually deplete the root reserves. Because mowing stimulates thistle roots to produce more shoots, incomplete treatment could result in a higher density of thistles after mowing than before.

    Most mowing studies are done in conjunction with forage crops and the mowing frequency is modified by the forage management practices. Frequent mowing (every 3 to 4 weeks) could limit forage production. Donald (1990) notes that most studies report that it took only 3 years of forage production along with mowing to severely reduce Canada thistle infestations, but the studies did not include data on how long control lasted or at what rate the thistle reinfested the fields.

    Cultural Controls

    Crop Competition in Combination with Other Controls
    Read (1996) suggests cultivating at the end of summer and preparing a good seedbed by harrowing and smoothing out the ground. Next, heavily seed the area to grasses, alfalfa, or other cattle or sheep forage (turnips work well because they quickly close up the canopy), or other desirable and competitive vegetation. It is important to put in the competitive vegetation soon after the cultivation; if you wait until spring, you will have hundreds more thistle shoots than you started with because you have broken up the roots.

    This method can also be used in backyards or in other small infestations. Use a rototiller to cultivate the soil and rake up and burn or otherwise destroy the roots and stems. You can use goats if you have access to them or you can simply hoe or cut the thistles back by hand.

    Chemical Controls
    In IVM programs, herbicides are considered transition tools that enable the manager to suppress weeds and replace them with desirable, competitive vegetation. Thus, it is important to select the least-toxic, low-residual herbicide that is effective against the target weed, and to apply them in a judicious manner.

    The following links discuss the primary considerations when using herbicides:  Use Herbicides Properly, Herbicide Information Resources, Criteria for Selecting an Herbicide, and Application Methods.

    Proper Timing
    Applying herbicide to plants when Canada thistle is most susceptible (preferably before seeds are produced) is crucial to the effectiveness of the treatment. The best time to treat thistles is in the early fall, when plants are still rosettes. During this time, they will be actively growing and transporting nutrients to roots for winter storage and systemic herbicides can be more effectively transported along with these nutrients (Leonard 1997).

    Educate Vegetation Management Personnel
    and the Public

    Evaluate the Vegetation Management Program


    Ang, B.N., L.T. Kok, G.I. Holtzman, and D.D. Wolf. 1994. Competitive growth of Canada thistle, tall fescue, and crownvetch in the presence of a thistle defoliator, Cassida rubiginosa Muller (Coleoptera: Chrysomelidae). Biological Control 4: 277-284.

    Cranston, R. 1996. Pers. Comm. Provincial Noxious Weed Specialist, Prov. of British Columbia. 17720 - 57th Ave., Surrey, B.C. V3S 4P9.

    Dewey, S.A. and J.M. Torell. 1991 What is a noxious weed? In: James et al., Noxious Range Weeds. Westview Press, Boulder, CO.

    Donald, W.W. 1990. Management and control of Canada thistle (Cirsium arvense). Review of Weed Science 5: 193-250.

    Donald, W.W. 1996. Pers. Comm. Res. Agronomist. USDA-ARS, Univ. of Missouri, Columbia, 269 Ag. Engineering Bldg., UMC, Columbia, MO.

    Fay, P.K., T.D. Whitson, S.A. Dewey, and R. Sheley, eds. 1995. 1995-1996 Montana-Utah-Wyoming Weed Management Handbook. Coop. Ext. Serv., Montana State University, Bozeman, MT. 245 pp.

    Fuller, T.C., and G.D. Barbe. 1985. The Bradley method of eliminating exotic plants from natural reserves. Fremontia 13(2): 24-25.

    Hodgson, J.M. 1968a. The nature, ecology, and control of Canada thistle. U.S. Department of Agriculture, Washington, D.C., USDA-ARS Tech. Bull. No. 1386. 32pp.

    Hodgson, J.M. 1968b. Canada thistle and its control. U.S. Department of Agriculture, Washington, D.C., USDA-ARS Leaflet No. 523. 8pp.

    Haderlie, L.C., R.S. McAllister, R.H. Hoefer, and P.W. Leino. 1991. Canada thistle control, pp. 260-263. In: James et al., Noxious Range Weeds. Westview Press, Boulder, CO.

    Hayden, A. 1934. Distribution and reproduction of Canada thistle in Iowa. American J. of Botany 21: 355-373. [Cited in Moore 1975].

    Jacobs, J.S., R.L. Sheley, and B.D. Maxwell. 1997. Yellow starthistle population dynamics model. Colorado Weed Management Association 1997 Annual Conference Proceedings. Granby, CO.

    James, L.F., J.O. Evans, M.H. Ralphs, and R.D. Child, eds. 1991. Noxious Range Weeds. Westview Press, Boulder, CO. 466 pp.

    Johnson, D. 1996. Pers. Comm. Research Associate, Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul.

    Lacey, C.A., et al. 1988. Bounty programs - an effective weed management tool. Weed Technology 2: 196-197.

    Leonard, N.R. 1997. Tame thistles: low-cost weed-control stategies that work wonders. Sustainable Farming Connection. Committee for Sustainable Farm Publishing. (visited Nov. 1997).

    Maxwell, B. " YST software." Personal e-mail (9 Feb 1998).

    Moore, R.J. 1975. The biology of Canadian weeds. 13. Cirsium arvense (L.) Scop. Canadian J. of Plant Science 55: 1033-1048.

    Magnusson, M.U., D.L. Wyse, and J.M. Spitzmueller, 1987. Canada thistle (Cirsium arvense) propagation from stem sections. Weed Science 35: 637-639.

    Pearson, W. 1998. Pers. Comm. County Weed Extension Agent. Stillwater County, Columbus, Montana.

    Peschken, D.P. 1984. Cirsium arvense (L.) Scop., Canada thistle (Compositae). pp 139-146. In: Kelleher and Hulme, Biological Control Programmes against Insects and Weeds in Canada 1969-1980. Commonwealth Ag. Bur., Farnham Royal, Slough, England. 410 pp.

    Randall, J.M. and J. Marinelli eds. 1996. Invasive Plants: Weeds of the Global Garden. Brooklyn Bot. Garden , Inc. Brooklyn, NY. 108pp.

    Read, R. 1996. Pers. Comm. Vegetation Biologist, B.C. Hydro, 400 Madsen Rd., Nanaimo, British Columbia V9R 5M3.

    Rees, N.E. 1996. Pers. Comm. Entomologist, retired. Rangeland Weeds Laboratory. USDA-ARS, Sidney, MT.

    Rees, N.E., et al.(eds). 1996. Biological Control of Weeds in the West. Western Society of Weed Science, USDA/ARS, Montana Dept. Agric., Montana State University, Bozeman, MT.

    Rees, N.E. 1992. Biological control of thistles, pp. 264-273. In: James et al., Noxious Range Weeds. Westview Press, Boulder, CO.

    Strand, O.E. 1982. An integrated approach for Canada thistle control on non-cropland. Proc. North Central Weed Control Conf. 37:113-114.

    Trumble, J.T. and L.T. Kok. 1982. Integrated pest management techniques in thistle suppression in pastures of North America. Weed Research 22: 345-359.

    University of Northern Iowa. 1993. Integrated Roadside Vegetation Management Technical Manual. Produced by the Roadside Management Program. To obtain, call Kirk Henderson at 319-273-2813.

    Further Information

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