United States Department of Agriculture 

NATURAL RESOURCES CONSERVATION SERVICE 

Invasive Species Technical Note No. MT-5 

September 2006 

Ecology and Management of Canada thistle [Cirsium arvense (L.) Scop.] 

by 

Jim Jacobs, NRCS Invasive Species Specialist, Bozeman, MT 

Joanna Sciegienka, Graduate Research Assistant, Montana State University, Bozeman, MT 

Fabian Menalled, Extension Cropland Weeds Specialist, Montana State University, Bozeman, MT 

Abstract 

A member of the Aster family, Canada thistle is a vigorous, highly competitive species. Occurring 

in a large range of habitats including croplands, ditch banks and riparian areas, gardens and pastures, 

this category 1 noxious perennial weed is particularly hard to control because of its deep, creeping, 

reproductive root system forming colonies. In general, infestations start on disturbed ground, with 

plants being able to colonize 10 to 12 feet per year. Canada thistle can grow in a variety of habitats, 

but it is best adapted to deep, well-aerated and productive soils. It prefers sunny and warm areas 

with 15 to 30 or more inches of precipitation or irrigation per year, but it can grow on dryer cropland 

and pasture sites with 12 to 13 inches of precipitation per year. When temperatures exceed 85o F for 

extended periods of time, it stops growing. 

Canada thistle threatens productivity in both crop and non-croplands. In cropland, Canada thistle 

causes extensive yield losses through competition for light, nutrients, and moisture. It also increases 

harvesting problems due to seed and forage contamination. In Montana, it is estimated that two 

shoots per square yard can reduce wheat yield by 15 percent and 25 shoots per square yard can 

reduce wheat yield by 60 percent. Other Montana crops seriously threatened by Canada thistle 

include peas, corn, beans, alfalfa and sugar beets. Heavy infestations are also commonly found in 

overgrazed pastures and ranges and may crowd-out and replace native grasses and forbs, decreasing 

species diversity in an area. 

Perennial forage plants and winter annual cereals compete most effectively with Canada thistle 

because they emerge early in the growing season and inhibit the emergence of late-emerging 

Canada thistle shoots. Cultivation increases Canada thistle in most situations. Herbicides that 

translocate and distribute into the root system including aminopyralid, clopyralid, or picloram 

applied when plants are in the rosette to bolting growth stage or in the fall, provide temporary 

suppression of Canada thistle. Biological control has not been effective in reducing Canada 

thistle populations; however, two insects (Hadroplontus litura and Urophora cardui) and two 

pathogens, (Puccinia punctiformis and Pseudomonas syringae pv. tagetis) have the highest 

potential for control. Sustainable management is most likely where cultural practices that 

encourage desirable competitive plants are integrated with herbicidal and biological control 

practices. 

(Disclaimer: Any mention of products in this publication does not constitute a recommendation by the 

NRCS. It is a violation of Federal law to use herbicides in a manner inconsistent with their labeling.)

NRCS−Montana−Technical Note−Invasive Species−MT-5 1
Biology and Identification 

Despite its name, Canada thistle is native to Europe, parts of North Africa and Asia, including 

Afghanistan, Iran, Pakistan and China. This species was introduced to North America from 

Europe in the 1600's as a contaminant of grain seed. By 1795, Vermont enacted noxious weed 

legislation against Canada thistle and, in the early 1900's, the currently named Noxious Weed 

Act gave a person the right to eradicate this species wherever they found it without fear of 

trespassing. It was first identified in Montana in 1881 in Musselshell County, and by 2000, it 

had been identified in every county in the State. Today, Canada thistle is prevalent all over the 

State of Montana covering 1.5 million acres with the potential for much more (see Figure 1). 

Figure 1. Probability of occurrence of Canada thistle in Montana, authors Frank Dougher, Lisa 

Rew, and Charles Repath. 

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Life Cycle, Identification and Growth 

In mid- to late-spring, Canada thistle emerges from the soil from either vegetative root buds or 

germinating seeds. Although Canada thistle seedlings grow slowly and are very susceptible to 

competition, vegetative buds can be formed on seedling roots seven to eight weeks after 

germination. Seedlings form a rosette with irregularly lobed spiny leaves (see Figure 2). If the 

rosette is formed early in the season, the plant gradually produces an upright, elongated stalk 

capable of flowering. Plants that are formed late in the summer or fall do not produce upright 

stems that year, but remain rosettes until a hard frost causes them to die back to the ground. 

Figure 2. Low-growing Canada thistle rosettes showing the irregularly-lobed, spiny leaves. 

Mature plants can develop stems up to five feet tall. A 16-hour day length initiates shoot 

elongation and flowering. Shoots that emerge in August or later when the photo-period is shorter 

than 16 hours will remain low-growing rosettes. Research has shown that herbicides applied 

during shoot elongation will only provide short-term suppression of Canada thistle because most 

of the toxic ingredient is moved to the shoot apex instead of the roots. The oblong or lanceshape leaves found in these mature plants are four and eight inches long, lack petioles, and are 

divided into spiny-tipped-irregular lobes (see Figure 3). Occasionally, plants with spineless and 

smooth leaves can be found. Leaves are alternately arranged on the stem and lateral branches 

grow from buds in the leaf axils. 

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Figure 3. Mature Canada thistle plants showing the oblong to lance-shaped, spiny leaves. 

Mature plants are imperfectly dioecious, meaning in most cases plants produce either male or 

female flowers, so that some colonies produce only pollen and some produce only seeds. Plants 

usually flower between June and August and seed producing flowers are pollinated by insects, 

primarily by honey bees that are attracted by the vanilla-like odor of the female flowers. Flower 

heads are small relative to biennial thistle flower heads, up to three-quarters of an inch in 

diameter, with the majority of them being purple or lavender, though white flower heads occur in 

approximately ten percent of the population (see Figure 4). Canada thistle stems produce many 

flower heads arranged in an open panicle. Each flower head has between 84 to 132 florets, and 

93 seeds have been counted from one flower head. 

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Figure 4. Budding Canada thistle flower heads showing the many, relatively small flower 

heads per plant. 

Female individuals are able to produce between 1,500 and 5,000 seeds per stem. Seeds are 

technically called achenes, and are approximately one-eighth inch long, flattened, brown, and 

have tufts of hairs at the top, called the pappus. The pappus is a poor aid to wind dispersal 

because it does not remain firmly attached to the seed. These tiny seeds mature quickly and are 

dispersed poorly by wind, but they float and are dispersed by moving water. Animals, their 

manure, farm equipment and other vehicles also contribute to the dispersal of Canada thistle. 

Although 90 percent of these seeds will germinate within a year, they can remain viable in the 

seedbank for up to 20 years if they are buried in the soil more than eight inches. Less that one 

percent of the seeds buried in the soil one to three inches survive more than five years. 

In the spring when the average weekly temperature reaches 41° F (5° C), Canada thistle can 

reproduce vegetatively from its extensive root system which can penetrate the soil to a depth of 

six to 15 feet. When roots are cut or broken, pieces are capable of developing new plants 15 

days after the fragmentation and can become fully developed plants within seven to eight weeks. 

Even small root pieces (0.25 inch long, 0.125 inch in diameter) have enough stored energy to 

develop new plants. In addition to absorbing water and nutrients from the soil, roots receive 

sugars produced in the leaves, store carbohydrate reserves, and can regenerate aboveground 

parts, making Canada thistle very difficult to control. Carbohydrate levels in the roots are lowest 

in the latter part of June. In the fall, carbohydrates in the roots are converted to sugar which 

prevents the formation of damaging ice crystals during winter and acts as anti-freeze protecting 

the roots from frost damage. Roots that survive the winter are capable of regenerating the 

Canada thistle population. 

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Figure 5. New Canada thistle shoots developing from buds on roots. 

Damage 

In alfalfa stands grown for seed production, Canada thistle can reduce yield by 48 percent. An 

extra ten percent yield reduction can occur in alfalfa seed production due to seed cleaning. In 

pastures, Canada thistle reduces productivity by crowding out forage species with spiny leaves 

that deter cattle from grazing. In non-cropland ecosystems, Canada thistle can crowd out and 

replace native grasses and forbs limiting land’s recreational use. In gardens, flower beds, and 

lawns, Canada thistle’s extensive root system makes it a hassle to control. Mowing or pulling 

this weed is not effective because it grows again from vegetative buds on the roots. In fact, 

improper cultivation can even worsen Canada thistle problems!

Management Alternatives 

Because of its extensive root system, complete elimination of Canada thistle usually takes 

persistent control over several years. Fortunately, there are several cultural, mechanical, 

biological, and chemical practices that can be combined to exhaust the nutrients stored in the root 

systems of Canada thistle. These methods should be adapted to the specific conditions of the 

infested area. 

Cultural 

Once Canada thistle has invaded, it is very important to maintain soil fertility and moisture at 

optimum levels to favor crop or pasture plant establishment and growth. Perennial forage plants 

and winter annual cereals compete most effectively with Canada thistle. 

 

NRCS−Montana−Technical Note−Invasive Species−MT-5 6
Cultivation. Because of its extensive root system and its ability to reproduce from small root 

fragments, cultivation has the potential to increase Canada thistle. To suppress Canada thistle, 

cultivation should be repeated at 21-day intervals over the growing season. Cultivation should 

be followed by the establishment of perennial forage plants or winter annual cereals because they 

emerge early in the growing season, inhibit the emergence of late Canada thistle shoots, and 

compete most effectively with Canada thistle. 

Mowing. Mowing has been used to manage Canada thistle. Canada thistle populations were 

observed to decline 86 percent after one year and 100 percent after four years in alfalfa that was 

mowed two times per year. In Ohio, mowing Canada thistle infestations three or four times per 

year nearly eliminated it after three years in some areas and only kept it from flowering in other 

areas. In Minnesota, mowing after the longest day in summer prevented re-growth during that 

year. In Colorado, control of Canada thistle using picloram, picloram plus 2,4-D, Clopyralid 

plus 2,4-D, or dicamba was enhanced if it was followed by mowing two or three times. Mowing 

Canada thistle in late June when root reserves are lowest may result in the greatest reduction of 

the weed. 

Prescribed Burning. In natural communities, fire has been used to manage Canada thistle 

infestations. However, while late spring burns (between May and June) can reduce Canada 

thistle, early spring burns are not recommended since they may increase sprouting and 

reproduction. 

Hand Pulling. For light infestations, pulling or hand-cutting can be effective if done several 

times each season to starve underground roots and stems. Hand pulling of Canada thistle is not 

considered a practical control method on well established infestations because of the extensive 

reproductive root system. As a general rule, pulling should be done in combination with other 

cultural, biological, and chemical practices to reduce the competitive ability of Canada thistle. 

Grazing. The spiny leaves of Canada thistle make it unpalatable to most classes of livestock and 

therefore grazing is not commonly used to manage Canada thistle. In Australia, intensive sheep 

grazing reduced Canada thistle spread compared to a pasture where sheep did not graze. 

Observations indicate that goats will also eat Canada thistle and can prevent it from flowering. 

Herbicide 

Cropland. In small grains, Affinity BroadSpec PP and Affinity TankMix (thifesulfuron + 

tribenuron) provide good control both at the pre-plant or pre-emergence stage and the postemergence stage. Clopyralid (Curtail® and Stinger®) can also be used during the postemergence stage with excellent control ratings. Finally, while paraquat (Gramoxone Max®) and 

glyphosate (Roundup®) can be used to control Canada thistle at the pre-plant or pre-emergence 

stage of lentil and pea, bentazon (Basagran®), imazamox (Raptor®) and MCPB (Thistrol®) can 

be used at the post-emergence stage. Remember that paraquat is a restricted-use product and 

special care should be taken when applying it. For additional crops, consult the 2006-2007 

Montana, Utah and Wyoming Weed Management Handbook.

Glyphosate can be used to control Canada thistle in hay or forage alfalfa during the pre-plant/ 

pre-emergence stage. Bromoxynil (Buctril®) and imazamox can be used as post-emergence 

products in alfalfa stands, but they only provide partial control. When using Raptor, remember 

that there must be at least 20 days between application and cutting or feeding alfalfa for forage or 

NRCS−Montana−Technical Note−Invasive Species−MT-5 7
hay and 70 days between application and seed used for feed or forage. Unfortunately, unless 

producers utilize Roundup Ready alfalfa, there are no products that can be used to control this 

weed in dormant established alfalfa stands. 

Non-cropland. The key to long-term herbicidal suppression of Canada thistle is to get a toxic 

level of the appropriate active ingredient in as much of the root system as possible. To do this, 

the appropriate herbicide needs to be applied, it needs to be applied to enough leaf area to absorb 

sufficient amounts of the herbicide, and it needs to be applied at a time when the herbicide is 

mostly translocated and distributed in the roots. To maximize herbicide performance on Canada 

thistle, it is important to avoid mixing fast-acting contact herbicides with the systemic product. 

By mixing fast-acting herbicides with systemic products, the weeds are burned back too quickly 

and are unable to absorb and translocate the slower performing herbicide that is utilized to kill 

the root system. 

Research has shown that aminopyralid (Milestone®), clopyralid (Transline®), and picloram 

(Tordon 22K®) provide similar suppression of Canada thistle in pastures and rangelands when 

applied at label rates and when the herbicides are translocated to roots. Formulations of 

clopyralid plus 2,4-D (Curtail®) and clopyralid plus triclopyr (Redeem®) are also labeled to 

control Canada thistle. Using current prices, cost comparisons of using these chemicals at the 

label recommended rates resulted in a range from $16.44 to $26.56/acre treated (not including 

application costs). Picloram is a restricted-use herbicide because it is persistent and mobile in 

the soil and cannot be used where there is a potential for water contamination including many 

areas where Canada thistle is common. 

Herbicides should be applied either when most of the Canada thistle plants in the population are 

in the bud stage or to fall re-growth. At the bud stage, leaf area for herbicide coverage and 

absorption is maximized, and root reserves are at their lowest. In the fall, translocation of the 

herbicide to the roots is the greatest. Recent research suggests that fall treatment using the 

translocatable herbicides mentioned above prevents Canada thistle plants from converting starch 

into sugar in the roots. The sugar in the roots acts as an anti-freeze, preventing the formation of 

membrane-destroying ice crystal in the cells. Without the formation of sugar in the root cells, 

the roots are susceptible to winter kill. These herbicides will injure desirable forbs, especially 

legumes. Individual Canada thistle plants can be treated in pastures, rangelands, and non-crop 

areas with a wick applicator or hand sprayer to reduce non-target effects. 

 

Biological Control Practices 

Many biological agents have been surveyed and tested for effective control of Canada thistle. 

Based on host specificity, availability, and degree of damage, few of them merit justification of 

their use (see Table 1). Among them, two insects (Hadroplontus litura and Urophora cardui) 

and two pathogens, (Puccinia punctiformis and Pseudomonas syringae pv. tagetis) are probably 

the ones with the highest potential for control. Unfortunately, U. cardui has not shown much 

success in the field and P. punctiformis poses many challenges to be manufactured for 

commercial use. 

Hadroplontus litura is a stem-boring weevil that attacks Canada thistle plants at the rosette stage. 

Adults lay an average of 2.5 eggs in a 1-2.5 mm cavity on the underside of leaves that are at least 

5 cm long. This continues for four to six weeks after rosettes appear. The larvae mine down the 

leaf mid-veins into the root crown, and sometimes into the upper root, feeding on callus tissue. 

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If there are only a couple larvae, the callus growth can kill them. However, a larger number of 

larvae can create more tunnels, which then coalesce to create a woody gall. It is important for H. 

litura to attack before stem growth starts in order to do the most damage. The larvae pupate in 

cocoons of soil particles and the adults emerge in late summer/early fall to feed on the upper 

leaves and stem of the Canada thistle plant. They over-winter in soil litter, and in the spring, 

come back to eat rosette leaves, making feeding punctures of 2-4 mm2. While this insect 

probably cannot control Canada thistle by itself, the exit holes left in the root crown by larvae 

can make the weed more susceptible to synergistic control with pathogens.

Pseudomonas syringae pv. tagetis (Pst) is a pathogen that causes leaf spot and apical chlorosis in 

some members of the Asteraceae family, including Canada thistle. Pst produces a toxin that is 

not host-specific and prevents the plant from photosynthesizing correctly. To infect Canada 

thistle, Pst must be used in conjunction with a surfactant such as Silwet L-77. Any damage or 

wounds on the plant can also provide pathways for the pathogen to enter. Growth chamber and 

field tests have shown variable results for the destructive action of Pst. However, combined 

action of this pathogen with other methods could show better results.

Integrated Pest Management 

An integrated Canada thistle management program will include prevention, early detection and 

small-scale eradication, containment, and large-scale population reduction. Prevention is guided 

by how Canada thistle spreads and its requirements for establishment and includes managing 

disturbance, maintaining competitive plant communities, and preventing seed imports by using 

weed-free feed and seed, cleaning equipment before application on weed-free areas, and a fiveday containment period for grazing animals that have fed in and potentially eaten seeds in 

Canada thistle infestations before moving them to weed-free areas. Early detection and smallscale eradication is achieved through persistent survey and herbicide application. Canada thistle 

populations are contained by herbicidal control of population boarders and satellite populations, 

control actions that reduced seed production such as sheep or goat grazing, and cultivation of 

competitive desirable plants. Large-scale population reduction is achieved over the long-term by 

applying management alternatives such as prescribed grazing that includes sheep or goat grazing, 

and management that reduces the Canada thistle population fitness and increases the fitness of 

desirable, competitive plant populations. 

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Table 1. Bio-control agents for Canada thistle. 

Species Common Name Details

Cassida 

rubiginosa 

leaf-feeding 

tortoise beetle 

Recorded on numerous species including globe 

artichoke (Cynara scolymus), an economically 

important crop. 

Larinus planus seed-feeding 

weevil 

Thought to prefer Canada thistle over other Cirsium

species for feeding and oviposition; will not feed on 

ornamental or economic species in the tribe 

Cardueae. Has been found to infest a native thistle. 

Terellia ruficauda Seed-head fly Reared from six Cirsium species in Europe; widely 

distributed across Canada; does minimal damage. 

Cleonis pigra Root-feeding 

weevil 

Attacks numerous species of Cardueae in Europe; is 

an economic pest of globe artichoke. 

Puccinia 

punctiformis 

Rust fungus Synergistic effects with other bio-control agents look 

promising. However, by itself it is not effective at 

significantly decreasing thistle populations. 

Pseudomonas 

syringae pv. 

Tagetis 

phytopathogenic 

bacterium 

Field tests in a commercial corn field resulted in 57 

percent mortality of Canada thistle as well as damage 

to several other weedy Asteraceae species. 

Surfactant is required to allow penetration of the 

weed cuticle. 

Rhinocyllus 

conicus 

Seed-head weevil Most widely distributed insect for thistle control in 

the US; can reduce thistle populations by 90-95 

percent in eight to ten years. Also feeds on native 

thistles and has a wider-than-expected host range. 

USDA has prohibited interstate movement of these 

weevils. 

Altica carduorum Leaf-feeding 

beetle 

Adults readily feed on all Cirsium species tested. 

Although risk analysis predicts that Canada thistle 

will be favored over other thistles; this biotype has 

not been approved for field release in North America. 

Hadroplontus 

litura, formerly 

Ceutorhynchus 

litura 

Stem-boring 

weevil 

Only known host plant in Europe is Canada thistle 

(minor exceptions). Releases began in 1971 and four 

releases were made in Montana. 

Urophora cardui Stem and petiolegalling fly 

Reported only on Canada thistle and closely related 

species. Evidence suggests this fly is not particularly 

effective for controlling Canada thistle. 

Tingis ampliata Lace bug Never released in North America because of concerns 

about possible attack on globe artichoke. 

Lema cyanella Leaf beetle Limited release in Canada; no further releases or redistribution are planned. 

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References 

Coombs, E. M., J. K. Clark, G. L. Piper, and A. F. Cofrancesco, Jr. (eds) Biological control of 

invasive plants in the United States. Oregon State University Press, Corvallis. Pages 345-378. 

Morishita, D.W. 1999. Canada Thistle. In: Biology and Management of Noxious Rangeland 

Weeds. Oregon State Press. Pages 162-174. 

Dewey, S.A., S.F. Enloe, F.D. Menalled, S.D. Miller, R.E. Whitesides, and L. Johnson. Weed 

Management Handbook 2006-2007 Montana, Utah, Wyming. Cooperative Extension Services 

Publication # EB0023, Bozeman, Montana. 272 pages. 

NRCS−Montana−Technical Note−Invasive Species−MT-5 11