Canada Thistle 

Cirsium arvense (L.) Scop.¹

Family: Aster family, Asteraceae¹

Other Common Names - creeping thistle, small-flowered thistle, perennial thistle, green thistle, field thistle, cursed thistle, corn thistle¹

History

Canada thistle probably originated in southeastern Europe and the eastern Mediterranean.¹ It was introduced to North America in the 1600s from Europe.² One of the earlier known introductions was Franciscan fathers who attempted to “improve” the agricultural efforts of the Indigenous peoples, brought it to the Hudson Bay area about 1750. There is good historical evidence that the largest early introduction to the United States, was in the horse feed used by British General John Burgoyne's army during the American Revolutionary War. The thistle trail from Montreal to Saratoga, New York, was visible for many years and neighboring states passed laws against its importation.³

It was probably brought to Idaho about 1880. Small patches existed near Boise and in Sandpoint in 1896.³ In 1959, Canada thistles covered about 230,000 acres. About half (110,000 acres) is concentrated in the Upper Snake River Valley, mainly on irrigated ground. The bulk of the remainder is in the higher rainfall area of northern Idaho.⁴ And that makes sense as Canada Thistle is best adapted to cool and moist areas and prevails throughout the northern half of the United States.³

Noxious Weed Status for Surrounding States:

Idaho - Yes   Washington - Yes    Oregon - Yes    Nevada - Yes    Utah -Yes    Wyoming - Yes   Montana - Yes

Quick Identification

General - Canada thistle is a prickly perennial weed spreading by deep thickened storage roots.¹ It can grow over 6 ft. tall (more commonly ~3 ft.). Each branch of the plant typically bears 3-5 dandelion-sized flowers that range from white to purple (more commonly pink). Canada thistle leaves are rigid and glossy, and stems are virtually spineless.²

Common Look Alikes - Many thistles and thistle-like plants have spiny leaves and similar flower heads to Canada thistle, and nearly all are biennial. Other thistles have singular taproots rather than the extensive storage root system of Canada thistle. Bull thistle (Cirsium vulgare) leaves have hairy upper surfaces and spiny, winged stems (ridges on the stems), while Canada thistle leaves are hairless above and the stems lack wings. Sowthistle (Sonchus) species may look similar to Canada thistle, but Sowthistles have yellow flower heads and bleed milky sap when damaged, while Canada thistle has pink/purple flower heads and clear sap.¹

Further Physical Description

Flower - Male and female flowers are produced on separate plants, and are located where the leaf connects to the stem and at the end of branches. Since male and female flowers occur on separate individual plants, the species typically cannot self-pollinate. Consequently, many infestations are single clones that cannot produce seeds. If male and female plants are present, Canada thistle is pollinated by insects, especially honeybees.¹

Male flower heads are round and 0.5 inch wide, while female flower heads are 1 inch wide and flask shaped. Each female flower head may produce as many as 45 seeds. The flowers of both sexes are purple-pink, with bases enclosed in overlapping triangular or diamond shaped, scale-like, spineless green bracts. A small leaf is present at the base of each flower stalk.¹

Leaf - Cotyledons (leaves directly from seed) of the seedling are stalkless, fleshy, hairless, oval to rounded and 0.2–0.6 inch long by 0.1–0.2 inch wide. The first true leaves of the seedling are egg shaped to pointed, hairy and have irregularly lobed, spiny edges. The early leaves join at a central point at first and then start growing up a central stem. Mature leaves are 2–6 inches long, alternate, oblong and irregularly lobed with sharp spines along the edges. Upper surfaces of the leaves are hairless, dark green and waxy, while lower surfaces are pale green and smooth to hairy. The leaves don’t have stalks and connect directly to the stem.¹

Stem - Mature plants are 1–5 feet tall and branched towards the top. Stems are hollow, grooved and sparsely hairy to smooth.¹

Fruit/Seed - The apparent seeds have a thin dry outer coating of fruit tissue. Seeds are brown, flattened, oblong and 0.1–0.25 inch long. Each seed is weakly attached to a feathery white fluff (called a pappus, like a dandelion seed head).¹

Roots - Seedlings begin to develop horizontal storage roots early, and a plant with only two true leaves may have a resilient storage root up to 6 inches long. Vegetative buds develop on the storage root by the third true leaf stage. The roots are an extensive system of white, fleshy storage roots that can extend to over 15 feet horizontally and vertically and can produce more than 200 new buds annually. Roots have approximately one to two root buds per 4 inches of root length throughout the year. The root system commonly extends to depths of 6.6 feet or more, and new shoots quickly reappear after the previous shoots have been removed. Root fragments larger than 0.1 inch or 0.05 ounces fresh weight will usually produce new plants, and smaller fragments may also if they are near the soil surface.¹

Dispersal and Propagation - Canada thistle reproduces both vegetatively and by seeds.¹

Shoots from rootstocks begin emerging in mid-spring and emerge continuously until frost. Shoots can emerge from roots several feet deep, but the optimum depth for shoot emergence and plant canopy spread is 4 inches. Vegetative shoots resemble seedlings but are larger and lack cotyledons.¹

Seedlings emerge primarily in the spring, with some emerging in summer. Optimum depth for seedling emergence is 0–0.4 inch, but seedlings can emerge from as deep as 2.⁴ inches.

Seeds are unable to mature and become viable when stems are cut in full bloom. Seed production varies from 0–5,000 seeds per shoot, depending largely on the thoroughness of pollination. Seeds mainly allow the species to disperse between locations and do little to maintain local populations. The seeds have a special feathery structure (called a “pappus”) that aids in wind dispersal.¹

Most seeds are ready to germinate when shed from the parent plant, however some investigators report the need for chilling and after-ripening to achieve full germination. Seeds germinate best at warm daytime temperatures (77–86°F) but are inhibited from germination by hot (104°F) temperatures. Light, day/night fluctuation in temperature, and to a lesser extent, nitrate, all increase germination. A few seeds may persist in the soil for 17–21 years, but most disappear within the first few years. Seeds persist better when buried over 1 foot, and more poorly when near the soil surface.¹

Dealing with Canada Thistle

Prevention

To stop the spread of Canada thistle, stop the seeds and the roots coming to the land. Making sure Canada thistle doesn’t take root in or around fields so they can’t blow seeds in. Seeds can also come from baled grain straw, manure with grain straw bedding, and seeds can blow into irrigation or drainage waters and then be transported long distances while maintaining 52–65% viability.¹

Roots can be brought in on farming machinery and they should have dirt and vegetation removed regularly. 

Susceptibility

Frost - Shoots tolerate light frost (as in spring) but are killed by hard frost (as in fall). The root system persists over the winter. Seasonally higher concentrations of sucrose and fructans in roots during fall and winter months allow the roots to adapt to cold temperatures. However, root fragments at the soil surface are killed by 14–21°F temperatures.¹

Drought - Well developed plants are drought tolerant due to their deep root systems. Shoot establishment and growth from root fragments are reduced by low soil moisture, suggesting that dry conditions can enhance control with tillage practices, and that areas with optimum soil moisture are more prone to proliferation of this weed. Competition for water between shoots and root buds is a primary cause of bud inhibition under dry conditions.¹

Shade - Seedlings will die when light falls below 20% of sunlight. The species is shade intolerant, but due to its extensive root reserves, shoots can often grow through competing vegetation. Shoots of established plants suffer when shaded by competing taller species.¹

Wet - It does poorly on wet soils, and is particularly vigorous on well drained, fine textured (clay to silt loam) soils suitable for agricultural production.¹

Depth - When the roots are cut up (like when a field is tilled), deeply buried root fragments (8 inches deep) have weaker regrowth potential than plants derived from shallow root fragments (2 inches deep).¹

Reserves - Food reserves in the roots reach a minimum near the onset of hot weather when the shoots reach about 1 foot tall and then increase as energy flows from the shoots to the storage roots. Consequently, shoots should be removed for the first time by at least late spring.¹

Mechanical 

Surface Removal - Repeated removal of the shoots before they attain several leaves will exhaust the storage roots within two years and eliminate the weed. Several studies found a 21-day weeding schedule was optimal. Since buds on the roots will continue to sprout well into the fall, persistence is required.¹

Mulch - the root reserves are sufficient to push the shoot through any amount of loose mulch, and Canada thistle growth will benefit from the soil moisture conserved by the mulching materials.¹

Tarping - A tough synthetic tarp can prevent shoots from reaching light. Transfer of energy into the dying shoots reduces vigor of the storage roots.¹

Tilling - Canada thistle is highly resistant to cutting, hoeing and even to deep tillage. A single cultivation event increases and disperses the population of propagating rootstock, thereby increasing the abundance of this species, so multiple cultivations or integration with additional suppression tactics are required for control. Shoot emergence from root fragments decreases in early fall, but production of underground shoots continues, suggesting that fall tillage could still be effective for reducing reserves.¹

Cultural

Cover Crops - Sudangrass and sorghum-sudan hybrids suppress Canada thistle. Summer mowing when Canada thistle is tall but not yet flowering can set it back (root reserves are near their minimum at this time). In Illinois, a sudangrass cover crop was mowed when it was several feet tall and thistle plants had 7-10 leaves, resulting in a 98% reduction in Canada thistle in a soybean crop the following year. Whereas neither buckwheat cover nor mowing alone reduced thistle in the soybean crop. Sudangrass regrows aggressively after mowing and releases an allelochemical (sorgoleone), thereby compounding the impacts of mowing on the weed.⁵

Pasture Crop - Competition from pasture species maintained low Canada thistle populations, but populations increased substantially when released from competition. Accordingly, high intensity, low frequency grazing over two to three years provided better control than high frequency grazing for a short duration.¹

Harvestable Crop - Both spring and fall sown grain crops tend to promote Canada thistle due to the long period in which the thistle shoots can grow without disturbance, but winter wheat is more competitive than spring grains. Because alfalfa is mowed several times per year over a period of several years, this crop is very useful for managing Canada thistle. Hay that is mowed only once per year is less helpful for managing this weed. On a Maryland organic farm, a program including two years of repeated summer cultivations followed by dense plantings of winter barley for haylage reduced a heavily infested field of Canada thistle by 76%, allowing transition to alfalfa followed by successful establishment of row crops with minimum Canada thistle populations after five years.¹

Biological

Birds - Goldfinches and other birds eat the seeds.¹

Insects - Four species have successfully established as biocontrol agents on this weed, including Canada Thistle Stem Weevil (Hadroplontus litura formerly Ceutorhynchus litura), Canada Thistle Gall Fly (Urophora cardui), Canada Thistle Bud Weevil (Larinus carlinae), and Rhinocyllus conicus, but their impact is typically limited overall.¹

Other insects being looked at are: Larvae of painted lady butterfly (Cynthia cardui) can completely eat the leaves of a Canada thistle plant. Larvae of Canada thistle midge (Dasypeura gibsoi) eat the seeds, and larvae of Orellia ruficauda attack the flowering heads. The shoot-boring weevil, Apion onopordi, infested and reproduced in Canada thistle shoots but had little effect on thistle performance. None of these organisms is sufficient to control Canada thistle alone.¹

Fungus - The Phoma macrostoma fungus is locally systemic in stems, but not roots, of Canada thistle in England and Canada, and causes a bleaching disease. A particularly active strain of this fungus has been registered as a bioherbicide in Canada.¹

Bacteria - Pseudomonas syringae pv. tagetis also can cause dramatic bleaching symptoms.¹

Thistle Rust - The use of the fungal pathogen Puccinia punctiformis, thistle rust, has shown potential as a Canada thistle biocontrol agent. Thistle rust exists in temperate climates with its only known host Canada thistle (Cirsium arvense), where the pathogen lives as a systemic parasite in roots. Infected individuals develop dark red pustules on the leaves and stem, causing stem dwarfing and tissue death, although plants can remain asymptomatic for one to two years after inoculation.⁶

In early spring, diseased Canada thistle shoots emerge that appear unusually tall, sparse, and are covered with yellow speckling (spores) on the underside of the leaves. During this time the diseased stems emit a sweet floral fragrance. In late spring to early summer, diseased shoots will cross with other nearby diseased shoots and the spores on leaf tissues will turn a rusty red-brown color. Spores from diseased shoots from the spring will infect neighboring Canada thistle stems throughout the summer via wind-blown spores. Finally, during late summer or fall, diseased stems die and the leaf tissue falls on late-season shoots that allow the fungus to quickly move to the roots where it overwinters. In past trials, the least effective case was a 45% reduction of thistles over five years, while the best case was 100% after 18 months… largely dependent on the current density of thistle and surrounding vegetation.²

The rust fungus does not spread far which limits its efficacy as a natural biocontrol method.⁶ The rust fungus is so effective at killing its host plant, it spreads slowly to ensure new thistles will be present to infect in later years.² For do-it-yourselfers the fungus can be spread to uninfected patches following these steps. First, identify diseased plants in the spring. Look for thistle plants that are yellowing and have black or yellow spores on the leaves. Infected plants at this stage will have a fragrance similar to thistle flowers. Mark infected plants with a flag or stake. In mid- to late-summer return to the marked sites and look for surrounding plants with dark red (rust-colored) spores. Collect the above ground material and put it in a paper bag. Let it thoroughly air dry and prevent it from rotting, then grind it up (could use a blender). Save this ground-up, infected thistle in a cool, dark, and dry place until fall when the temperatures begin to cool.  Apply a teaspoon to healthy rosettes and mist it with water from a spraybottle when the weather is stable, temperatures are between 50-65° F and there is potential for high relative humidity but no rain.⁶ Do not be concerned if there appear to be very few diseased plants after rust spore application. The real killing work is done in the root system, so above-ground symptomatic stems are typically not an accurate representation of the infection rate overall.² To see the step by step process, you can watch this video from MSU https://www.youtube.com/watch?v=QukA6h3xFas.⁶

Sources:

1. Mohler, Charles L. et al. “Canada Thistle”, Sustainable Agriculture Research and Education Outreach, 2021 https://www.sare.org/publications/manage-weeds-on-your-farm/canada-thistle/ Accessed June 2026

2. “Canada Thistle Biocontrol”, Colorado Department of Agriculture https://ag.colorado.gov/conservation/biocontrol/canada-thistle Accessed June 2026

3. Pennington, Lawrence R. et al. "Canada Thistle Charactistics and Control", Agricultural Experiment & UI Extension Publications, October 1966, University of Idaho Library Digital Collections, https://www.lib.uidaho.edu/digital/uiext/items/uiext33135.html Accessed June 2026

4. McKay, Hugh C. et al. "Control Canada Thistle for Greater Profits", Agricultural Experiment & UI Extension Publications, December 1959, University of Idaho Library Digital Collections, https://www.lib.uidaho.edu/digital/uiext/items/uiext18114.html Accessed June 2026

5. “Weed Management for Canada Thistle”, Organic Farming Research Foundation, 25 May 2022 https://ofrf.org/news/weed-management-for-canada-thistle/  Accessed June 2026

6. Seipel, Tim et al. “Thistle Rust: A Potential Biocontrol Agent to Help in the Management of Canada Thistle”, Montana State University https://www.montana.edu/extension/ipm/cropweeds/research/canada-thistle-rust-fungus.html Accessed June 2026