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Plant Conservation Alliance®s Alien Plant Working Group 

Xeeds HoneXild: Alien Plant Invaders of Natural Areas 

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FACT SHEET: CANADA THISTLE

Canada Thistle 

Cirsium arvense (L.) Scop. 

Sunflower family (Asteraceae) 

NATIVE RANGE 

Temperate regions of Eurasia 

DESCRIPTION 

Canada thistle is an herbaceous perennial with erect stems 

11⁄2-4 feet tall, prickly leaves and an extensive creeping 

rootstock. Stems are branched, often slightly hairy, and 

ridged. Leaves are lance-shaped, irregularly lobed with 

spiny, toothed margins and are borne singly and alternately 

along the stem. Rose-purple, lavender, or sometimes white 

flower heads appear from June through October, generally, 

and occur in rounded, umbrella-shaped clusters. 

The small, dry, single-seeded fruits of Canada thistle, called achenes, are 1-11⁄2 inches long and have a feathery structure 

attached to the seed base. Many native species of thistle occur in the U.S., some of which are rare. Because of the 

possibility of confusion with native species, Canada thistle should be accurately identified before any control is attempted. 

ECOLOGICAL THREAT 

Natural communities that are threatened by Canada thistle include non-forested plant communities such as prairies, 

barrens, savannas, glades, sand dunes, fields and meadows that have been impacted by disturbance. As it establishes 

itself in an area, Canada thistle crowds out and replaces native plants, changes the structure and species composition of 

natural plant communities and reduces plant and animal diversity. This highly invasive thistle prevents the coexistence of 

other plant species through shading, competition for soil resources and possibly through the release of chemical toxins 

poisonous to other plants. 

Canada thistle is declared a "noxious weed" throughout the U.S. and has long been recognized as a major agricultural 

pest, costing tens of millions of dollars in direct crop losses annually and additional millions costs for control. Only recently 

have the harmful impacts of Canada thistle to native species and natural ecosystems received notable attention. 

DISTRIBUTION IN THE UNITED STATES 

Canada thistle is distributed throughout the northern U.S., from northern 

California to Maine and southward to Virginia. It is also found in Canada, for 

which it was named. Canada thistle has been identified as a management 

problem on many national parks and on preserves of The Nature Conservancy 

in the upper Midwest, Plains states, and the Pacific northwest. 

HABITAT IN THE UNITED STATES 

Canada thistle grows in barrens, glades, meadows, prairies, fields, pastures, 

and waste places. It does best in disturbed upland areas but also invades wet 

areas with fluctuating water levels such as streambank sedge meadows and 

wet prairies. 

BACKGROUND 

Canada thistle was introduced to the United States, probably by accident, in the early 1600s and, by 1954, had been 

declared a noxious weed in forty three states. In Canada and the U.S., it is considered one of the most tenacious and 

economically important agricultural weeds, but only in recent years has it been recognized as a problem in natural areas. 
20 May 2005 Page 2 of 3 

Plant Conservation Alliance®s Alien Plant Working Group 

Xeeds HoneXild: Alien Plant Invaders of Natural Areas 

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BIOLOGY & SPREAD 

Canada thistle produces an abundance of bristly-plumed seeds which are 

easily dispersed by the wind. Most of the seeds germinate within a year, but 

some may remain viable in the soil for up to twenty years or more. Vegetative 

reproduction in Canada thistle is aided by a fibrous taproot capable of sending 

out lateral roots as deep as 3 feet below ground, and from which shoots sprout 

up at frequent intervals. It also readily regenerates from root fragments less 

than an inch in length. 

MANAGEMENT OPTIONS 

Management of Canada thistle can be achieved through hand-cutting, mowing, 

controlled burning, and chemical means, depending on the level of infestation 

and the type of area being managed. Due to its perennial nature, entire plants 

must be killed in order to prevent regrowth from rootstock. Hand-cutting of 

individual plants or mowing of larger infestations should be conducted prior to 

seed set and must be repeated until the starch reserves in the roots are 

exhausted. Because early season burning of Canada thistle can stimulate its 

growth and flowering, controlled burns should be carried out late in the growing 

season for best effect. 

In natural areas where Canada thistle is interspersed with desirable native 

plants, targeted application of a systemic herbicide such as glyphosate (e.g., 

Roundup® or Rodeo®), which carries plant toxins to the roots, may be 

effective. For extensive infestations in disturbed areas with little desirable 

vegetation, broad application of this type herbicide may be the most effective 

method. Repeated applications are usually necessary due to the long life of seeds stored in the soil. 

USE PESTICIDES WISELY: Always read the entire pesticide label carefully, follow all mixing and application instructions and wear all 

recommended personal protective gear and clothing. Contact your state department of agriculture for any additional pesticide use 

requirements, restrictions or recommendations. 

NOTICE: mention of pesticide products on this page does not constitute endorsement of any material.

CONTACTS 

For more information on the management of Canada thistle, please contact: 

• Daniel Roddy, National Park Service, Wind Cave National Park, SD; dan_roddy at nps.gov 

• U.S. Geological Survey, Biological Resources Division, Flagstaff, AZ 

http://www.nbs.nau.edu/FNF/Vegetation/Exotics/Cirsium/cirsiumarvense.html 

OTHER LINKS 

• http://www.invasive.org/search/action.cfm?q=Cirsium%20arvense 

• http://nbii-nin.ciesin.columbia.edu/ipane/icat/browse.do?specieId=46

AUTHORS 

Gwendolyn Thunhorst, The Nature Conservancy, Arlington, VA 

Jil M. Swearingen, National Park Service, Washington, DC 

PHOTOGRAPH 

John M. Randall, The Nature Conservancy, Davis, CA 

REFERENCES 

Evans, J.E. 1984. Canada thistle (Cirsium arvense): a literature review of management practices. Natural Areas Journal 

4(2):11-21. 

Hutchison, M. 1992. Vegetation management guideline: Canada thistle (Cirsium arvense (L.) Scop.). Natural Areas 

Journal 12(3):160-161. 
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Plant Conservation Alliance®s Alien Plant Working Group 

Xeeds HoneXild: Alien Plant Invaders of Natural Areas 

http://www.nps.gov/plants/alien/

Swearingen, J. 2009. WeedUS Database of Plants Invading Natural Areas in the United States: Canada Thistle (Cirsium 

arvense). http://www.invasive.org/weedus/subject.html?sub=2792. 

The Nature Conservancy. Canada Thistle: Element Stewardship Abstract. In: Wildland Weeds Management & Research 

Program, Weeds on the Web.

USDA, NRCS. 2009. The PLANTS Database (http://plants.usda.gov). National Plant Data Center, Baton Rouge, LA 

70874-4490 USA.