NOXIOUS WEEDS
Status: Introduced

CANADA THISTLE
(Cirsium arvense)

 Description

In the scientific name of Canada thistle, "arvense" is Latin for "of the field" or "arable field," which is where Canada thistle is commonly found. The relevance of the word "Cirsium" is more obscure. It is from Greek meaning, "a swollen vein." This term probably refers to the fact that thistles, not necessarily this one, were used in medical treatments. 

Characteristics of the plant include smooth, green, spine-tipped leaves with crinkled margins. The leaves arise alternately along the stems. Growing 2 to 5 feet (0.6 to 1.5 m) in height, the Canada thistle has many flowers on branching heads, about 3/4 inch (1.9 cm) in diameter. The color of the flowers ranges from lavender to pink. Canada thistle is often confused with other common thistles. Some thistles have a white cottony material on their leaves and stems, while the Canada thistle is dark green in color with smaller leaves than other thistles.
 Distribution 

Canada thistle, despite the name, was probably introduced into North America in the 18th century as a seed contaminant from
Eurasia. It is native of southeastern Eurasia and northern Africa. Today it is found throughout South Dakota and is estimated to infest nearly 2 million acres of land. This prickly weed grows in all cultivated crops, pasture, range, woodland, and wetland habitats.

The Canada thistle in South Dakota is considered a noxious weed. Unlike other common thistles that are biennial, Canada thistle is a perennial plant with pink to lavender flowers that bloom in June and July. It is unique among the true thistles in that male and female flowers are located on different plants.

Therefore, flower pollination is dependent on wind and insects. Seeds are only about 1/8 inch (0.3 cm) long, dark brown and are attached to a tannish down called pappus. This down allows the seeds to be easily scattered by the wind. The root system of the Canada thistle can reach 10 to 15 feet (3 to 4.6 m) deep. Canada thistle sends out horizontal roots, which classifies it as a "creeping" perennial weed. Horizontal roots and the capability to quickly produce seeds, give the plant the ability to spread quickly. Each Canada thistle produces up to 680 seeds, which can mature in 7 to 10 days after the first flower. One plant, left undisturbed, can multiply to cover a one-half acre patch within 3 years.

Management Considerations

Early detection and control of Canada thistle is essential, especially during times of abundant moisture. One method of control is by the cultivation of a crop such as alfalfa. The repeated cuttings help control the weed. Another method is the introduction of a biological control agent. With this method, the natural control agent of Canada thistle, in many cases an insect, is introduced. In South Dakota the use of biological control is just in the experimental stage.

The South Dakota Weed and Pest Control Commission has the legal responsibility for developing and implementing a statewide control program for all noxious weeds. South Dakota law requires that all land owners control noxious weeds on their land. If land owners do not comply with noxious weed control requirements, fines, tax levies, and liens against their land could result.

Glossary


Biennial - a plant that requires two years to complete growth.
Noxious weed - designation of the State Weed Control Board. They are weeds that have become difficult to control once they are established.
Perennial - a plant that lives more than two years.

References

Agricultural Extension Service, South Dakota State University South Dakota Weeds, 1974. South Dakota Weed Control
Commission.
Cooperative Extension Service, South Dakota State University, U.S. Department of Agriculture, Canada Thistle Bulletin FS 896 CT. SDSU Cooperative Extension Service.
Klingman Glenn C., Weed Science 1982 John Wiley & Sons Inc. NY.
U.S. Department of Agriculture. Noxious Weeds of South Dakota Extension Special Series 34. SDSU Cooperative Extension
Service.
Zimdalhl, Robert L., 1989. Weeds and Words: the Etymology of the Scientific names of Weeds and Crops. Iowa State University Ames.

Selected Resources For Teachers

Noxious Weeds of South Dakota , Extension Special Series 34, 1993. Cooperative Extension Service, SDSU, Brookings, SD 57007.
South Dakota/Nebraska Weed Identification Guide, 1994. Black Hills RC&D Office, 515 9th Street, Rapid City, SD 57701-2663.
 

Written by:
John Neff, USDA Americorp member, Fort Pierre, SD. 57532. 1996.
 

Reviewed by:
Leon Wrage, Extension Weed Specialist, SDSU, Brookings, SD.

Publication of the Canada Thistle fact sheet was funded by the Northern State University CUEST Center for Environmental
Education, Aberdeen, SD.