By Scott Mcintaggart, Master Gardener In-Training, SCMG
Scientific Name: Cirsium arvensis Family: Asteraceae
Common Names: Canada thistle, Creeping thistle, California thistle, Field thistle, Cursed thistle
Canada Thistle is not Canadian! Originally from Northern Africa, Western Asia and Europe, it was introduced to North America in the early 1600’s as a seed contaminant. It is believed that it was one of the first weeds imported by ship. It is called Canada thistle because early residents of the New England States in the United States of America, blamed its emergence in North America on French traders from Canada.
Canada Thistle is an invasive broadleaf perennial weed that grows along roadsides, stream banks, in pastures, cultivated fields, on lawns and in gardens. Once established, it firmly anchor’s itself by establishing a tap root that can grow 6 to 10 feet deep. The thistle then spreads sideways by establishing a creeping horizontal root system (rhizomes) that often grow an additional 12 feet per year. The mature plant can grow up to three feet tall and produces beautiful purple, pink or white flowers that bloom from June to October. Each of the flowers can produce up to 5,000 seedlings annually. Spread by wind, these seedlings can remain viable in the soil for 20 years prior to germination.
Due to the plant’s aggressive nature, it can quickly infest large portions of land. This infestation replaces native plants, crowds out forage grasses used by grazing animals in pastures, and reduces use of fields for crop production. If left unchecked, Canada thistle will completely dominate and leave land useless.
Canadian thistle is classified as a noxious weed (meaning harmful to crops and /or natural areas) in much of the USA and in six Provinces in Canada including Ontario. With this classification Canada thistle, by law, must be controlled. It can be managed by hand pulling (making certain that all of the extensive root system is extracted) and by repetitive mowing or cutting. If these methods prove ineffective use of a premium herbicide is warranted.
References:
Canada thistle (Cirsium arvense (Sunflower Family: Asteraceae)). Weed Gallery: Statewide Integrated Pest Management Program University of California Agriculture & Natural Resources. https://ipm.ucanr.edu/PMG/WEEDS/canada _thistle.html
Canada Thistle Factsheet (March 2019). Invasive Species Council of BC.
BCINVASIVES.CA/Info@BCINVASIVES.CA
Churchill, J. (March 13, 2023). What is Canada Thistle and How Do I Get Rid of It?
Vanover, L. (September 29, 2022). Should You Get Rid of Canada Thistle?https://www.birdssandblooms.com/gardening/gardening-basics/canad…
