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Canada Thistle Identification

Updated on September 15, 2012
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Sean has been in the industry of gardening and landscaping since 2006. He is also a Certified Arborist that tends to focus on plant health.

Thistle flower heads
Thistle flower heads | Source

Identify Canada Thistle

Common Name
Canada Thistle

Botanical Name
Cirsium arvense

Canada thistle dispersing seed in autumn.
Canada thistle dispersing seed in autumn. | Source

Canada Thistle Characteristics

Canada thistle is an herbaceous perennial with upright, grooved, hairy stems that branch towards the top of the plant. It can grow up to 6 feet tall. Canada thistle is invasive and legally classified as a 'noxious weed' in many regions of the U.S., but land owners are not legally required to remove them.

Forest entrances, oak savannas, prairies, dunes, agricultural fields, pastures, roadsides, and disturbed soils.

Simple and lance-shaped. Irregular lobes with spiny, toothed edges. Alternate leaves are smooth when young and become hairy when mature.

Small, numerous flowers that are purple to pink in color. Flowers are terminal (at the tip of stem). Flowers bloom from June to September.

Fruit/Seeds: The seeds are small and light brown with a tuft of hair loosely attached to the tip. This tuft enables wind dispersal. Seeds are easily spread by mowing after flowering has begun.

Roots: The tough taproot can grow more than 6 feet deep. Roots can also produce clones of the mother plant when they reach the surface of the soil. Roots can grow up to 12 feet horizontally in a single year. Fragments of roots can also regenerate new plants.

Ecological Threats
Invasions can disrupt agriculture and prairies. Canada thistle can quickly take advantage of disturbed and exposed soils. Rapid spreading forms monospecific stands of dense thistles that crowd out native plants.

Thistle seed dispersal
Thistle seed dispersal | Source

Eradicate Canada Thistle

Eradication & Control
Mechanical eradication consists of pulling and removing as much of the taproot as possible to decrease root reserves. Mowing over thistles before flower buds bloom also reduce the thistle's ability to regenerate. Controlled burning of infested areas during late spring is effective in eliminating germination and clones. Burning should be done for 3 to 4 years. Established areas of turf and native lands suppress thistle growth as well. Mechanical eradication is very labor intensive.

Chemical eradication is most effective when using a foliar spray of glyphosate while the thistle is young, and preferably under 12 inches tall. Metasulfuron-methyl is a residual broadleaf herbicide that inhibits cell division in roots and new shoots. It has a very low toxicity to mammals, birds, fish, and insects when applied correctly. Always follow herbicide directions closely and use proper personal protective equipment.


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