Weed class: C
Year Listed: 1988
Native to: Europe and Asia
Is this Weed Toxic?:
not known to be
Why Is It a Noxious Weed?
Once established, it spreads quickly replacing native plants. It grows in circular patches, spreading vegetatively through roots which can spread 10 -12' in one season. It poses an economic threat to the agriculture industry by reducing crop yields.
How would I identify it?
Canada thistle is a rhizomatous, perennial, herbaceous plant. It grows 2 to 5 feet tall with slender grooved stems that branch only at the top. It has dense clonal growth of male and female plants.
Flowerheads many, in clusters at tips of branched stems. Flowerheads are 1/2 to 3/4 inch in diameter and bloom June to October. Bracts under flowerheads many, with some having a weakly spined tip. Flower petals purple to pink (sometimes white).
Leaves are alternately arranged and are lance shaped to narrowly oval. Margins range from smooth to spiny to somewhat toothed or lobed. Surfaces have variable hairs from densely to thinly hairy to no hairs present, with hairs typically on leaf undersides.
Stem shoots begin as a rosette and then grow upright stems after 2 to 4 weeks. Stems can be slightly hairy or without hairs. Stems are not winged as they are in the Carduus genus.
Fruit Seed Description
Seeds are around 0.16 inches (4 mm long) with tufts of bristles that help them disperse. Plants produce 1,500 seeds on average and may produce up to 5,300 seeds.
Where does it grow?
Canada thistle is common in pastures and croplands. It invades natural areas such as prairies and dunes that have disturbance. It also invades wet areas with fluctuating water levels such as stream banks, sedge meadows and wet prairies. Please click here to see a distribution map of Canada thistle in Washington.
How Does it Reproduce?
It reproduces by tufted seeds dispersed by the wind. They can remain viable in the soil for over 20 years. Each plant has a fibrous taproot with deep, wide spreading horizontal roots. Small pieces of root can also grow into new plants.
How Do I Control It?
General Control Strategy
Canada thistle is difficult to control once it is established on a site. One treatment or a combination of treatments may need to be used to control populations depending on the site.
Repeated tillage at 7 to 28 day intervals for up to 4 years can be effective on infestations of Canada thistle. Repeated mowing to weaken stems and prevent seeding is also effective in low level infestations. Frequently tilling may also reduce populations if continued for a few years.
Planting competitive crops, such as alfalfa and forage grasses can be very effective in controlling an infestation of Canada thistle.
Fly larvae of the stem gall fly Urophora cardui impact plant vigor in Canada thistle by inducing the plant to divert energy away from root and flower production to produce gall tissue in stems. Growth and flowering can be retarded, but this agent alone does not kill plants or prevent spread. For more information about the biological control of Canada thistle, please visit WSU Extension Integrated Weed Control Project.
For More Information
See our Written Findings for more information about Canada thistle (Cirsium arvense).
Control Options for Canada thistle from Whatcom County NWCB
Cowlitz County NWCB Fact Sheet on Canada thistle
Clallam County NWCB Fact Sheet on Canada thistle
Spokane County NWCB Fact Sheet on Canada thistle
King County NWCB Fact Sheet on Canada thistle
Jefferson County NWCB Fact Sheet on Canada thistle
Lincoln County NWCB Fact Sheet on Canada thistle
Clark County NWCB Fact Sheet on Canada thistle
Whatcom County NWCB Fact Sheet on Canada thistle
San Juan County NWCB Fact Sheet on Canada thistle
Franklin County NWCB Fact Sheet on Canada thistle