Creeping Thistle
Cirsium arvense
Washington State Classification
Class C — Widespread, County-Selected Control
Once established, it spreads quickly replacing native plants.
Quick Reference
Identification
Growth Habit
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.
Leaves
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.
Flowers
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).
Fruit & Seeds
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.
Impact
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.
Ecology & Spread
Habitat
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.
Spread Mechanisms
Reproduction
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.
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Control Methods
Mechanical
- 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.
Cultural / Prevention
- Inorganic mulches, such as plastic, commercial “weed barrier” fabrics and other materials such as roofing paper, is an effective weed management option. Cover inorganic mulches with a thin layer of soil or organic mulch.
Biological Control
- 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.