Canada Thistle

Canada thistle is an herbaceous perennial in the Asteraceae (daisy/aster) family. It produces erect stems that may reach 3 to 5 feet tall. These stems are smooth or slightly hairy and may be branched toward the top. Leaves occur alternately on the stem, but leaf color and shape vary widely from light to dark green, oblong to lance-shaped, and lobed or smooth. Spines frequently occur at points along the leaf margin. Canada thistle flowers from early to late summer. Flowers occur in clusters at the ends of stems and branches. Flowers are composite flowers consisting of clusters of light pink to deep purple, and occasionally white, petal-like flowers protruding from an involucre (a bulb shaped base covered in bracts). Canada thistle has an extensive rhizomatous root system reaching up to 15 feet in depth and 15 feet wide. Seeds are small, light to dark brown, and have tufts of hairs or bristles that allow the seeds to be wind dispersed.

Canada thistle is native to Eurasia. The date of introduction to North America is unknown with records indicating possible introduction in the 17th or 19th century, likely as a seed contaminant. It creates dense infestations by spreading rhizomes and seeds and spreads to new locations primarily by wind-dispersed seeds. Canada thistle can be found in a variety of areas including riparian and damp areas, roadsides, forests, rangeland and pastures, gardens, landscapes, and lawns. Canada thistle is currently found on every continent except Antarctica.

Prevention strategies and cultural control strategies should be utilized as much as possible.

There are several biological control agents available for Canada thistle. Agents in Wyoming include rust fungus (Puccinia punctiformis), gall forming flies (Urophora cardui), and weevils including stem-miners (Hadroplontus litura) and seedhead feeders (Rhinocyllus conicus).

Hand pulling and digging out of rhizomes is not recommended unless conducted every 2 to 3 weeks throughout the growing season because rhizome fragments are capable of resprouting. Mowing at regular, 3 to 4 week intervals can effectively deplete food stores in roots.

Larger infestations can be controlled with herbicides. Early to mid-summer treatments prior to bloom can be effective; fall treatments with systemic herbicides can also effectively kill the root stock. Read the label before using any herbicide. Contact TCWP if you have any questions about application rates or how to use an herbicide.

• Canada thistle (2013) DiTomaso, J.M., G.B. Kyser et al. Weed Control in Natural Areas in the Western United States. Weed Research and Information Center, University of California.
• King County Noxious Weed Alert: Canada Thistle (2018). King County Noxious Weed Control Program, King County, Washington.
• Canada Thistle Fact Sheet (2013) Beck, K.G. Colorado State University Extension.