Canada thistle competes directly with native plant species, decreasing species diversity and altering habitat structure. An economically significant agricultural weed, Canada thistle displaces forage species, decreases crop yield, and serves as a host to bean aphid, stalk borer, and sod-web worm, which cause damage to corn and tomato plants. Canada thistle has a large rhizomatous root system, allowing it to spread quickly and making eradication efforts difficult.
Canada thistle is native to Europe and Asia. It was introduced to the northeastern United States in the 1600s and is considered a noxious weed in most states. Possession with the intent to sell, import, purchase, transport or introduce Canada thistle is prohibited in New York State.
Canada thistle can be distinguished from other thistles by its creeping horizontal roots. It is not shade tolerant and therefore is found in open disturbed areas such as abandoned lots and fields, pastures, and cleared forests. It presents a problem in prairies and riparian habitats.
Canada thistle reproduces sexually by seed and vegetatively by creeping roots. Flowering is incited by 15 hour day length, occurring from June to August. A Canada thistle shoot will produce and average of 32 to 69 purple to pink to white flowers. Canada thistle is dioecious, a distinguishing factor from other thistles. Male and female plants must be within a few hundred yards of each other for pollination and seed set to successfully occur. Female flowers are fragrant and flask-shaped while male flowers are smaller, more globular in shape, and not fragrant. Its flowers are primarily pollinated by honeybees. Up to 26% of male plants are self-fertile hermaphrodites that will occasionally produce seed. Seeds are released 2-3 weeks following pollination. The seeds feature an aerodynamic pappus, facilitating wind dispersal for up to several kilometers. Most seeds germinate the following spring.