HPIPM:Canada thistle

Taxonomy

Domain	Eukarya
Kingdom	Plantae
Phylum	Magnoliophyta
Class	Magnoliopsida
Superorder	Asteranae
Order	Asterales
Family	Asteraceae
Subfamily	Carduoideae
Tribe	Cardueae
Subtribe	Carduinae
Genus	Cirsium

Scientific Name

Cirsium arvense

Scientific Name Synonyms

Serratula arvensis

Cirsium setosum

Cirsium incanum

Breea incana

Breea arvensis

Carduus arvensis

Common Name

Canada thistle

Canada thistle (Cirsium arvense)

Author: George Beck, Colorado State University Cooperative Extension

Adapted from the Colorado State University Extension Natural Resources Fact Sheet "Canada Thistle"

Identification and Life Cycle

Canada thistle (Cirsium arvense) is a creeping perennial in the sunflower family (Asteraceae). Flowers are usually purple but occasionally white and 1/2 to 3/4 of an inch in diameter. Seeds are about 1/8 of an inch long, flattened, brownish in color with a tuft of white hairs (pappus) at the top. Leaves are alternate, oblong or lance-shaped, and divided into spiny-tipped irregular lobes. Stems can reach 1 to 4 feet tall. Canada thistle has an extensive creeping root system. Seedlings form a rosette with irregularly lobed spiny leaves.

Habitats

Canada thistle invades crops, pastures, rangeland, roadsides, and noncrop areas. Generally, infestations start on disturbed ground, including ditch banks, overgrazed pastures, tilled fields, or abandoned sites. The weed grows in a variety of soils and can tolerate up to 2% salt content. It is most competitive in deep, well-aerated, productive, cool soils. It usually occurs in 17- to 35-inch annual precipitation zones or where soil moisture is adequate. It is less common in light, dry soils.

Impacts

Canada thistle can form dense monocultures, displacing desirable vegetation and reducing available forage for livestock and wildlife. It also has a large economic impact due to its impact on crop yields and control costs.

Biology and Ecology

Canada thistle develops from seed or vegetative buds in its root system. Horizontal roots may extend 15 feet or more and vertical roots may grow 6 to 15 feet deep. Canada thistle emerges from its root system in mid- to late spring and forms rosettes. The greatest flush of root-derived plants occurs in spring, but another flush occurs in fall. A flush can occur anytime during the growing season when soil moisture is adequate. This is particularly a problem when Canada thistle growth is disturbed by tillage or herbicides. Seedlings emerge at about the same time as root-derived shoots. Seedlings grow slowly and are sensitive to competition, particularly if shaded. Canada thistle seedlings develop a perennial habit (the ability to reproduce from their root systems) about 7 to 8 weeks after germination. Canada thistle begins to flower in late spring to early summer in response to 14- to 16-hour days. Plants are male or female (dioecious) and grow in circular patches that often are one clone and the same sex. Canada thistle develops seed sparingly. It may produce 1,000 to 1,500 seeds per flowering shoot. Generally, vegetative reproduction from its root system contributes to local spread, and seeds contribute to long distance dispersal. Seed can remain viable in soil up to 20 years, and deep burial promotes seed longevity. Canada thistle allocates most of its reproductive energy into vegetative propagation. New shoots and roots can form almost anywhere along the root system of established plants. Tillage segments roots and stimulates new plants to develop. Shoots emerge from root and shoot pieces about 15 days after disturbance by tillage.

Management Approaches

The key to Canada thistle control is to stress the plant and force it to use stored root nutrients. Canada thistle can recover from almost any stress, including control attempts, because of root nutrient stores. Therefore, returning infested land to a productive state occurs only over time. Success requires a sound management plan implemented over several years.

Biological Control

Three biological control agents have been released to control Canada thistle. The weevil, Ceutorhyncus litura, lays eggs underneath Canada thistle leaves in early spring. Larvae bore into the main leaf vein, then down into the plant's crown area. If the weevil population is high enough, plant death can occur, otherwise Canada thistle is stressed and less vigorous. C. litura alone will not effectively control Canada thistle. It must be combined with other methods to be successful Such as Combining the weevil with cultural techniques that allow for maximum desirable plant competition. The Canada thistle stem gall fly, Urophora cardui, lays eggs on apical meristems of developing shoots. Larvae burrow into shoots, and their feeding triggers large galls to form that stress the plant, perhaps killing it. Galls that form near the terminal meristems (e.g., where flowers develop) keep the weed from flowering and reduce seed set. The Canada thistle flea beetle, Altica carduorum, feeds on the leaves of Canada thistle but has not established successfully.

Mechanical and Cultural Control

Mowing alone is not effective unless conducted at one-month intervals over several growing seasons. Combining mowing with cultural and chemical control can be effective. Mowing at hay cutting stimulates new Canada thistle shoots to develop from its root system. In irrigated grass hay meadows, fall herbicide treatments that follow mowing can be an effective management system because more Canada thistle foliage is present after cutting to intercept herbicide. Additionally, root nutrient stores decrease after mowing because the plant draws on them to develop new shoots.

Chemical Control

Consult herbicide labels for additional rate, application, and safety information. Additional herbicide information can be found at http://www.greenbook.net.

Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.

References

For more information and images please visit IPM Bugwood. http://www.ipmimages.org/search/action.cfm?q=Canada%20thistle