Invasive Plant Species Control
Canada Thistle
(Cirsium arvense)
Native Substitutes:
- Blue giant hyssop (Agastache foeniculum)
- Purple prairie clover (Dalea purpurea)
- Canada tick trefoil (Desmodium canadense)
- Rough blazing star (Liatris aspera)
Description
Appearance: Perennial forb, 2-5' tall with slender grooved stems that branch only at the top. It has male and female plants.
Leaves: Alternate, smooth, oblong, tapering, and directly attached to the stem, deeply divided with prickly margins.
Flower: Numerous small purple flowers appear on top of the upper branched stems between June and September.
Seed: Small light brown seeds are tufted for wind dispersal. Seeds remain viable in the soil for over 20 years.
Root: Each plant has a fibrous taproot with wide spreading horizontal roots. Each small section of root can form a new plant, enabling the species to spread vegetatively.
Ecological Threat
Canada thistle invades natural areas such as prairies, savannas, glades and dunes if some degree of disturbance already exists. It also invades wet areas with fluctuating water levels such as streambanks, sedge meadows and wet prairies. Once it has established itself it spreads quickly, replacing native plants and diminishing diversity. It grows in circular patches spreading vegetatively through horizontal roots which can spread 10-12' in one season. Canada thistle occurs in Canada and throughout the northern U.S. from northern California to Maine and southward to Virginia.
Legal Status: Canada thistle has been declared a noxious weed in 43 states as one of the most tenacious and economically important agricultural weeds. This plant is a prohibited noxious weed in Minnesota and must be controlled or eradicated as required by Minnesota Statutes, section 18.78.
Control Methods
| Mechanical | Chemical | Biological |
|---|---|---|
| Late spring burns May/June are most detrimental, consecutively for 3 years | Spot application with glyphosate or clopyralid, a broadleaf selective herbicide | None |
| Mowing time is critical or will resprout and flower again; cut when flowerbuds are just about to open | Basal bark treatment on stems less than 3" | |
| Repeated pulling and cutting will weaken roots | ||
| Maintain a buffer around an infestation to prevent vegetative spread |
Sources
Wisconsin Manual of Control Recommendations for Ecologically Invasive Plants, 1997
Minnesota Department of Natural Resources, Restore Your Shore CD, 2001
