Terrestrial invasive species

Nature

Most of these invasive plant factsheets are created from the booklet Minnesota invasive non-native terrestrial plants, an identification guide for resource managers.

Check the additional resources and herbicides table for more information.

Canada thistle (Cirsium arvense)


 

Description:

Appearance: Perennial herbaceous plant, 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.

Flowers: Numerous small purple flowers appear on top of the upper branched stems between June and September.

Seeds: Small light brown seeds are tufted for dispersal by the wind. Seeds remain viable in the soil for over 20 years.

Roots: Each plant has a fibrous taproot with wide spreading horizontal roots. Each small section of root can form a new plant enabling the plant 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, diminishing diversity. It grows in circular patches spreading vegetatively through horizontal roots which can spread 10 -12' in one season.
  • Canada thistle occurs throughout the northern U.S. from northern California to Maine and southward to Virginia and in Canada.
  • It has been declared a noxious weed in 43 states as one of the most tenacious agricultural weeds.
  • Canada thistle is a MDA Prohibited noxious weed (Controlled List) in Minnesota.

 

Control Methods:

Mechanical

Repeated pulling and mowing will weaken roots, mowing when flower buds are just to open

Late spring burns May/June are most detrimental, but also stimulate seed germination; burn consecutively for 3 years

Chemical

Spot application with glyphosate or with selective herbicide clopyralid, or metsulfuron

Biological

Stem weevil, bud weevil and stem gall fly are commercially available

 

 

Native Substitutes:

Additional Resources