United States Department of Agriculture
NATURAL RESOURCES CONSERVATION SERVICE
Invasive Species Technical Note No. MT-5
Ecology and Management of Canada thistle [
Cirsium arvense (L.) Scop.]
Jim Jacobs, NRCS Invasive Species Specialist, Bozeman, MT
Joanna Sciegienka, Graduate Research Assistant, Montana State University, Bozeman, MT
Fabian Menalled, Extension Cropland Weeds Specialist, Montana State University, Bozeman, MT
A member of the Aster family, Canada thistle is a vigorous, highly competitive species.
in a large range of habitats including croplands, ditch banks and riparian areas, gardens and pastures,
this category 1 noxious perennial weed is particularly hard to control because of its deep, creeping,
reproductive root system forming colonies.
In general, infestations start on disturbed ground, with
plants being able to colonize 10 to 12 feet per year.
Canada thistle can grow in a variety of habitats,
but it is best adapted to deep, well-aerated and productive soils. It prefers sunny and warm areas
with 15 to 30 or more inches of precipitation or irrigation per year, but it can grow on dryer cropland
and pasture sites with 12 to 13 inches of precipitation per year.
When temperatures exceed 85º F for
extended periods of time, it stops growing.
Canada thistle threatens productivity in both crop and non-croplands.
In cropland, Canada thistle
causes extensive yield losses through competition for light, nutrients, and moisture.
It also increases
harvesting problems due to seed and forage contamination.
In Montana, it is estimated that two
shoots per square yard can reduce wheat yield by 15 percent and 25 shoots per square yard can
reduce wheat yield by 60 percent.
Other Montana crops seriously threatened by Canada thistle
include peas, corn, beans, alfalfa and sugar beets.
Heavy infestations are also commonly found in
overgrazed pastures and ranges and may crowd-out and replace native grasses and forbs, decreasing
species diversity in an area.
Perennial forage plants and winter annual cereals compete most effectively with Canada thistle
because they emerge early in the growing season and inhibit the emergence of late-emerging
Canada thistle shoots.
Cultivation increases Canada thistle in most situations.
translocate and distribute into the root system including aminopyralid, clopyralid, or picloram
applied when plants are in the rosette to bolting growth stage or in the fall, provide temporary
suppression of Canada thistle.
Biological control has not been effective in reducing Canada
thistle populations; however, two insects (
) and two
Pseudomonas syringae pv. tagetis
) have the highest
potential for control.
Sustainable management is most likely where cultural practices that
encourage desirable competitive plants are integrated with herbicidal and biological control