Common Name: Canada thistle
Scientific Name: Cirsium arvense
Code: CIAR4
Group: Dicot
Family: Asteraceae
Growth Habit: Forb/Herb
Duration: Perennial

What does it look like?

Canada thistles have rigid stems which are smooth to slightly hairy and branched at the top, and they grow approximately 1 to 4 feet tall. The leaves extend directly from the stem, without a stalk, vary in color from light to dark green, and are oblong with spiny tips. The flowers can either be male or female, are 3/8 to 3/4 inch in diameter, and exist in several to many head clusters. Flowers are purplish lavender, and in rare cases, white. The seeds are 1/8 inch long, brownish, with a protrusion at one end and a tuft of hairs at the top. The extensive root system extends into the soil as deep as 10 feet and can then grow laterally 12 to 15 feet a year. It can sprout randomly placed root buds in the right conditions. Canada thistle flowers bloom July to August (BLM 1995).

The characteristics that distinguish Canada thistles from other thistles, especially bull thistle, are numerous flower heads at the end of the smoother stems that lack spiny wings. Flowers are smaller and spineless, and though similar in color to bull thistle, are a noticeably lighter shade of purple (Bossard et al. 2000).

Where does it grow?

This species is native to Europe, and like bull thistle, it tends to invade open, disturbed areas such as cropland, meadows, pastures, roadsides, clearcuts, and waste areas. Though it cannot survive in saturated soils, this species is commonly found in moist habitats such as meadows, ditches and along streambanks. Canada thistle thrives in open areas with good light conditions (Bossard et al. 2000).

Is it in our watershed?

Canada thistle grows extensively throughout California and in the King Range. It also grows throughout the Mattole watershed along roadsides, cleared fields and other sites with disturbed soils.

Why is it a problem?

Canada thistle is a perennial with an extensive underground root system that may penetrate the soil to a depth of 10 feet or more and grow laterally 12- 15 feet per year. This thistle propagates by both seed and root buds and is extremely difficult to eradicate due to its extensive root nutrient storage. One of the most serious pests to agricultural lands, it often outcompetes native plants and crops for vital nutrients and water supplies. Growing infestations of Canada thistle also reduce the amount forage area in pastures because most cattle will not graze near these plants.

How do you get rid of it?

Removal Techniques: Canada thistle is more difficult to control than Bull Thistle because it is a perennial and it has an extensive root system that must be removed or depleted to ensure successful eradication. Persistance and a combination of control techniques are vital to successful control and eradication projects for this thistle.

Manual Control: Cut or pull plants using gloves after the plants bolt but before the flowers go to seed, around June or July. This requires care and persistence to exhaust roots and nutrient supplies while preventing spread of seed.

Mechanical Control: Repeated mowing can prevent spread and protect desired species. Mowing every three weeks will weaken the plant, prevent flowering and seed production (Bossard, 2000).

Grazing: Studies in New Zealand show that goat grazing can work well in areas of thistle infestations. Cattle and sheep will only graze on young and tender Canada thistle plants. Continuous grazing can deplete root reserves (Bossard, 2000).

Habitat Manipulation: Alfalfa and grasses in pastures can outcompete Canada thistle populations where management favored new growth of these species.

Biological Control: A certain species of weevil, Ceutorhyncus litura, is currently in use as a biocontrol agent in Colorado. Urophora cardui, another biocontrol insect, is available from the Colorado Department of Agriculture.

Author:
Unity Peterson, Invasive Plant Program Coordinator, Mattole Restoration Council, Petrolia, CA.

Photographs:
Molly Elizabeth Bagley 2002.

References:
Bossard, CC., J.M. Randall and M. Hoshovsky. (eds.) 2000. Invasive Plants of California’s Wildlands. University of California Press, Berkeley, CA. Online version: http://groups.ucanr.org/ceppc/Invasive_Plants_of_California's_Wildlands/