Cirsium arvense (L.) Scop.

Cirsium arvense (L.) Scop.
Canada Thistle

Category
Forb/Herb

Description

Height

Canada Thistle grows to 4 feet (1.2 m).

Stem

The plant stem is rigid and hairy.

Leaves

Leaves are lance-shaped, irregularly lobed 2 to 6 inches (5 to 15 cm) in length with prickly margins.

Flowers

The flowers are purple to white in color and up to 0.5 inch (1.8 cm) in diameter. Flowering occurs in late June to August.

Fruit

Small fruit, called achenes, are 1 to 1.5 inches (2.5-3.8 cm) long. They have a feathery pappus which enables them to be dispersed by the wind.

Images

Photo:Dan Tenaglia, Missouriplants.com, Bugwood.org
More images of Cirsium arvense

Life History

Canada Thistle is a perennial herbaceous plant that grows to 4 feet (1.2 m) tall. Spiny, tall, erect thistle is a member of Asteraceae. Unlike other thistles the male and female flowers appear on separate plants in the Canada thistle. (Male and female parts may appear together, but only one sex is fertile.) This dioecious, weedy perennial occurs in patches, commonly in disturbed areas. Most seeds germinate within one year. Some seeds immediately produce rosettes before winter and emerge to flower the next spring. Seeds remain viable in soil up to 20 years in some cases.

Weed control legislation for Canada thistle was passed as early as 1795 in Vermont and 1831 in New York. Canada thistle is a problem even in Europe where natural enemies abound. Once established it spreads rapidly and is hard to remove.

Canada thistle is distinguished from other thistles (Cirsium spp.) by its deep-running perennial rootstocks, more slender stems, and small compact heads.

Habitat

Canada Thistle invades open areas such as prairies, savannas, fields, pastures, wet meadows and open forests. It can form dense stands that shade out and displace native vegetation. The plant does not do well in tight and healthy ecosystems.

Origin and Distribution

It is native to Europe and Asia and was first introduced by accident in the 1600s to North America. It is an invader of open areas such as prairies, savannas, fields, pastures, wet meadows and open forests throughout parts of North America. Introduction to new areas occurs mostly by windborne seed or sometimes by run-off in ditches. Seeds can be blown a half mile in the wind. It spreads rapidly by rhizomes or root segments.
Other States Where Invasive: AK, AL, AZ, CA, CO, CT, DE, IA, ID, IL, IN, KS, KY, MD, MI, MN, MO, MT, NC, ND, NE, NM, NV, OH, OK, OR, PA, SD, TN, WA, WI, WY.
Federal or State Listed as Noxious Weed, Prohibited, Invasive or Banned: AK, AZ, AR, CA, CO, CT, DE, HI, ID, IL, IN, IA, KA, KY, MD, MI, MN, MO, MT, NE, NV, NM, NC, ND, OH, OK, OR, PA, SD, UT, WA, WI, WY.

Sources

"Cirsium arvense." Species Abstracts of Highly Disruptive Exotic Plants at Effigy Mounds National Monument. Northern Prairie Wildlife Research Center: http://www.npwrc.usgs.gov/resource/othrdata/explant/cirsarve.htm

Erich Haber. "Canada Thistle Fact Sheet." Invasive Exotic Plants of Canada Fact Sheet No. 8: http://infoweb.magi.com/~ehaber/facthstl.html

Max Hutchinson. "Canada Thistle." Vegetation Management Guideline. Vol. 1, No. 7. Approved 02/06/90. Illinois Nature Preserves Commission. http://www.inhs.uiuc.edu/edu/VMG/cthistle.html

Management Recommendations

Biological Control

Five European insects (Ceutorhynchus litura, Rhinocyllus conicus, Altica carduorum, Lema cyanella and Urophora cardui) have been introduced into this continent to help control Canada thistle. Only the first of those has become established and begun to suppress the plant. A type of fungus (Puccinia spp.) weakens the thistles enough to make them more susceptible to 2,4-D and more susceptible to other insects. Thistles treated with this fungus and a weevil (Ceutorhynchus litura) had a 50% increase in damage over untreated plants. Canada thistle can be partly controlled with the aid of "smother crops" which develop earlier in the season, forming a dense cover to shade thistle seedlings. Smother crops are most effective if combined with regular cuttings, such as alfalfa, or forage grasses. Cutting Canada thistle on a regular basis depletes carbohydrate food reserves in the roots. Seeding intermediate wheat grass and tall fescue reduced Canada thistle density by 60 percent when combined with mowing. Sugar beets, buckwheat and small grains have also been planted as smother crops.

Mechanical Control

Cutting Canada thistle on a regular basis depletes carbohydrate food reserves in the roots. Seeding intermediate wheat grass and tall fescue reduced Canada thistle density by 60 percent when combined with mowing.

Herbicidal Control

For small infestations, spot applications of glufonsinate-ammonium (Finale®) or glyphosate (Roundup®) are effective means of control, especially early in the season. These are non-selective herbicides, so care should be taken to avoid hitting non-target plants. The herbicides can be dribbled on the plants for wiped on with a sponge, but be sure to wear rubber gloves and goggles. Large infestations of Canada thistle require selective herbicides to avoid killing desirable plant species. The herbicide 2,4-D amine is effective during the bud to flower stage when root carbohydrate reserves are low. Triclopyr and dicamba herbicides may be more effective, but also more persistent in the soil. These herbicides can migrate into the groundwater, especially in sandy soils. Spraying should ideally be done when there is plenty of moisture for growth, otherwise the herbicides may not translocate well through the plants. Also note that there are many varieties of Canada thistle, some with resistance to certain herbicides. If one herbicide fails to get results then consider trying another. Nitrogen fertilizer may be added to stimulate better translocation of the chemicals.