Canada Thistle (Cirsium arvense L.)

Family: Asteraceae

Description and Variation: Canada thistle is a perennial herb with a deep-seated complex system of roots spreading horizontally which give rise to aerial shoots. The one to four foot tall stems are slender, green, and freely branched. The leaves are alternate, sessile, and deeply lobed. The leaf margins have stiff yellowish spines. The heads are many and relatively small. The plants are dioecious (all flowers on a plant are either male or female). The flowers are purple. The fruits are about 1/8 inch long, somewhat flattened, and brownish with an apical circle of long hairs, these eventually falling. Four varieties of C. arvense have been recognized based on variation in leaf characters, texture, vestiture, segmentation, and spinyness.

Economic Importance: Detrimental: C. arvense is found in virtually all crops, and is also found in pastures and rangelands where it reduces forage. The weed is an effective competitor for light, moisture, and nutrients and is thus able to reduce crop yields. C. arvense also serves as an alternate host for insects and pathogenic microorganisms that attack various crops.

Beneficial: C. arvense is regarded as a good source of nectar and pollen for honey bees. The shoots and roots are consumed by some people in Russia and by North American Indians.

Habitat: Climatic requirements: C. arvense is found in open mesophytic areas. It does best with a temperature range of 0-32 degrees Centigrade and a rainfall range of 400 mm - 750 mm per year. In northern Canada, it exists where winter temperatures of -27 to -35 degrees Centigrade are common. C. arvense can grow on a wide variety of soil types; clay loam, sandy loam, sandy clay, and sand dunes. It does poorly on wet soils without much aeration.  C. arvense is found in almost every plant community disturbed by man. It is common to roadsides, railway embankments, lawns, gardens, abandoned fields, sand dunes, agricultural fields, margins of forests, and waterways. It grows poorly in shaded conditions and produces few flowers.

Geographic Distribution: In North America it occurs approximately from latitudes 37degrees North to 59 degrees North. It does not survive in the southern United States. C. arvense is probably native to southeastern Europe and the eastern Mediterranean area. It now occurs throughout Europe, North Africa, Asia Minor, and across central Asia to Japan. It is also naturalized in South Africa, New Zealand, and southeastern Australia.

History: Canada thistle was probably introduced to North America by early colonists in the 17^th Century. Control legislation was enacted in Vermont in 1795 and by New York in 1831. It was not reported west of the Allegheny Mountains until after 1835.

Growth and Development: The survival and spread of this species is due to the highly successful vegetative propagation carried on by the creeping horizontal roots which survive winters and continue to give rise to numerous aerial shoots year after year. The plants can survive indefinitely through the root system. The shoots emerge in the spring when the mean weekly temperature reached five to eight degrees Centigrade. Different ecotypes vary as to when they emerge. After shoot emergence, rosettes develop, followed by rapid vertical growth for several weeks. Growth slows somewhat and flowering commences in early summer and continues for several months.

Reproduction: Since Canada thistle is dioecious, it is mainly insect pollinated. Many insect visitors have been reported. The average seed production is about 1,530 seeds per plant, but exceptional plants may produce up to 5,300 seeds. The mechanism for long distance seed dispersal is not known. Irrigation water is one possible means of transport and wind transport is another. Studies have shown that freshly collected seeds had germination rates of up to 95 percent. Different ecotypes of this species have different germination rates. Two year old seeds had a 38-71 percent germination rate. The seedlings of Canada thistle develop a fibrous taproot, and within a few months, the main root thickens and develops lateral roots. After growing 6-12 cm, the horizontal roots bend downwards, growing towards the water table. A new horizontal root develops at this point of bending and continues the horizontal spread. Aerial shoots develop from the original vertical root or from buds on the arching branches of the horizontal system. The weedy nature of this plant is also due to the ability of the root to regenerate from small pieces. Root fragments as small as three to six mm thick and eight mm in length have been found to produce shoots about 15 percent of the time. Fragments of 12.5 mm produce shoots 100 percent of the time.

Hybrids - In Europe, C. arvense hybridizes with nine other species of Cirsium. It has been found to hybridize with only C. hookerianum Nutt. in North America.

Population Dynamics - By vegetative propagation, a single seedling can establish a large patch of stems. However no seeds will be produced because seed production requires the presence of both sexes. More than one introduction is thus needed. Plant competitors vary in their effectiveness. In one study, the percentage of thistles increased by 192 percent in four years in a continuous cropped spring wheat, but decreased to one percent in alfalfa grown for hay.

Response to Herbicides: Effective control can be achieved by using several broad-leaved herbicides that do not harm grasses. For more site specific control recommendations, please refer to the latest edition of the Pacific Northwest Weed Control Handbook.

Response to Cultural Methods: Planting competitive crops, such as alfalfa and forage grasses can be very effective in controlling an infestation of Canada thistle.

Response to Mechanical Methods: Repeated tillage at 21-day intervals for about four months can be effective on minor infestations of Canada thistle. Repeated mowing to weaken stems and prevent seeding is also effective in low level infestations.

Biocontrol Potentials: Many insects, a few nematodes, and the American Goldfinch have been reported to feed on various parts of Canada thistle. Most of these do very little damage. Three insects from Europe have been studied for biological control - Altica carduorum Guer (flea beetle), a leaf feeder, has not established itself well. Adults of the beetle Ceutorhynchus litura F. eat young thistle shoots, but do little damage. The fly, Urophora cardui L. is the most promising biological control agent. Eggs are laid in the terminal buds and galls develop which divert nutrients and stress the plant. Many microorganisms have been found associated with Canada thistle, but no potential biocontrol agents are known.

References:

Moore, R.J. 1975. The Biology of Canadian Weeds. 13. Cirsium arvense L. Scop. Can J. Plant Sci. 55: 1033-1048.

This page was last updated 03/31/07