Cirsium arvense (L.) Scop (Family: Asteraceae)

Canada thistle is an aggressive cool-season perennial that reproduces from seed or by vegetative buds in its creeping root system. Canada thistle root systems are quite extensive; taproots can send out lateral roots as far as 15 feet horizontally and 6 feet vertically! Also, each plant can produce up to 5,000 feathery seeds which are easily dispersed by wind. It is especially important to control Canada thistle before beginning a restoration project, even on sites that are within wind dispersal range.

Also known as: Californian thistle, creeping thistle, field thistle, Canadian thistle, corn thistle, perennial thistle
 

Origin: Canada thistle is native to the temperate regions of Eurasia and was accidentally introduced to Canada in contaminated crop seed in the 17th century. It quickly became an agricultural pest, and by 1954 it had been declared a noxious weed in 43 states, including Indiana. It has only recently been recognized as a threat to native species in natural ecosystems.

Description:

• Canada thistle stems can grow from 1 ½ to 5 feet tall. Stems are erect, branched, slightly hairy, and have ridges.

• Leaves are lance-shaped, irregularly lobed, have spiny toothed margins, and are found singly or alternately along the stem.

• Flower heads are rose-purple or lavender, in rounded umbrella-shaped clusters, and fragrant.

• Canada thistle blooms from June through September, and seed production is usually completed by July.

• Plants that spring up from the root system form basal rosettes.

Distribution: Canada thistle is distributed throughout the northern United States, from northern California to Maine down to Virginia. It does best in disturbed upland habitat: open areas, roadsides, logged sites, agricultural fields, prairies, savannas, and sometimes in wetlands if dry periods occur, or in wet areas with fluctuating water levels such as streambank sedge meadows or wet prairies.

Problem: Canada thistle is a major agricultural pest that costs farmers greatly in terms of direct crop losses and control efforts as well as a management problem on many national parks and preserves. Canada thistle threatens non-forested natural communities such as prairies, barrens, savannas, glades, sand dunes, fields, meadows, and generally any area that has been impacted by natural disturbance. It can reduce species diversity in these ecosystems by crowding out and replacing native plants through shading or competition for soil resources. It can become established very quickly and can be difficult to eradicate, due to the fact that damaging established plants only stimulates new growth from underground buds on its creeping root system. These vegetative buds cause the plant to become re- established after attempts at control.

Control: The best method for controlling Canada thistle is prevention, that is, encouraging healthy, dense native prairie vegetation. The thistle is sensitive to shading and may be able to be shaded out by a vigorous crop or other healthy vegetation. However, if the plant has become established in an area, mechanical or chemical control may be necessary. Mowing is a good option, but it is important to do so several times during the summer before the plants’ seeds have formed; mowing after flower buds open will only cause the seed to spread further. Herbicides may be used in conjunction with mowing, and you will want to choose an herbicide that will move down through the plant and cause substantial injury to the creeping root system, such as Clopyralid or Mesulfuron-methyl. These herbicides can kill most other broadleaf plants, so spray thistle in October when adjacent native plants are dormant, but thistle is still green. Even using these techniques, spots will likely require re-treatment in successive years for complete control. If you are working in a high-quality natural area, herbicides are not recommended because the amounts needed for complete control will do more damage to the ecosystem than the thistle.

USE PESTICIDES WISELY: Always read the entire pesticide label carefully, follow all mixing and application instructions and wear all recommended personal protective gear and clothing. Contact your state department of agriculture for any additional pesticide use requirements, restrictions or recommendations.