History

Canada thistle arrived in North America from Eurasia by the early 17th century. By 1954 it had been designated as a noxious weed in 43 states.

Biology

A single Canada thistle plant can produce more than 5,000 seeds, with the seeds remaining viable in the soil for 20 years or more. Canada thistle reproduces by sending out shoots from lateral roots, and can regenerate from root fragments less than an inch in length. The plant prefers wet conditions throughout the United States and Canada. Within one to two years a single plant can colonize an area up to 6 feet in diameter.

Ecology

Midwestern prairies have been impacted by Canada thistle as have savannas, sand dunes, fields, meadows and crops. Canada thistle will crowd out and eventually replace native vegetation.

Impact

Canada thistle is considered highly invasive as it prevents other plant species through shading and competition for soil resources. It has proven successful at changing the structure and species composition of natural plant communities and reducing plant and animal diversity. The plant's behavior costs tens of millions of dollars in crop losses annually and additional millions in control costs.

Control

Herbicides offer only a temporary solution in problem areas. Biological controls such as the gall fly (Urophora cardui), and the stem mining weevil (Ceutorhynchus litura) have become common methods of control along with hand-cutting, mowing, and controlled burning.

References on the Web:

Colorado State University

North Dakota Department
of Agriculture

National Park Service

Alien Plant Working Group

The Nature Conservancy