Canada thistle is a member of the Aster family. Canada thistle was introduced from Europe. It is a creeping perennial, which reproduces by seeds and fleshy, horizontal roots. The erect stem is hollow, smooth and slightly hairy, 1 to 5 feet tall, simple, and branched at the top. The color is primarily lavender, pink, or purple. Canada thistle emerges in June in most parts of Pitkin County. It is one of the most widespread and economically damaging noxious weeds in Colorado. Infestations are found in cultivated fields, riparian areas, pastures, rangeland, forests, lawns and gardens, roadsides, and in waste areas. Because of its seeding habits, vigorous growth, and extensive underground root system, control or eradication is difficult.

Comments: Canada thistle is best managed through an integrated management system that emphasizes competitive, desirable plants.

Biological controls: Three insects currently available. It is best to release a complex of insects (different insects that will stress differest parts of the plant.)

Ceutorhyncus litura – a weevil that stresses the crown of the plant.

Urophora carduii – a stem and shoot gall fly.

Cassidia rubiginosa – leaf beetle.

Chemical controls: There are many effective herbicides that are labeled for Canada thistle control.

Cultural controls: Maintain soil fertility and moisture at optimum levels to favor grass growth. 

Mechanical controls: Research indicates that mowing of Canada thistle may be effective when done repeatedly at two week intervals over a period of several years. Pulling and digging up Canada thistle has been shown to be ineffective as the plant has such an extensive root system.

Locations of Infestations of  Canada Thistle on Roads and Open Space in Pitkin County:

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