Canada Thistle in Lawns

One of the most common yard and garden thistles in Montana is the Canada thistle. This plant begins growing each year as a cluster of leaves close to the ground (rosette) and has the typical prickly leaves characteristic of the thistles. If the plant is allowed to mature, it reaches a height of about 2-4 feet, and has lavender blossoms that are similar in appearance to dandelion blossoms, but half as large. 

Canada thistle is a deep-rooted perennial, which means it lives for several years. Mowing or pulling of this weed is not effective because it grows again from vegetative buds on the roots. The roots can extend into the ground 10 feet or more, so cultivation in gardens and flower beds will not help to control this weed. In fact, cultivation can even worsen a Canada thistle problem because when roots are cut into pieces, pieces only one-half inch long can produce new plants from the countless vegetative buds in the root system.

To manage Canada thistle, use an herbicide that will not only kill foliage, but will also move down into the root system to kill the root buds. One of the most effective herbicides for this purpose is glyphosate (e.g. Roundup). Glyphosate is relatively safe for both humans and the environment, it breaks down quickly in the soil, and does not leach through soil into ground water. Remember, however, that glyphosate will kill any plant it touches, including desirable species like grass and trees. So apply it very carefully, allowing it to touch only the plants you wish to kill. One method is to dip a sponge into the mixed solution, using rubber gloves and goggles, then apply a small amount to the Canada thistle plant. A few drops are enough. Another method is to put a cloth glove over a rubber glove, and dip the glove directly into the glyphosate, then rub it onto the thistle. Don't allow the solution to drip onto you or
the ground. Apply glyphosate twice a year, once in early summer, and again in fall to catch plants that have been produced by seed or other rootstock.

 Broadleaf herbicides which contain triclopyr or dicamba (e.g the Trimec products) are probably more effective than those commonly used containing 2,4-D or MCPP. However, triclopyr and dicamba herbicides have the potential to remain in the soil longer than glyphosate and can move the soil into groundwater, especially in sandy soils. So be sure to apply them in the correct amounts, following label instructions carefully. At this time, there are no biological control agents available for Canada thistle that are appropriate for yard and garden settings. 

References: 

Farm Chemicals Handbook. 1997. Meister Pub. Co. Willoughby, OH 44094-5992. 537 pp. 

Fay, P., S. Dewey, T. Whitson, R. Sheley. 1995-1996. Montana Weed Management Handbook. Extension Service Pub. Montana State Univ - Bozeman. Bozeman, MT 59717. 245 pp. 

Pacific Northwest Weed Control Handbooks. 1997. Extension Services of Oregon State University. Corvallis, OR 97331-0817. Ross, M., and C. Lembi. 1985. Applied Weed Science. Macmillan Pub. Co., New York, NY 10022. 340 pp. 

Shurtleff, M., W. Thomas, and R. Randell. 1987. Controlling Turfgrass Pests. Prentice- 

Hall, Inc. Englewood Cliffs, NJ 07362. 449 pp. 

Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997