Canada thistle, a noxious weed common in Colorado, is an aggressive and creeping perennial that spreads from its root system. Shoots become 1 to 3 feet tall and its purple flowers are about the size of a dime. It can also be recognized by its spiny leaves and stems.
Hand pulling Canada thistle isn't an effective control because of its extensive root system. Herbicides such as Round-up and 2,4-D or related chemicals can be effective. Controlling Canada thistle with herbicides may take several applications over several years. Fall applications are most effective. Use caution when using any herbicide near desirable plants. Follow label directions carefully.
To treat Canada thistle in a lawn, 2,4-D and related herbicides are preferred. Be sure to apply 2,4-D when temperatures are between 65 and 85 degrees Fahrenheit and when winds are less than 5 mph.
In other areas, such as flower beds and vegetable gardens, Round-up, 2,4-D or related chemicals can be used. It's important not to spray the foliage of desirable plants with these herbicides. Sponging or painting a herbicide is best when Canada thistle and desirable plants are in close proximity.
For more information, see the following Colorado State University Extension fact sheet(s).
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Updated Tuesday, October 13, 2015