It is considered to be a noxious weed which is difficult to eradicate because of its extensive root system. When mowed in a lawn, it will not develop full height and flower. Canada thistle is invasive due to shoots from its root system. Pulling generally is not effective due to the tremendous reserves in the root system, and new shoots appear soon after pulling. Regular, persistent pulling may gradually starve the root system. Shoots should be pulled as they are noticed, since all shoots (leaves) are producing food reserves.
Turf density and competetiveness can be increased with the proper cultural practices described in CSU Fact Sheet 7.202, Lawn care.
Herbicides such as glyphosate can be painted on thistle leaves. Repeat applications will be needed. Herbicides such as triclopyr + clopyralid or 2,4-D combinations can be sprayed on thistle foliage; repeat applications may be needed at 6 week intervals. The most effective times for herbicide applications are spring, just after the green shoots appear, or in August/September. Always read the label before applying any pesticide.
Photographs courtesy of Judy Sedbrook.
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010