Canada thistle (Cirsium (Breea) arvense)
Aggressive creeping perennial.
Reproduces by seed and also readily from roots. In Gilpin County, plants start going to seed in August.
It is difficult to control because its extensive root system allows it to recover from control attempts.
Combining control methods is the best form of Canada thistle management.
The key principle to Canada thistle control is to stress
the plant and force it to exhaust root nutrient stores.
Mowing or pulling alone is not effective unless conducted at one-month intervals over several growing seasons.
If you do spray, do it in the fall after pulling or mowing once earlier in the season. Do not allow the plant to go to seed.
Glyphosate (Roundup) is fairly effective against Canada thistle, although repeat applications may be necessary. It is most effective when combined with cultural and/or mechanical control. Aminopyralid (Milestone) has been shown to be very effective.
Please note that there are some native thistles, which are not noxious plants.
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