Answer: Canadian thistle is one of the most difficult perennial weeds to control, because it spreads by underground rhizomes. The only thing I can suggest is to continue your regimen of observing and removing the plant. You might try waiting until the thistle just begins to flower, then cutting the plant off at ground level. At this stage the plant has used up the most reserves from its roots, and has not yet formed seed heads. You'll need to continue this until the roots have spent all their energy--which may take a long time; rhizomes can lay dormant for 3 or more years. And, of course, this is not practical when the plant is invading your garden beds. Sometimes a thick (3 or more inches deep) can help slow down the invasion. If you keep the stems of the plants cut down to ground level you can try, in the fall, painting Round Up on the freshly cut stems. In the fall the plants are busy collecting energy to store over the winter months. Round Up is most likely to travel down to the roots and kill them in the fall season. Good luck with your garden!
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