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 maytake 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 peony or iris planting. Sometimes a thick (3 or more inches deep) can help slow down the invasion.
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