The Q&A Archives: Dealing with Canadian thistle

Question: For the past three years I have been trying to eleminate Canadian thistle from my perennial garden. I've tried pulling it up and Roundup every 7 days. What can I do, especially when it's coming up in the center of a peony or in between the irises?

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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