Bermuda grass vs Yarrow

I have a small central area in my garden that I seeded with yarrow two years ago to create a small yarrow lawn. Unfortunately, it is starting to be invaded by bermuda grass. I thought the yarrow was successfully competing with the bermudagrass, but lately it looks like the bermuda grass is starting to establish itself. Can anyone offer suggestions as to how to help the yarrow win?
Thanks!
Andy
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