Hi Kim,
I had the same thing happen to mine. When I weeded out the crab grass this spring past I had lost 75% of my yarrow. I don’t think there is any way around simply hand weeding it out. I replanted what was left and started over. The ones that are left are now thriving. I’d weed out the crab grass as soon as possible before it chokes out the yarrow.  I’ll be more diligent with my weeding going forward. Good luck. 🙂
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