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The best time to transplant most perennials is either early spring when growth is just starting or fall. Since your Shasta Daisies are blooming, it is a little risky but not impossible. You should wait for a cloudy day, dig up as much of the root ball as possible and water it well after it's transplanted. It may wilt some for a day or so but should bounce back. You may also loose blooms due to transplant shock.
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Alan is right on I did this myself last Friday, weather was exactly as he said and I had fantastice results too. The key is to avoid the shock of transplant. Just get lots of original dirt with the root and you are home free. I love shasta daisies because they are so prolific.