Answer: In my experience the best time to divide and transplant most perennials is in early spring, although many can be successfully moved in early September (when the summer heat diminishes) and some, such as peonies, prefer it. A few, such as bearded iris, should be moved in early to midsummer, and some such as daylilies can be moved with care any time.
Water the plants well the day before you plan to move them and water them in well when you replant them. Prepare the new planting spot before you dig up the plant and when you dig, take as large a rootball as possible. It's best if you can work on a day when rain is in the forecast, too. Be sure to keep them adequately watered up until the ground freezes, (use your finger to check the soil to see if it is moist) and check them for frost heaving during the winter.
Good luck with the rearranging -- it's a big job!
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