Answer: The best time to divide them is early spring or late summer, but you can do it any time during the growing season if you are careful. Water the clump well the day before. Dig up the clump; next, some gardeners wash the soil off the roots. Cut off half to two thirds of the top foliage so you can see what you are doing with the crowns.
Depending on the variety, some clumps will naturally separate themselves into individual crowns but others need to be untangled. This can be quite a job. Some gardeners use an improvised tool such as a screwdriver to help with this, others simply use a knife and cut them apart. Each individual crown will make a new plant as long as it has roots attached, but for larger plants faster you can leave several crowns together as a larger division. Depending on the size of divisions you take, you can produce as many plants as you need.
Replant into prepared soil placing the crowns at the same depth as they grew before. (To do this with bare rooted plants, dig the hole, make a mound of soil in the center, and drape the roots over the mound.) Fill in around them carefully and water them in well. Finally, a layer of mulch is a good idea. Keep an eye on the watering for the rest of the season while they reestablish, and if planted in fall, also check for frost heaving during the coming winter.
Enjoy your new plants!
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