It is general gardening wisdom to transplant spring-blooming plants in the late summer or early fall, and fall-blooming plants in the spring, just as growth starts. If you use care, however, you can move a plant at almost any time. It is best to wait until after a plant is done blooming before moving it, so it can put all its energy into creating new roots.
Transplant when the weather is cool, even a little rainy. Avoid hot, windy days, as these cause the plant to suffer more moisture loss. It's a good idea to water the plant the night before you plan to move it.
Dig the new hole before you take the plant from the ground. Mix organic material, such as compost or shredded leaves, into the soil to provide nutrients.
Cut back top growth to about 6 inches when moving plants in the fall--this makes the plant easier to handle and reduces its need for water.
Dig a large hole under and around the plant. Barbara Bates, horticulture educator with the University of Illinois, recommends using a rounded shovel or spading fork when transplanting perennials.
Take as much of the root ball as you can when you lift the plant. If you can't put it in the soil right away, keep the plant in a shaded spot and cover the root ball to help prevent moisture loss. You can leave plants on the ground for several days as long as the roots are kept moist.
Place the plant in the new hole at the same level it was before you moved it, then gently tamp the soil around it. If the soil is only placed loosely around the plant, air pockets around the roots will cause them to dry out.
Water the newly transplanted perennial well, though not to the point that the soil is soggy. Check the plant's water needs every other day by sticking your finger in the soil; water when the top 2 or 3 inches becomes dry.
Apply 3 to 4 inches of mulch around a perennial when you transplant in the fall. It will help keep the soil warm and give the roots more time to become established before the ground freezes. Don't place mulch directly on the plant, as this can cause crown rot.
Don't fertilize the plant until it has time to become established. If transplanted in the fall, fertilize it the next spring, and if transplanted in the spring, wait a couple of months before feeding.