The peony has been ubiquitous in American gardens since the 1800s. Peonies are hardy, long-lived and available in two types -- herbaceous peonies and tree peonies, which are larger and have woody stems that do not die back in winter. Although herbaceous peonies can be left undisturbed for many years, it may become necessary to move established plants to a sunnier site or to transplant older, overcrowded peonies that are flowering poorly.
The best time to move or divide herbaceous peonies is in the late summer or early fall, according to Cornell Cooperative Extension. This gives the replanted peony adequate time to establish roots before winter. Moving established plants is not difficult. Simply cut the peony stems near ground level in September, leaving enough stem with which to handle the plant. Using a spade, shovel or spading fork, carefully dig around the plant about 6 inches from the outside and then dig under the plant, retaining as much of the root system as possible. Gently lift the peony out and replant it immediately.
Herbaceous Peony Division
To divide a herbaceous peony, wash soil off of the roots with a hose after lifting the plant from the ground. With a sharp butcher's or hunting knife, carefully cut all roots back to 4 to 8 inches. Choose sections from the outer root area of large plants for best results. The roots should be joined to the crown at or near the base of the eyes, which are small reddish buds from which a new plant grows. Make cuts so that each division has three to five healthy eyes and several roots 4 to 6 inches long. Remove any rotted areas. Plant new divisions immediately.
Herbaceous Tree Peonies
Once established, a tree peony should not be moved, according to the University of Vermont Extension. Some tree peonies can live as long as 90 years or more, so it is wise to carefully consider where to initially plant a tree peony. Tree peonies are divided differently from herbaceous peonies. Divide older tree peonies when several new branches appear off of the main root system. Divide only branches that have a root on them.
Herbaceous peonies like full sun and well-draining soils. Plan to space plants about 3 to 4 feet apart. Dig the planting hole so there is ample room for the root system. Place the peony in the hole so that its eyes are 1 to 2 inches below the soil surface and are pointing upward. Water the plant thoroughly. In late fall, apply 2 to 4 inches of mulch. Remove the mulch in early spring. Transplanted peonies do not produce many blooms their first spring. The Iowa State University Extension even recommends removing flower buds during the first year to encourage plant growth. The peonies will provide numerous blooms in two to three more years. Plant tree peonies 4 to 5 feet apart in a location that provides afternoon shade and well-draining soil.