Garden Guides states that it is best to divide peonies at the root clump using a sharp knife. Each root clump has red growths that look similar to the eyes on potatoes. Cut up the root clumps, leaving three to five “growing eyes” per piece, then plant the root pieces with the eyes turned up, approximately 24 to 36 inches apart, and about two inches deep in the soil.
In September, cut the plants down to four inches. Don’t leave the trimmings in the bed because peonies are prone to infection. Always remove cuttings from pruning to prevent molds, mildews and the spread of other plant diseases.
Next, dig out the surrounding soil and scoop the plant from underneath the roots. Once the plants are dug out, check the root clumps for good health and divide them. Replant the divided roots in the freshly turned bed. Some gardeners first re-compost and till beds down to 18 inches deep to prepare for the next growing season. Water the bed until it is moist, but not soggy. Later in the year, mulch the bed to keep the peonies’ roots warm during the winter.
Peonies don’t need to be divided very often and may not bloom the first year after division. That is normal. Other than this involved process, peonies are generally easy to grow. Peonies need well-drained soil, plenty of organic matter and a spectrum of sun that includes full exposure and partial shade.