Peonies can be easily divided after five years or more of growth.
image by Public Domain--Russia, DRW & Associates Inc
Peonies are garden favorites because they bloom early in profusion. They are also some of the longest-lived perennials, blooming for decades and demanding little care. Peonies generally do not grow true from seed and those that do take up to seven years to bloom. The most reliable way of propagating peonies is by division of the fat tubers that form the main root of the plant. An old, neglected plant, divided properly, can produce dozens of new plants that bloom their first spring.
Propagate peonies in the fall as peony foliage is dying down. This can happen anywhere from mid-August to late September, depending on climate and precipitation. Although it's possible to divide mature peonies every two to three years, their bloom will possibly decline. The peonies most of us grow are stay-at-home folks; let them stay in the same location for at least five years before digging them up again.
Cut foliage down to about 2 inches to make the clump more manageable and buds easier to find. Dig in a circle around the peony crown---the base of the bush---about 8 inches out. Keep working around and under the crown until you have dug down between 8 inches and a foot. Gently test to see if you can lift the crown out of the ground.
Clean the dirt (most of the dirt should fall off as the crown is lifted) off the crown when it comes loose by spraying it with tepid water. Count the eyes or buds on the fleshy crown and divide by three. Cover the crown with a towel if the air is cold. It needs to dry a bit before it can be divided.
Dig as many holes as you have divisions of three eyes. Dig them at least 2 feet deep (3 for heavy soils) and at least a foot square. Fill holes to a depth of 8 inches with a mix of garden soil and manure, compost or peat moss to provide a light, well-drained base for your new peonies. Set the extra soil aside and work in amendments, breaking the soil up into extra-light soil.
Cut the crown into pieces with one to three crowns each with a sharp spade or knife. Dust every cut with a fungicide to protect tender flesh from insects looking for a winter home. Set each new crown in the soil so that there is no more than an inch of soil above it. Fill the remaining hole with amended soil, spreading roots out and back-filling gently around them. Water thoroughly and check the position of the crown. Adjust its height if necessary.