Peonies can be planted in fall or spring, but there are a few reasons why fall is the better choice—so when you're purchasing bulbs to plant this fall, pick up a few bare-root peonies, too.
Fall planting of peonies can occur in September and October in northern climates (or even into November in a mild fall), and in November and into December in southern regions. Fall-planted peonies tend to establish themselves more quickly than those planted in the spring, and they will flower sooner. Spring-planted peonies seem to run about a year behind fall-planted peonies in terms of maturity, growth and flowering.
When planting peonies, choose a site in full sun. Peonies need good drainage; drainage can be improved by adding compost or other organic matter to the soil. Dig a hole that will fit the bare-root peonies, with some extra space; you shouldn't need to bend the roots to get them to fit. Situate the roots so that the topmost eye (the point from which stems will grow) is just an inch or two below the soil surface. Fill in the hole and water well. Keep the site watered until the ground freezes and be sure to add several inches of mulch to conserve moisture and keep the soil temperature more consistent through the winter. If you are planting more than one set of peony roots, leave three feet between them.
Existing peonies can also be divided in fall, which will rejuvenate them and improve flowering in the future. Cut their foliage down to ground level, dig up the roots and divide them into several clumps. Then follow the above instructions for planting.