Large, silken-textured flowers of many colors grace peony (Paeonia spp.) bushes during the spring. Fall care helps spring blooms, but how you should care for peonies in the fall depends on what type of peony you're growing. Most common is the non-woody herbaceous peony (Paeonia lactiflora), hardy in U.S. Department of Agriculture plant hardiness zones 3 through 8. It dies back in fall and renews its growth from underground tubers each spring. Tree peony (Paeonia suffruticosa), growing in USDA zones 4 through 8, forms a deciduous woody shrub that remains in the garden year-round.
Cutting Back Foliage
If you have herbaceous peonies, remove all the old stems in late fall after the first frost turns the foliage yellow. Before cutting the stems, dip clean, sharp pruner blades in rubbing alcohol to lessen the chance of spreading disease. Discard all the cut foliage to prevent a fungal disease called botrytis blight or gray mold, which affects peonies and can survive the winter months in composted old stems.
Providing Winter Protection
Protect peonies from the winter cold, especially in colder areas of their hardiness zones. In late fall, give herbaceous perennials a mulch layer 2 to 3 inches thick, using an organic material such as shredded bark or straw. You can wait until after the ground freezes to apply mulch. For tree peonies in colder areas, such as zones 4 and 5, wrap the bush with burlap in late fall to provide protection against winds as well as cold temperatures.
Although tree peonies usually don't need much pruning, older bushes may become leggy. To renew the growth, prune the oldest tree peony branches back to the base in fall after the leaves drop. Clean the pruners with rubbing alcohol before pruning. However, if your tree peony is grafted rather than growing on its own roots, it's best not to do renewal pruning. Instead, moderately cut back some of the lower branches to fill out the base of the bush.
If you want to use an organic fertilizer such as aged compost or aged manure, in late fall add a 2-inch-thick layer of either material on top of the soil around established peony bushes. If you use inorganic fertilizers, wait until spring.
If you need to relocate either herbaceous or tree peonies, fall is the best time. Carefully dig around and then under the roots, taking care not to damage the fleshy tubers. Lever the peony out of the ground, disturbing the root mass as little as possible. Transplant it in the new location, which should be in sun with well-draining, rich soil. Keep the plant at its original soil level, and water it well.
Large, established herbaceous peonies can be divided in the fall to renew growth or to make new plants, although clumps can grow in place for 40 or 50 years without division. To divide a plant, cut back the foliage, and then carefully dig up the peony and shake or wash the dirt away from the root system. Use a sharp knife cleaned with rubbing alcohol and cut the clump into divisions, each holding three to five eyes and several roots. Eyes are the buds on top of the crown that grow into new stems. Replant each piece in its new garden location, placing the buds 1 to 2 inches below the soil surface. Water the divisions thoroughly.