The recommended time to prune your peonies depends entirely on which of the two major varieties of peonies you are talking about. The pruning of herbaceous peonies is done in two stages at the opposite end of the growing season from tree peonies. Tree peonies are only trimmed once and are far more carefully managed than the wholesale cutting of the herbaceous variety.
Herbaceous peonies are showy perennial flowers, growing back to full size from bulbs each year. The buds begin to appear in late April, early May and June, and can be harvested, refrigerated and opened later. This makes them an ideal flower for commercial harvesting for the florist industry. If you bloom them on the stalk in mid-summer, you’ll have a beautiful display of huge blossoms for two to three weeks. When the blooms die back, they leave behind a pod that can be harvested for seed. Whether you retrieve the seed pods or not, you should do your first trim at this time, removing about a foot of the stalk below the seed pod. Leave the foliage behind. It not only strengthens the bulb for over-wintering, but also leaves you with a lovely display of greenery.
When the remaining peony foliage begins to change color and wilt in the autumn, it is time to prune the plant back. Herbaceous peonies should be cut back to ground level for the winter. Rake away all the plant cuttings from the plants and rake some dirt up over the cut stalks to protect it during the cold season.
Tree peonies produce a woody stem and root system that must be preserved in order for the plant to come back in the spring. Some gardeners advise you not to cut back peonies, allowing the leaves and stems to fall naturally. Cornell University Extension Service recommends that you never cut back tree peonies to the ground unless the plant is diseased. New buds are formed from old wood grown the previous year. If you cut away all the buds, you’ll have no flowers in the spring. Instead, clean up the fallen leaves and stems. Then come back in the late winter to early spring and thin out any new shoots from near the center of the remaining stems to reduce crowding. Tree peonies produce better if the stems are not too close together. If the peony is grafted onto another variety, remove any stems that spring from the old ungrafted root system.
Cleaning up after grafting is critical to preventing diseases like boytritis, a fungal infestation to which both tree and herbaceous peonies are particularly vulnerable. Bag and remove the cuttings or burn them. Never use peony cuttings for compost. Leave the soil around the base of the plants bare for the winter. In spring, cover the soil around the plants with a thin layer of sharp sand to bury and prevent any fungus spores that may have been left behind from infecting the new crop of peonies..