Knowing when to prune peonies can help these herbacious perennials reach their true potential and bear the large, colorful flowers that they are known for. To avoid problems in June, prune peonies to the ground in October or November after the first killing frost. You'll know it's time to prune when the leaves mature and look dry.
Using sharp pruning shears, cut every stalk to just below ground-level until there is no sign that a flower was ever there. Remove all dirt and debris, and do not compost the cuttings if they are likely diseased. Apply a liberal helping of ground limestone along the border. Add some composted cow manure above the line and gently rake the two into the soil. Never mulch peonies in the winter. It is okay to mulch them in the spring. In March or April, before the peony buds appear, sprinkle a layer of sharp sand on top of the soil.
The tree peony, unlike the herbacious variety, should never be pruned to the ground. Prune these plants after the leaves appear and remove any damaged or broken stems. Because tree peonies are grafted on herbacious root stock, the tree peony may occasionally send out a shoot from the rootstock. These should be removed as soon as they appear. Proper pruning will yield healthier blooms when the flowers are in season.
Peonies are prone to diseases such as bud rot, stem rot, leaf blotch, and anthracnose. Bud rot, also known at botrytis, is particularly aggressive and peonies left to over-winter without pruning will succumb. In the spring, the flower buds will turn black and fall from the plant, never to bloom.