Peonies have been grown in Minnesota for more than 100 years. Pioneers carried peony starts with them as they settled the region, beginning a tradition of growing this herbaceous perennial in the cool Minnesota summers where peonies thrive. Cool summers also mean cold winters, however, and peonies die to the ground with the first hard frost. The hardy crown survives under the snow and sprouts readily the following spring, but despite their tough nature, peonies do need care when temperatures drop. Preparing your peonies for winter will keep them blooming next spring.
Cut back peonies within 2 inches of the ground after the first hard frost, when the plant shows obvious damage. Cutting down a peony earlier prevents the plant from storing food for the following spring. Although peonies survive an occasional early trimming, they are slowly weakened by overly eager gardeners who cut them down too early year after year. To prevent transferring potential disease from plant to plant, the University of Minnesota Extension suggests spraying or dipping pruning shears with a 10 percent bleach solution between cuts, especially if a plant shows signs of disease.
Clear any fallen leaves or other debris from the peony. Toss the cuttings in the trash. Keeping the area clear is the best prevention against botrytis and other fungal diseases. Don't compost peony trimmings; fungal diseases may be dormant and could re-infect the plant.
Work compost into the first 2 inches of soil around the peony. Peonies are fast-growing plants that bloom heavily, and, although they are tough plants, they respond well to the extra boost. Compost will provide nutrients as the ground warms in spring, when the peony puts on the most growth.
Surround the peony with mulch. Deb Brown of the Minneapolis-St. Paul "Star Tribune" recommends placing 4 to 6 inches of mulch over perennials to insulate the ground and prevent peony crowns from heaving up during freeze-thaw cycles. It also provides extra protection for the peony during early thaws, when the plant may send up shoots if the ground is warmed too quickly. A thick layer of mulch also keeps the soil from drying in case of little or no snow.