Peonies are popular perennials because of their magnificent flowers in spring and early summer and their leaves that remain attractive throughout the season. These instructions apply to herbaceous peonies (USDA Zones 3–8), which die completely to the ground in winter. Tree peonies, shrublike plants with woody stems that remain visible all year (Zones 4–9), have slightly different requirements.
It's best to plant herbaceous peonies in fall, when they are dormant. Spring-planted peonies may fail to flower for several years, and may even die if they put out new shoots before their roots become established. Mail-order and local nurseries offer bare-root divisions in the fall. A bare-root peony division is a section of rooted crown with several dark red growth buds called eyes. Each bud gives rise to a stem in the spring. The more eyes a division has, the fuller it will appear, so look for a division with at least 3 to 5 eyes.
To plant a bare-root herbaceous peony in the fall:
1. Choose a site with well-drained soil away from the competing roots of trees and shrubs. Give each plant an area about three feet in diameter. Avoid windy areas or provide a windbreak, since peony stems are vulnerable to breakage when heavy with buds and flowers. In most regions a site in full sun is best. If your spring tends to be hot and dry, choose a site with afternoon shade.
2. Prepare the soil. Peonies bloom most prolifically if left to grow undisturbed in the same location for many years, so planting in well-prepared and enriched soil is important. Dig a planting hole about 2 feet across and 18 inches deep. Mix several shovels of compost or well-rotted manure into the bottom of the hole. Fill the hole halfway with topsoil mixed with more compost and a cup of bonemeal. If your soil is acidic, also mix in about a cup of ground limestone. Peonies like a pH of between 6 and 7.
3. Plant the peony. Setting the division too deep prevents flowering; the eyes should be covered by no more than 1.5 to 2 inches of soil. If you garden in a mild-winter climate, position the eyes so they will be only .5 to 1 inch below the soil surface, thus exposing the roots to as much winter cold as possible. Once you've positioned the peony in the hole, fill the soil carefully but firmly around the roots. Make sure there are no air spaces that could allow the plant to settle too deeply.
4. Water the plant in with a gentle stream from a hose. Then surround it with a light mulch, such as chopped leaves or evergreen branches, to help reduce competition from weeds and regulate soil temperature and moisture. Remove the mulch in winter so that the plant gets properly chilled. The peony may not bloom its first year, but by the third year it should reach top form.
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