The best time of year to plant peonies depends on the type of peony. Four types of peonies exist, three of which are herbaceous. The other is the tree peony, and is handled differently. Planting peonies at the right time of year and planting them properly helps them thrive, producing beautiful flowers for years to come.
Chinese, common and fern leaf are herbaceous peonies. The foliage on these types of peonies dies back in the fall, and new foliage is produced each spring. These peonies should be planted in the fall.
Tree peonies are ornamental trees that grow similarly to other ornamental trees. They are deciduous, meaning they lose their foliage in the winter, but the woody trunk and stems remain year round. Plant this type of peony any time between April and November, but early April is preferable.
All types of peonies need full sun exposure to thrive. Six to eight hours or more of direct sunlight is considered full sun exposure. Avoid planting them close to large bushes, trees or other plants that will compete for water and nutrients.
Prepare the planting location ahead of time. Till the soil to a depth of 1 foot and mix in compost or well-aged manure. The soil should be slightly acidic with a pH of about 6.0. Have the soil tested by your local county extension office to determine if the pH needs to be adjusted prior to planting the peonies. Follow their recommendations to adjust the pH before planting.
Plant herbaceous peonies at a depth that will put the "eyes" or leaf buds about 2 inches below the surface. The hole should be between 12 to 18 inches deep and approximately 18 inches wide. Put approximately 8 inches of loose fill dirt back into the bottom the hole. The loose dirt beneath the tuber will facilitate root growth. Place the tuber into the hole and gently fill in under, around and over it with loose dirt. Ensure there's no more than 2 inches of soil covering the "eyes."
Plant tree peonies in the same manner, but ensure the hole is deep enough to accommodate the tree roots. The graft union where the peony is grafted onto root stock should be 5 to 6 inches below the surface. The peony will eventually grow roots of its own instead of relying entirely on the root stock.
Water the freshly planted peonies thoroughly and mulch them to a depth of 1 to 2 inches. The mulch should be removed from herbaceous peonies in the spring, making it easier for the plants to sprout.