Ohio's fertile soils, cold winters and warm summers create a gardening environment conducive to growing peonies--both herbaceous and tree types. Situated in USDA Hardiness Zones 5 and 6, peonies (Paeonia spp.) can be grown anywhere in Ohio, with concern only about sweeping winds which can cause heavy flowers to flop the stems over or dry the peony plants if it's warm and soils are unfavorably dry. Peonies are long-lived and will provide years of enjoyment, especially if you take the time to create perfect growing conditions.
Time of Year for Planting
Bare-rooted or freshly dug-up peony plants can be planted at two different times of the year. Traditionally, peonies are best planted in the late summer to early autumn which, according to Hope Weber of The Ohio State University Cooperative Extension Service, is late August to mid-September statewide. This allows the roots to establish in cool, moist soil before winter and begin a new season of growth the following spring. Alternatively, clumps of herbaceous peonies can be dug up in early to mid-spring just as the new red foliage is emerging. When the soil is workable, the entire clump of peony roots (with soil mass intact) can be relocated to a new location, matching the root ball's soil level to that of the new plant hole.
Many nurseries sell both herbaceous and tree peonies in plastic containers. Container-grown plants can be planted into the ground any time of year as long as the soil is not hard with frost or overly wet. Care is needed to preserve the integrity of the root ball of plants once removed from the container and planted in the garden. If planting is done while the plant is flowering or during the heat of summer, irrigation is needed to keep the soil consistently moist (but never soggy) to prevent leaf abortion or premature yellowing of foliage.
Ideal Soil Conditions
Peonies require a fertile, moist soil that has good drainage after rain or irrigation. The soil pH ideally is slightly acidic to neutral (6.5 to 7.5). Locate a garden bed that receives at least six hours of direct sunlight daily. Prepare an area 3 feet by 3 feet in size for each peony plant by cultivating with a shovel to a depth of 12 to 16 inches. Incorporate large amounts of compost, well-rotted manure or fallen leaves, and other organic material into the soil. Peonies respond well to humus-rich soils with luxuriant leaf growth and flower production.
Caveats to Planting Peonies
Herbaceous peony roots look like beige to white-colored carrots, with small growing points at the top called "eyes." These eyes must be between 1 and 2 inches below the soil surface. Planting them too deeply prevents production of flowers. Tree peonies typically have graft-unions on their roots. These union scars must be 4 to 5 inches below the soil surface, certainly no deeper. Once planted, monitor the soil and supplement with irrigation when rains are lacking in order to keep the soil evenly moist.
Generally speaking, peony plants resent root disturbance. Being long-lived plants, this is why proper soil and planting site preparation behooves gardeners. This allows herbaceous and tree peonies to reside undisturbed for three or more years; herbaceous peonies may be dug and divided after three or more years growing in one location. Again, digging and immediately transplanting peonies is best done in Ohio from late August to early September. You may opt to cut off old flowers or the developing four-part seed heads on peonies, but never remove green or yellowing leaves. Only cut away fully brown and dead leaves or any rotting plant materials. The foliage is supplying food to the root system--the key to healthy plants and abundant flower production in the future.