Once planted in well-drained soil, peonies bloom for decades with little attention, gracing old and new gardens. They even bloom on abandoned sites where the owners’ homes have long since disappeared. To find the cause of a peony’s failure to bloom, review its cultural requirements and determine what has changed in its environment.
Peonies grow best when planted in well-drained soil in full sun although dappled shade at mid-day benefits plants during hot spells. Garden peonies require a slightly acidic to slightly alkaline soil with a pH of 6.5 to 7.5 but tree peonies prefer a slightly alkaline pH. Herbaceous peony tubers planted deeper than 1 to 2 inches, or tree peonies with grafts planted deeper than 3 to 4 inches, have to pull themselves up in the ground and will not bloom until several eyes rest just below the soil’s surface. Heavy soil, too much shade, extremely alkaline soil — or acidic soil for tree peonies — may keep peonies working hard to grow leaves instead of blooms.
Peonies need very little fertilizer, and if they are over-fertilized, especially with nitrogen-rich fertilizer, they may not bloom. Organic matter added to the planting hole and an annual feeding of a complete 5-10-5 garden fertilizer will nourish plants, while bone meal or other forms of phosphate encourages strong roots and flower set. Existing plants that are over-fertilized, possibly by a new homeowner, may stop blooming.
Peony plants can suffer setbacks through natural causes. Heavy spring rains leave sodden soil that can rot tubers, halting bloom until healthy parts of the root can replace rotted sections. Divided tubers that are re-planted may not bloom until the plants mature or because the tubers have been planted too deeply or in an area with less than four to six hours of sun. Tree or shrub roots that grow into a peony’s area may interfere with its ability to gather enough moisture and nutrients to flower. Aging trees may shade out over peonies, limiting the amount of sunlight that reaches their leaves. Re-setting the tubers to another location or fertilizing properly placed tubers lightly may help the plant to bloom next season.
Although peonies are exceptionally long-lived plants, very old clumps may become cluttered and blooms may taper off. In this case, dividing the clump should solve the problem. However, peonies can suffer setback because of excessive division, too. If the tuber has been divided into roots with fewer than three to five eyes — the little buds that become growing shoots in spring — only time will tell if the plant will start to flower again. Overenthusiastic cleanup that removes leaves in summer instead of late fall removes the peony’s food-making leaves and there may not be enough energy stored to make flowers the next spring. A hard late freeze may stop buds that have formed from flowering.