Flower buds form on my peonies, but don't open. Why?
There are several possible explanations why peony buds fail to open.
Peonies bloom best in full sun. In partial to heavy shade, plants are weak and may not be able to supply adequate food to the developing buds. As a result, the undernourished buds don’t develop fully. Peonies growing in partial to heavy shade should be transplanted to a sunny site in September.
Fungal diseases, such as Botrytis blight, may infect peony buds. Infected buds turn brownish black and wither. Fungal diseases are most common during cool, wet, spring weather. Sanitation is the most effective means of controlling Botrytis blight and other peony diseases. Cut off peony foliage at ground level in fall. Remove the diseased plant debris from the area and destroy it.
The flower buds may have been damaged by a late freeze. While the plants themselves can tolerate a light freeze, the buds are more susceptible to cold injury. Unfortunately, little can be done to avoid this problem.