Peonies are one of those herbaceous plants that can usually be relied on to produce their big, blowsy blooms year after year as regular as clockwork. However sometimes the flowers will fail to appear for no apparent reason. In fact there are a number of factors that may be responsible as Margaret Roach explains in her article which I found on her A Way To Garden website.
W HEN I GET OVERWHELMED AS GARDENERS DO ABOUT NOW, I think of the peonies that grew in the narrow space between the flagstone walk and a stucco wall of the home of my youth. No matter that there was hardly room for anything in that spot, or that they’d probably been there 30 years already. Each year, during the week of my birthday, they bloomed like mad. “Onward,” they seemed to say. “Keep at it.” Sometimes, though, peonies don’t bloom well, or the buds just turn black and dry up, I’ve learned since. Want to know the reasons why?
Blooming in herbaceous peonies (and this is true for most plants) is controlled by factors like light, nutrients, the premature removal of foliage, recent transplanting, and also various plant diseases (often triggered by weather conditions). Planting depth can also affect peonies adversely. The details:
Are your peonies getting enough sun, or has a nearby tree or shrub grown and reduced the amount over the years (hence a recent decline in bloom, perhaps)? Nearby trees can pose another challenge: When peonies try to compete with extensive root systems of large woody plants, they can lose…meaning reduced bloom. Peonies ideally want a minimum of six hours of full sun a day (you may be able to skimp a little in the more southern part of their hardiness range, Zone 8).
See more at A Way To Garden