Peonies (Paeonia spp.) have been garden staples for centuries. Hardy throughout U.S. Department of Agriculture zones 2 to 8, they thrive in all but the warmest areas of the United States. Few plants can match a peony's extravagant flower display, so it's disappointing if they don't bloom well. Fortunately, the reason is usually easily determined and remedied.
Planted Too Deeply
Peony roots must be planted close to the surface, with the buds no more than 2 inches below soil level. When planted deeper than that, the roots produce all foliage and no buds, although no one really knows why. Lift your roots so they're closer to the surface and never cover the crown of the plant with mulch.
Too Much Shade
Peonies bloom best in full sun or light shade, requiring a minimum of four to six hours of direct sunlight daily. If your peony is being shaded by a tree, it's also likely that the tree's roots are competing with the peonies for water and soil nutrients. Transplant your peony to a site away from trees where it will get enough sunshine. Peonies are transplanted most successfully in fall but can be moved while dormant in early spring as well.
Peonies are stressed by drought, so supplemental watering is necessary during dry spells to ensure good bud formation for the following year's flower display. Buds that have started to open can be damaged by late freezes. If a freeze is forecast, cover the bushes with a tarp or blanket before sunset and remove it the next morning.
Although uncommon, there are several diseases that can infect peonies. Botrytis blight is a fungal disease common during cold, wet weather. Buds turn black and stem tissue collapses. Phytophthora blight causes black spots on the buds, and stems that turn brown and leathery. Peonies can also be infected by a leaf spot fungus. For all diseases, remove infected stems when seen and prevent spores from overwintering by removing all debris from peony beds in the fall.
Scales are the only insects likely to infest your peony. They'll overwinter on dead stalks so the best prevention is a thorough fall cleanup. Contrary to widespread belief, ants neither harm nor benefit peonies. They just like to feed on the sweet liquid that occurs naturally on peony buds.
Peonies that have been recently divided or transplanted may not bloom for several years. This is normal, so just be patient.
Feed your peonies with a low-nitrogen fertiliser such as 5-10-10. Too much fertiliser results in leaf growth at the expense of flower production.
Peonies grown from seed will not bloom for at least four or five years. When dividing peonies, each section should have at least three to five eyes; less than that will delay flowering.
No matter how ratty it gets in late summer, leave the foliage on your peony until it's blackened by a heavy frost. Removing the foliage too soon doesn't allow the plant enough time to build up its reserves for the next season.