The peony is a beloved spring perennial, known for its lush, fragrant blooms. It is frustrating when promising flower buds fail to open, instead turning dark and shriveling on the stem. This failure, often termed “bud blast,” is a common problem. While the symptom is consistent—a brown, aborted bud—the underlying cause is not singular. Issues ranging from fungal disease to environmental stress can lead to this failure, and diagnosis is the first step toward a successful bloom season.
Botrytis Blight: The Most Common Cause
The primary reason for peony buds turning brown is Botrytis blight, a fungal infection caused by the fungus Botrytis paeoniae. This pathogen thrives in the cool, moist conditions typical of early spring. The fungus overwinters as small, black resting structures called sclerotia, which reside in the soil and on old plant debris.
In the spring, these sclerotia germinate, releasing spores that are easily spread by wind and splashing water. Infected young shoots may turn black, wilt suddenly, and collapse, sometimes with a water-soaked appearance near the base. When the fungus attacks the flower buds, they turn a dark brown or black and feel soft or mushy to the touch. A distinguishing sign of Botrytis is the appearance of a fuzzy, dense gray mold on the rotting tissue, especially during periods of high humidity. This gray mold signals an active infection and spreads the disease to nearby buds and leaves.
Environmental Factors Causing Bud Blast
When brown buds lack the gray mold of Botrytis, environmental stress is often the underlying issue, sometimes referred to as true bud blast. Peonies require consistent resources to develop large flowers, and any major interruption can cause the plant to abort the buds. A lack of adequate moisture, particularly during bud formation in late spring, is a frequent cause.
Nutrient availability plays a role in bud development. Insufficient levels of potassium, for instance, can weaken the plant’s ability to support and develop the buds. Too little phosphorus can also hinder the energy transfer necessary for flower production. Sudden temperature changes, such as a late spring frost after the buds have begun to swell, can physically damage the delicate internal tissues. The buds may appear waterlogged initially, then turn dark and desiccated, failing to open because their development was prematurely halted.
Recognizing Pest and Physical Damage
Beyond disease and environmental stress, physical injury from pests or external forces can cause buds to brown and fail. One common insect culprit is the thrips, a tiny insect that feeds by rasping the plant tissue and sucking the contents. Thrips feed on the developing buds, leaving behind scarring or distortions that result in a brown, withered appearance.
Another damaging pest is the root-knot nematode, a microscopic worm that attacks the plant’s roots. Damage to the root system inhibits the uptake of water and nutrients, creating a stress response that mimics bud blast. Physical damage, such as strong winds or hail, can also bruise the buds and cause them to brown and fail to open. Careless use of gardening tools or late-season application of harsh herbicides can also cause localized injury, resulting in disfigured or aborted buds.
Corrective Care and Cultural Prevention
Effective control begins with strict sanitation, especially if Botrytis blight is suspected. Immediately remove and destroy all infected shoots, leaves, and buds as soon as they are noticed. In the fall, cut all peony stalks down to the ground and dispose of the debris away from the garden to prevent the overwintering of sclerotia.
Improving air circulation around the plants helps to reduce the high humidity that Botrytis requires to thrive. This can be achieved by ensuring proper spacing between plants and selectively thinning some stems in dense clumps. Avoid overhead watering, which keeps foliage wet for extended periods, and instead water deeply at the base of the plant, ideally in the morning.
Address nutrient deficiencies by applying a balanced, low-nitrogen fertilizer early in the spring, as excessive nitrogen encourages leafy growth at the expense of flowers. If thrips are a recurring issue, a horticultural oil or insecticidal soap can be applied to the foliage and buds. For persistent fungal problems, a preventative fungicide can be applied when new shoots emerge, and again every two weeks during cool, wet weather.