What Is a Chemical to Treat Peonies?

Thoughts of peonies (Paeonia spp.) evoke memories of big, bowl-like blossoms overflowing with petals and fragrance. Hardy in U.S. Department of Agriculture plant hardiness zones 2 through 8, these long-lived garden classics normally require little maintenance. In certain conditions, however, peonies may be susceptible to disease and pests. Understanding common peony challenges, proper cultural practices and chemical remedies helps keep peony problems at bay.

Fungal disease can hit peonies any time, but the battle often begins with cool, wet springtime weather. Evidenced by fuzzy, gray mold spores, botrytis blight attacks peony buds, stems and foliage. The disease rarely extends down into the plant's crown, so chemical treatment with a fungicide can effectively limit botrytis blight's reach. Phytophthora blight doesn't exhibit fuzzy mold, but instead turns infected areas dark brown or black. This disease usually impacts peonies both above and below ground. Although fungicide can be applied for phytophthora, less success generally results. To limit and prevent these blights and other fungal diseases, early intervention works best.

Preventive care early in the growing season helps protect against blight and other fungal disease. Once fungus is present in a peony, fungicide can keep blight from spreading but will not eliminate the disease. The chemical mancozeb can be used to begin treating peonies in early spring, once shoots reach 6 inches tall. For general treatment, mix 3 teaspoons of mancozeb with 1 gallon of water and spray the plant thoroughly. Repeat this treatment at seven- to 10-day intervals throughout the growing season. In early spring and again in fall, drench the soil around peonies with this same solution. Always wear protective gloves, clothing and eyewear while working with chemicals. Restrict children and pets from the area and follow your product's label instructions explicitly.

Peonies have few pests, but interlopers such as thrips and scale insects can strike. Insecticidal soap treats both. Mix 4 teaspoons of insecticidal soap concentrate with 1 quart of water. Mix well and spray the plant thoroughly to fully cover all affected leaf surfaces. Repeat treatment once every week or two weeks, as needed. Follow your product's label instructions, and protect yourself and others as you would with mancozeb. While much folklore exists about ants and peonies, these insects have no significant impact. Ants feed on a sweet substance emitted by peony buds, without a negative or beneficial effect.

Proper culture remains the best weapon against peony problems. Peonies thrive in full sun and well-drained soil. Plant peonies in areas free from root competition with trees and shrubs. Place new peonies where peonies have not grown before. Provide air circulation and water at midday so foliage dries thoroughly. Do not use overhead watering; instead, water the plant's base. If disease hits, remove infected plant parts immediately. Work when peonies are dry. Sterilize garden shears with household disinfectant before and after each cut to prevent disease from spreading. In autumn, cut peonies to the ground. Dispose of all debris. Do not put it in the compost pile. Fungal disease can overwinter there and in the soil.