Peonies are classic, long-lived perennials. Given their basic cultural requirements, they bloom reliably with little care for many years. After several years, peonies will require fertilization. A careful application of food will ensure many more years of abundant blooms from your peonies. Always water immediately after fertilizer applications.
Peonies require well-drained soil rich in organic matter. Organic matter, such as compost, holds plant nutrients and releases them over time to peony roots. Soil pH should be neutral to slightly alkaline--about 6.5 to 7.0.
After many years, the soil may become nutritionally deficient as peonies use the nutrients and water washes them away. Often, old plantings of peonies build up with organic matter, raising soil acidity. Old peonies with leaves turning yellow or slowing growth but no signs of insect damage or disease need fertilizer.
Peony plant food should be low in nitrogen. Too much nitrogen will result in abundant foliage growth with little or no flower production. Fertilizers should contain no more than half as much nitrogen (N) as phosphorus (P) and potassium (K) (5-10-10, for example).
Apply fertilizer in spring when peony shoots are about 2 to 3 inches tall. Use according to label directions. Top-dressing with 1/3 inch of compost keeps the plant food in place without scratching tender roots.
Wood ash contains potassium and is a good peony food. Wood ash is alkaline and naturally modifies acidic soils. Containing no nitrogen, wood ash will not cause excessive leaf production. Mix a small amount of ash with compost (1 to 9 ratio) and top-dress to a depth of about one-half inch. The compost helps keep the ash in place, warrants against excessive soil alkalinization and naturally adds nitrogen to the soil.