It should be repeated when the plants are about half grown and again just before they bloom. To prevent leaf spot they can also be sprayed several times after blooming. If the peony was planted in properly enriched, fertile loam to start with, the plant should not need additional fertilizer for 3 to 4 years. Over fertilized peonies will produce foliage instead of flowers, so fertilize with discretion. The type of soil will determine the need for additional fertilizer.
Porous soils lose their fertility more rapidly, as nutrients are leached out by rain.
Keep all fertilizer away from the crowns of the plants. Apply fertilizer in the fall. After September the tops of herbaceous peonies should be cut to the ground. Never cut tree peonies back to the ground, unless the stem is diseased, as the next year's growth starts from the buds formed on the old wood. All foliage of both, herbaceous and tree peonies, should be collected and destroyed or composted to prevent diseases.