Peonies should not need regular fertilization if a laboratory soil test (performed every ~3 years or so) hasn't indicated that a particular nutrient is deficient in the soil. If any are deficient, the lab will usually provide application dosage recommendations, though a precise N-P-K analysis isn't necessary as long as something comes close for supplying the needed nutrient(s). Established plants that are growing well and not showing indications of a foliage nutrient deficiency (like chlorosis) rarely need supplemental fertilizer. When soils are not deficient, it means the roots have plenty of what they need, so adding more nutrients won't spur extra growth or provide other benefits. In addition to contributing to stormwater pollution, over-fertilization can detriment plants, like increasing the risk of pest or disease damage, making stems floppier, reducing flower formation, or causing root stress from the higher mineral levels, so we generally do not recommend fertilization for ornamental plants unless there is a demonstrated need for it (like soil test results).

Miri