Why are my peonies failing?

Healthy, well sited plants are able to ward off pests and disease most easily. Be sure to water early in the morning, so the plant has a chance to dry off before nightfall to prevent fungus/black spot.  Make sure your plant is sited in full or morning sun, in deep rich soil, with plenty of room for the roots to grow, and with good air circulation. They like a deep, moist, but well-draining soil with lots of organic matter. Make sure there is not a lot of root competition with other plants or trees.  Cut them back when they are dormant. This could be in late fall or up to early spring. Cut them to the ground. Fertilize with a good organic fertilizer when you see flower buds, and once in the fall.