It is difficult to exaggerate the effect a tree peony flower has on a gardener. Big and beautiful enough to be seen clear across the nursery, gardeners will make directly for these spectacular bloomers as quickly as any bee. With hundreds of fragrant, long-lived (up to one hundred years) varieties to choose from, good luck leaving the garden center without one.
The best times to plant tree peonies are in early spring, as soon as soil is workable, and in the fall before permanent frost hardens the ground. Choose a site, free of other tree roots, that gets either 4-5 hours of morning sun or has dappled shade all day, as strong sun will prematurely wilt flowers. Tree peonies will do best in fertile soil that drains well; excessive water around roots will cause rot. Dig a hole two feet deep by one foot wide and amend with plenty of organic matter. Water well and cover base with compost or other organic mulch. Don’t use wood chips, which harbor a fungus that can infect tree peonies.
Water peony trees weekly until well established, but don’t overdo it. Powdery mildew is a common problem for tree peonies. Caused by excessive dampness, powdery mildew usually appears in summer, causing a discoloration of the leaves that won’t kill the plant but is unattractive. Remove any affected leaves, and remove all leaves in fall to prevent fungus from overwintering. Then treat plant with a copper spray in spring. Fertilize every 2-3 weeks with a low nitrogen fertilizer, until a month before frost. Add an extra few inches of mulch for winter protection. Tree peonies bloom on old wood, so prune only after flowering and only if necessary, either to improve shape or to remove dead, crossing, or inward growing branches, preventing disease. Make angled cuts just above a bud, cleaning pruners with a solution of one part bleach to nine parts water after each cut.