The blooms on a tree peony arrive earlier than those on herbaceous peonies, typically around April and May. The blooms are on a single woody stem that stands upright, unlike herbaceous blooms that tend to flop over once they become full. Tree peonies grow three to six feet tall.The leaves on tree peonies are also unique. They are bigger and more sculptured than regular peonies, and some of the edges have a reddish-purple tint when they first come out in the spring.
Peonies in containers can typically be purchased anytime between March and November. The best time to plant them is in the spring or fall. The location is very important--they don't like full sun or deep shade. Pick an area dappled with light or partial sunshine. The soil should be well drained. Enrich it with three magic ingredients: compost, peat moss, and cow manure.
The hole you dig for your tree peony should be as deep as your container and about twice as wide. Before you place the plant in the hole, add a high phosphorus, time-released fertilizer. Just scratch it into the soil.
Now gently remove the plant from the container and carefully pull the roots apart. Put the plant in the hole and cover up the roots with soil and a layer of mulch.
Bare-root tree peonies, which have to be ordered through a catalog or online, can be planted only in the fall when they are dormant. The plants will come with instructions that indicate where the top of the soil surface should be. Dig the hole about two feet deep and two feet wide with a mound in the center. Carefully place the roots on top and then check the depth with a shovel handle. Make sure the bud graft union is at least five to six inches below the soil surface. Then just cover up the roots with soil.
Whether you plant bare-root or containerized peonies, be patient. They take two to three years before they actually bloom, but it's worth the wait.