Question
How far back should you prune and what season?

Answer
Hi Brad,
Thanx for your question.  After the tree peony is several years old and has many branches, you may find that you need to prune out extra growth in the interior of the bush. Select 6 to 10 of the strongest branches as the major stems of the plant. This will allow the peony to concentrate its strength on flowering in its main branches, rather than excessive leaf growth. It will also eliminate chances for fungus to grow, allowing more light and air to circulate in the plant. If you prefer to see the wood of the tree peony at the base, you can prune out leaf buds low to the ground. This is a Chinese style of shaping. The natural habit for most Chinese tree peonies is a very dense, spreading, rounded form, though some varieties, like White Screen Reflects a Blue Jewel, Phoenix White and Lotus that Shines in the Sun are naturally tall and woody at their base.  Prune in the early spring before the last frost.

I hope this helps.
Tom