Plant it NOW; don't wait til fall. Hopefully the vendor has sent planting instructions.

If the vendor did not send planting instructions please plant with the graft and part of the stem under the surface of the dirt. Most tree peonies are grafted plants. To insure the grafted plant(the part that you want to grow) sends out it's own roots part of the stem must be below ground. Some people actually bury with only the tip showing. I only do this if the grafted stem is very short.

A word of advice I find with many of the grafted peonies the stem that is grafted will die this winter. However if I leave the plant alone a new stem may come up not next spring but the following spring. So patience is the word. I find this is most often true with the tree peonies that are imported directly from China. The stem dying may happen several years in a row but once the plant has established itself the resulting plant is quite vigorus. This is from my experience in my area and may not be the same for everyone.

With herbaceous peonies one should always plant them in the fall, but with tree peonies, I personally, have better luck if I do plant them in the spring. Just remember when planting tree peonies you need to plant them much deeper than herbaceous peonies.