I am growing a tree peony in a pot as its permanent home. It was bought as a bareroot in spring of 2005 and planted in a 10" pot. After a winter outside (out of the direct weather and with some pine needles mounded around the woody stems), it came back nicely this spring and grew well during this summer. In fact, its leaves are putting on their version of fall coloration in preparation for dropping right now.

Regarding pot size, I would progressively pot it up as it grows. I put my bareroot graft (this is a japanese tree peony - "Seidei") into a 10" pot and once it outgrows that, I'll move it up a size or so.

The below is a pic of it after its first winter and when it first started sprouting:

This is a pic a couple weeks later with its new leaves and growth:

As I understand, tree peonies have been grown in pots for centuries in Asia so it definitely is do-able. You'd just want to make sure that your growing media is well draining. I am using straight Promix, which is a soiless container mix, and it seems to work okay with this. Since this one was grafted, I tried to make sure the graft was buried about 4" - 6" beneath the top of the soil so that it forms its own roots. For non-grafted, planting at the level it was growing should be sufficient. If you want to use a large decorative pot, you could plant the peony in a smaller one and sink that into the larger pot until it grows large enough to plant directly.

Tree peonies can be potted up for a few years. And then they may deteriorate due to being cramped unless you keep on giving them bigger and bigger pots every few years. The herbaceous ones are more forgiving becuase you can always divided them into smaller clumps.

To keep in pots for a long time you will need to root prune the plants. Removing enough growth to keep the plant in balance. I consider it better to keep potted tree peonies as a single or double trunk plant rather than allowing multiple trunks in a pot. If the top growth to too big for the root structure they will deteriorate but if the root structure is big enough to support more stems the plant will attempt to produce them. The balance is learned by trial and error.

Try cutting a block of roots from under the pine tree large enough for a big pot (5 gallon maybe). You may need a piece of rebar to probe for the roots before you dig or the base of your tree will look like squirrels have been active. Some pine trees have a lot of surface roots and few deeper than 2-3 feet. If you can sink a pot to below the surface of the roots and plant in the pot. You might be able to slowly remove the pot as the tree peony becomes more established and better able to compete with the tree roots. Many of the tree peonies are from wooded areas.

one can purchase root bags at Cricket Hill Gardens for 10 bucks apiece. they not only make transfers easier but i think they limit the size of root ball as well.

after i lost 4 tree peonies one winter to hungry pine voles -they ate up all the roots - i became a fan of container planting!