Herbaceous peonies are already sufficiently beautiful that it seems sinfully excessive to move to tree peonies where both the flowers and foliage are even more attractive. But indeed, bountiful beauty is part of the pleasure of gardening and one should look at tree peonies as simply the logical progression after you have enjoyed the herbaceous types. The flowers are large and incredibly exotic looking. See the recent post by Melissa Clark at Garden Shoots for a more extensive description. I have yet to acquire any tree peony with a more exotic name than “purple” or “white” but I suspect the time will come.
One of the attributes of the tree peonies is that they are more shade tolerant (indeed almost happy) in shade that would be troubling for an herbaceous peony. Too much shade will limit the flowers but the plant grows well in semi-shade. They also flower much earlier than the herbaceous lot so that there is still very much a role for the peonies of the other persuasion. In fact the old fashioned Festiva Maxima, a herbaceous type, still has a fragrance unmatched in the peony kingdom. More recently we have been trying to get started with some of the species types and the Itoh intersectional hybrids, both of which have wonderful foliage (which is important since most of the time the plants are not flowering). I look forward to sharing flowers from both next year.