Frank from CAN, for some reason the Costco in the US does not sell the tree peonies that you are talking about. I remember last year someone said that they were offered in CAN and I checked for them but found that the offer was not available in the US.
First of all, I would like to clarify about the bareroot peonies planting in the spring. I am assuming that Frank (US) is referring to bareroot peonies purchased from vendors and planted in spring. THese roots are most likely dug in the fall, stored in a cool area, and then shipped in spring for planting. Most of these vendors such as Gilbert Wild will only ship them in late April or early May (at least for my zone). By the time I receive these roots, my peonies in my garden have already grown pips and stems. These roots if planted will not grow any feeder or tuberous roots during the summer eventhough some of them may send up stems and leaves. They won't grow any new roots until late summer or fall when their upper parts begin to go dormant.
In the case of moving a peony plant, I guess you can do it anytime but the earlier you do in their growth cycle, the more disrupted and shocked they will be. I try not to move any peonies in early spring while they are actively growing. The earliest I ever move a peony is right after its blooming.
