I believe that you can plant it now provided that you disturb the root as little as possible. I planted a potted itoh peony in April and it is doing fine.

Make sure you don't plant it any deeper than it is in the pot. You might want to brush the potting soil away gently from the base of the stems and make sure it isn't in the pot too deep to begin with, probably 1 - 1 1/2 inches from the top of the root to the surface of the dirt in your zone. You can plant a potted peony any time from spring on to fall, you'll just have to water more often until it gets established. Until then it'll probably droop enough to make you worry whenever it gets really hot but don't panic; it'll perk right back up as the temps drop in the evening. Just water at ground level in the early morning or evening or whenever you see it drooping too much. Good luck with your new peony! Alexander Fleming is supposed to be really fragrant and it sure is pretty!

Thanks! I will make sure to plant it at ground level.

Sometimes a potted peony sold at this time of year is terribly root bound. I bought one a few years ago and when unpotted it was so tight, the pot needed to be cut off, I did not want to disturb the roots, so planted as it was. I had to put an emitter directly on the root ball to keep it alive until the cool weather of the fall would allow it to be dug up and the roots to be opened up, and replanted at the correct height for this area. I hope yours was in better condition. Al