I can't advise about the tap root, but have a little experience with over-eager tree peonies here in coastal NJ.

They do seem to be more hardy than they appear, but are so lovely, who wants to take a chance? I was worried these past two winters about flower buds swelling and leaves unfurling when some pretty severe freezes were still anticipated. I made a "tee-pee" with bamboo stakes and some clear solar pool cover material I had left over from my greenhouse-insulating project ... I imagine other materials would work. I put a few water-filled 1-gallon milk jugs inside, and piled some shredded leaves inside (not too high) and around the base of the tee-pee. Each year they've come through just fine, with neither frostbite nor fungus. I do watch on sunny days, though, and remove or vent, to be sure temperatures in there don't get too high.

Good luck with your tree peonies ... maybe with care and time that potted one will be fine.

I see that we are in the same zone Andrea. I think the one you potted will be okay. I moved two peony trees from my mothers yard (one was huge) after she passed away. One was about 40 years old and I wondered if I had gotten enough big roots for it to survive but, it did and will bloom for the second year in a row. I covered all three of my peony trees last night with sheets and will continue to for the next several nights. I see that is was about 30 degrees here last night. The next several nights will be the coldest they say. Just cover them will sheets, newspaper, burlap or something to protect them. They should be okay too. By the way, what colors do you have?