I find that all peonies seem to do best when they are planted in pots with good soil for the first couple of years, and then it will prefer going into the ground when there is no more room to grow in the pot.

I think a pot provides a more controlled environment as peonies hate wet feet, and you can also put as many nutrients as you want in there. I believe that you can get a lot more progress at least in the 1st year if they start out in pots, and the chances of survival are much better.

Here's a blog on potting up itohs. There's a link to an earlier blog when they were just potted up.

Here is a link that might be useful: Itoh peony blog

Okay, controlling the environment makes sense. Do you keep them in part sun during the spring/summer/fall? How often do you have to water them? What's the best way to overwinter them in pots? I assume the pots get sunk into the ground? I want to make sure I'm giving my plants the best start that I can...I certainly appreciate your expertise, your blog is very impressive! Thanks for taking the time to educate me!

In my climate "upstate NY" it never gets hot, and the sun never gets at a high enough angle to damage any type of peony so I just grow them in full sun. I'm guessing if you are in zone 5, you might be in the same situation. I usually watch the soil to see when it gets too dry. Peonies don't like to be too wet, but I make sure the soil has some moisture. In the summer I sometimes water every other day when we don't get rain.

Over the winter, I will put all my temperate plants including peonies together, then place a tarp over them, and surround the perimeter with leaf bags. This allows them to experience the heat from the ground. I usually do this once lows start dropping below 20F. You can probably get away with digging and sinking pots into the ground, but that's a lot of work given that I have too many pots.

Thanks for the advice, Steve!
Our local nursery says our micro-climate is closer to a 4B because of the elevation (around 6500 ft) and on the tail end of the Palmer Divide, also considered high desert. Summers here can be pretty brutal during the day and cools off quite a bit at night. My peonies in the ground don't burn during the summer, though. Does that mean the potted ones wouldn't, either?
I just received 3 Itohs and 2 tree peonies in the mail, Canary Brilliants, Pastel Splendor, Hillary, Hanakisoi and Kinkaku and have 3 more Itohs coming, Bartzilla, Old Rose Dandy and Scarlet Heaven. It snowed here yesterday and is supposed to again tomorrow so I put the 5 in pots, at least for the short term. Most were small enough to fit in quart size pots but not a lot of room left over (it's what I had on hand and wanted them out of the plastic bags). I assume I need to get them in at least gallon size pots?

If they are about the size of mine, and dormant, they won't flower anyways on their first year so I would suggest splitting them up in half or thirds ensuring each piece has eyes, then plant them in 2-3 gallon pots so you can have more plants. 1 gallon pots may dry up very quickly under full sun, and they prefer to have more root space. They can be easily split as long as they are still completely dormant.

This year, my larger pieces do seem to be bigger than the originals from last year with even more buds, some that look like they might flower.

Here is a link that might be useful: The size of my itoh peonies last year