Mary, I also read that tree peonies do better in pots than herbaceous peonies. Evidently, the herbaceous peonies roots can get tangled and choke the plant to decline or death.
Here is a pic of the Lollipop itoh peony that I got from Michigan Bulb. Look at the small root system, perhaps grown in a 2.5 inch pot. Like Neal said, it is like a young seedling which probably requires a few years to mature enough to produce flowers.