I have seen standard peony roots 12 inches and more for the large storage roots and another 12 inches for the feeder roots. I think a lot depends upon the soil, temps, and how well the initial hole was dug. Ticker and Magnolia had some good info concerning the correct way to plant peonies and how the preparation of the hole area was critical for successful growth. Perhaps they can repost the info since it can make a major difference in the survival of the peony.
