Answer: Peonies often flop over when they are heavy with foliage, buds and bloom and/or later in the season. For this reason they are often staked, sometimes by using a peony ring, sometimes by creating a supporting network of string or twine and sticks. Next spring, place a peony ring or a piece of chicken wire in place before the plant starts to grow. Allow it to grow up through the support;it will become invisible when the plant leafs out.
Planting correctly would mean selecting a location in full sun with good soil and good drainage, meaning not soggy. The "eyes" are usually planted about two inches below the surface and peonies are sensitive to being planted too deep. Since yours was in a pot, the depth is probably okay as long as you planted it at the same depth as it grew in the pot. The main result of planting too deep is a lack of bloom, it will not kill the plant.
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