Potted peonies need moisture around their roots, but waiting for deep cracks or leaving the pot saturated can both cause problems. Check below the compost surface and make sure excess water can drain freely.
Check below the surface
A potted peony depends on a limited volume of growing medium. Check moisture below the top layer and lift the pot where practical. Do not wait for deep cracks in the compost as a signal that watering is due.
Wet the root zone and let it drain
When water is needed, apply it slowly enough to reach the root ball rather than running down a gap at the edge. Check that excess water can leave the container. Empty collected water from a cover pot or saucer instead of leaving the roots standing in it.
Adjust as the season changes
Warm growth, wind and a leafy canopy can increase demand, while cool conditions slow drying. Established plants in garden soil behave differently from those in pots. Keep that distinction in mind when reading watering advice. If foliage wilts while the compost is still wet, inspect the roots and drainage rather than assuming that another watering is the answer.
Related reading: Peony Flower Forms: Single, Lotus-Shaped and Double Blooms.