Calla lilies are subtropical perennial flowering bulbs that are hardy in USDA zones 9 to 11. In climates cooler than these, calla lilies are grown as annuals and the bulbs are dug up in the fall and overwintered in a protected location. They are replanted into the garden soil in the spring after the last frost has passed, leafing out in foliage in the late spring and blooming again in the summer.
Cut off the spent blooms and wilting foliage of your calla lilies after the first hard frost in the fall. Cut down to the crown of the plant with secateurs (pruning clippers) to just above the soil line.
Excavate around the plant crown and bulb with your hand trowel until you can easily lift the bulb from the soil without damaging it. Dust the soil off of the bulb roots and crown and lay out the bulbs on a few layers of newspaper to dry out for a few days, up to a week. After the drying period, carefully cut off any loose roots and any remaining foliage attached at the top of the bulb.
Prepare a shallow storage container to hold your calla bulbs in--no more than two layers deep. Fill the storage container three-quarters full with clean, dry sand or peat moss. Nestle the bulbs in the storage medium so that the bulbs are each surrounded with medium but not touching one another. Lay a half-inch layer of storage medium over the bulbs.
Cover the container lightly if desired by resting a lid on top. But do not seal the storage container. Place the container in a dry, low-lit and cool but frost-free location for the winter rest period. Inspect your stored calla bulbs periodically to look for rot or mold. If you see either, lift the bulb out with the immediate surrounding storage medium and discard.
Take your bulbs out of storage in the spring after the last threat of frost has passed. Plant in moist, nutrient-rich soil and water deeply. New green shoots should appear within a few weeks.