Calla lilies are tender, perennial, summer-blooming bulbs that belong to the arum family. They are very sensitive to cold and hardy only in USDA zones 9 through 11. Calla bulbs can be dug, overwintered indoors and replanted in the late spring to be grown as annuals in cool climates beyond their natural range. The best time to harvest calla lily bulbs for spreading, moving or winter storage is in the early fall before the first frost.
Excavate the soil very carefully from around the perimeter of the calla lily plant or bulb. Begin digging a perimeter trench at least 6 inches from the main stem of the plant. Dig a trench at least 8 inches deep to ensure that you can get under the bulb with your shovel or trowel, allowing a buffer of an inch or two of soil.
Use a hand cultivator or soil fork to carefully loosen the roots from the soil. Lift and lever the bulb and root mass gently in several spots around the trench perimeter to loosen the roots. Removing a large amount of roots with the bulb will increase the success of transplanting.
Lift the root mass and bulb from the loosened soil of the trench. Pull the calla lily bulb and the attached roots from the soil. Gently brush off any excess soil from the bulb, but do not rinse them. If you are going to transplant the bulb with the plant top attached, place it in moist, rich soil immediately so that it does not dry out. If harvesting the bulb for winter storage, cut off the the top foliage and store it in a dry, dark and cool place at approximately 60 degrees F.