Calla lilies (Zantedeschia aethiopica) have large green leaves and thick stems that bloom large, trumpet-shaped flowers. While called a lily, calla lilies are part of the genus Zantedeschia, not Lilium-like lilies. Their solid root structures are called bulbs and often are classified as bulbs. Technically, they are rhizomes. Calla lilies are winter hardy in USDA hardiness zones 8 to 10. In colder zones, the bulbs must be dug up in the fall and stored during the winter.
Cover the planting area during the growing season with about 2 to 3 inches of mulch; bark mulch or wood chips will do. This will help retain water and keep the soil from getting too hot during the summer. Give calla lilies with about an inch of water a week, and only when rain is scarce.
Allow the foliage to remain intact, even after the blooming season. The foliage is an important part of a calla lily. As long as it is remains green, the plant is absorbing sunlight, which is converted into sugar. This sugar is used for propagation and helps to promote a healthier and larger plant, including blooms, the next year.
Cut the foliage only after a frost has killed it in USDA zones colder than Zone 8 so that only 1 or 2 inches remain above ground. This makes it easier to see where to dig and easier to handle the bulb without damaging it. In warmer zones, just clip back individual leaves as they die off and keep the bulbs underground during the winter months.
Dig the bulbs carefully in USDA zones colder than Zone 8. Calla lily bulbs are planted as deep as 7 inches, so dig about 9 inches for the first plant to prevent damage to the bulb. Dig a circle 9 inches deep around the entire plant, about 2 inches from the stem. Then push down on the shovel or garden fork's handle in a few spots to gently lift the bulb out of the soil. If necessary, adjust digging depths based on the depth of the first bulb.
Keep the bulbs for one to two weeks in a dry, warm location but out of the sun. A spare room away from animals and children is best. When the stems can easily be pulled off of the bulbs, they are dry enough for winter storage.
Store the bulbs in a cool, dry location at 50 to 60 degrees F. Bury them in an open container filled with dry vermiculite or peat moss. Replant in the spring, after the last frost.