Calla lilies (Zantedeschia species) will survive winter in the ground in warm zones 9 and 10, but they must be lifted and stored indoors in your cooler zone 6. If you can get them to go dormant by withholding water in the fall, that's great. Otherwise, let a light frost kill the tops. Then gently pull the bulbs from the soil and set them out on a tray to dry for a day. Slip bulbs into small paper bags and put them into a cool, dry place for winter storage. An unheated basement is ideal. Pot them up again in March or April for blooms next summer.