The winters in U.S. Department of Agriculture hardiness zone 7a, where temperatures fall to 0 to 5 degrees F, are too cold for pink calla lilies to survive outdoors in the ground. Lift bulbs in fall and overwinter indoors.
In USDA zones 8 and warmer, pink calla lily rhizomes overwinter outdoors in the soil. A thick dry mulch over the soil provides extra insulation. In colder regions, the extent of freezing temperatures penetrates deeper and kills the dormant calla lily.
Once the fall frost kills back the pink calla lily foliage in USDA zone 7a, carefully dig up the rhizomes, cut off stems and roots and brush off soil. Place rhizomes in a cool, dry location over the winter and replant next spring.
Warmer microclimates in a garden may provide ample protection from cold to overwinter pink calla lilies outdoors in the soil in USDA zone 7a. This is not unheard of, but also is neither reliable nor likely to work. Moreover, newer cultivars of pink calla are typically bred for greenhouse culture, and thus their true hardiness outdoors is never absolutely known.