In general, the colorful hybrid Callas are less hardy and more apt to go dormant quite quickly after blooming. The white species type, Z. aethiopica, can remain evergreen throughout the season in areas where the summer heat is not too punishing and the soil is moist, but still must be dug if winters are very cold.
In some areas, such as the Pacific Northwest, Callas are a regular sight in gardens and along roadside ditches, spreading with abandon and needing the judicious hand of a gardener to pull them. Most of us however don’t have this problem and must overwinter our plants (north of Zone 8).
To do this, either keep individual pots plunged in garden beds for easy removal, or dig the clumps before the first frost. As Callas usually die back in the garden, it’s easy to forget about them. By putting a marker near the clump when in flower, you can help yourself locate them for overwintering.
Keep them just-damp during the winter months to prevent desiccation in an area that stays frost-free. When all danger of frost has passed, the rhizomes can be put outside for a new growing season. Don’t forget to refresh your potting soil and/or garden soil – Callas like a rich environment with plenty of moisture-retaining organics. If a longer growing season is desired, repot earlier in winter and put in a warm (65°F), well-lit spot indoors, keeping well-watered until plants can be set out in the late spring.
A good choice for aquatic gardens
As callas are originally a marsh-growing plant, they respond well to marginal, edge of pond situations and within the pond itself, growing in rich soil in a pot weighted with gravel.