Calla lilies (Zantedeschia spp.) aren't true lilies, but they feature large lilylike flowers in spring. They grow as perennials in U.S. Department of Agriculture plant hardiness zones 8 to 11, although you can treat them as annuals in some conditions. Callas fall into two main categories. White calla lilies (Zantedeschia aethiopica) do best with moist or wet soils but are invasive in some areas. Hybrid varieties, such as pink calla lily (Zantedeschia rehmannii), prefer drier soil and are better suited as perennials outdoors.
Callas grow as perennials and can remain in the ground year-round when grown in mild climates within their zone range. They may even remain green year-round, depending on winter temperatures and sun. Extreme heat or cold can damage or kill the rhizomes so they don't return the next year. To improve perennial growth, keep the soil evenly moist at all times so the top 6 inches don't dry out completely. Provide callas with full, all-day sun or with some light afternoon shade so they can produce healthy leaves to fuel their tuberous roots for ongoing growth.
Potted callas also grow as perennials, whether grown indoors or out. Outdoor pots are more prone to drought stress, so check the moisture in the pot daily and water when the top 1/2 inch feels dry. Indoor plants may need to be watered only every two or three days. Low temperatures may kill back the tops of outdoor potted callas, but you can bring the containers indoors in fall and grow them in a sunny window so they stay evergreen.
Sometimes calla plants begin to decline. They may produce sparse or weak foliage or few flowers. This occurs when the perennial tuberous roots, called rhizomes, become crowded. Lifting garden callas every 2 to 5 years or potted callas yearly and dividing them ensures they continue to grow as healthy perennials. Dig up the roots after they finish flowering in summer. Cut them apart into 2- to 4-inch sections, with each containing at least two buds on the upper portion of the root. Replant immediately with the top of the roots 4 to 6 inches beneath the soil surface.
An Annual Affair
Some varieties of calla may not survive in areas with wet winter weather or low temperatures. Overly wet, cool soil can result in root rot, so the rhizomes don't survive winter to regrow in spring. You can treat the callas as annuals and replace them each year if this is a concern in your climate, or you can dig up the roots after flowering and store them indoors for winter. Allow the roots to dry, then remove the dead leaves. Store the calla rhizomes in peat moss in a dry, 55-degree Fahrenheit location until you replant in spring.
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