Calla lilies (Zantedeschia) are admired for their elegant, funnel-shaped blooms and glossy foliage. Originating in subtropical southern Africa, these plants prefer warm, frost-free conditions. Understanding their temperature limits is necessary for successful cultivation outside their native climate. While the common white calla lily (Z. aethiopica) and colorful hybrids have slightly different needs, all require protection from cold to thrive.
Ideal Temperature Range for Vigorous Growth
The optimal temperature range for active growth and flowering is relatively narrow, mirroring their mild native climate. Calla lilies flourish during the day when temperatures remain between 60°F and 75°F (15°C to 24°C). This range promotes the development of strong, healthy foliage and stems, allowing the plant to support its distinctive blooms.
For the best flower development and color saturation, the plants benefit from a slight temperature drop at night. Nighttime temperatures around 55°F (13°C) help prevent flower stalks from becoming too long and weak, resulting in a more compact plant. When temperatures exceed 70°F (21°C), flowering can decrease due to heat stress. However, the plants will tolerate warmer conditions if they receive sufficient water and afternoon shade.
The Critical Low Temperature Threshold
Calla lilies are tender perennials, meaning freezing temperatures directly threaten their survival. The foliage is extremely sensitive and is typically damaged or killed immediately by the first light frost at 32°F (0°C). Tender hybrids, especially colorful varieties, will not survive sustained temperatures below 41°F (5°C) without protection.
The underground rhizome, or tuber, is the plant’s storage organ and holds a slightly higher cold tolerance than the leaves. However, it is not freeze-proof and will likely die if subjected to prolonged freezing soil conditions. Most varieties are reliably winter hardy only in USDA Hardiness Zones 8 through 10, where the soil rarely freezes deeply.
In colder climates, sustained soil temperatures below 32°F (0°C) are fatal to the dormant rhizome. The risk of the tuber freezing and dying rises significantly when temperatures drop below 30°F (-1°C). Gardeners in zones below 8 must assume the plant cannot survive the winter outdoors without intervention.
Preparing Calla Lilies for Cold Weather
For gardeners outside the reliably warm Zones 8 to 10, protecting calla lilies requires specific actions based on expected winter conditions. The plant naturally responds to shortening days and cooling temperatures by entering dormancy. This transition is encouraged by reducing or stopping watering in the fall.
In marginal areas, such as Zone 7, some gardeners attempt to overwinter plants in the ground using heavy insulation. This involves cutting the foliage back after the first light frost and applying a four-inch layer of dry mulch, such as straw or shredded leaves. This method aims to keep the soil temperature consistently above freezing, but success is not guaranteed if deep freezes occur.
The most secure method for Zones 6 and colder is to lift and store the rhizomes indoors. Tubers should be dug up after the foliage is killed by a light frost and then allowed to cure in a warm, dry location, ideally between 65°F and 75°F (18°C to 24°C), for one to two weeks. This curing process toughens the outer skin, which helps prevent rot during storage.
Once cured, the tubers should be stored in a dark, dry, and cool location, such as a basement or unheated garage. The storage temperature must remain consistently between 50°F and 60°F (10°C to 15°C). Storing them in a breathable medium like peat moss or vermiculite ensures air circulation and prevents them from drying out completely while dormant. Container-grown calla lilies can simply be moved indoors to a frost-free location and allowed to go dormant in their pots.