Calla lilies (Zantedeschia) are admired for their elegant, trumpet-shaped blooms and glossy foliage. They grow from a fleshy underground rhizome and are classified as tender perennials. A tender perennial lives for more than two years but cannot survive freezing winter temperatures. In USDA Hardiness Zone 6, Calla Lilies are not reliably cold-hardy and require intervention to return the following spring.
Hardiness Status of Calla Lilies in Zone 6
The USDA defines Zone 6 as having average annual minimum winter temperatures between \(-10^{\circ}\text{F}\) and \(0^{\circ}\text{F}\). This range is consistently lethal to the rhizomes of most Zantedeschia species, which are native to warmer Southern Africa. Most colored Calla Lilies are reliably hardy only in Zone 8 and above, where winter temperatures remain above \(10^{\circ}\text{F}\).
Leaving the rhizomes in the ground in Zone 6 exposes them to prolonged freezing temperatures. The plant’s root structure requires soil temperatures well above freezing to survive dormancy and initiate new spring growth. While a thick layer of mulch might offer protection in a very mild winter, the risk of total loss is high during a typical cold snap. Therefore, Calla Lilies should not be treated as reliable perennials in this climate.
Overwintering Calla Lily Rhizomes
Lifting and Curing
The most reliable way to ensure Calla Lilies return is to manually lift and store the rhizomes indoors during winter dormancy. This process begins in late autumn, immediately following the first light frost. The frost signals dormancy, causing the foliage to yellow and collapse, ensuring the plant moves energy reserves back into the rhizome. Once the foliage has browned, gently lift the rhizomes using a garden fork, avoiding punctures.
Cut the remaining stems and leaves back to one or two inches above the crown. Next, the rhizomes must undergo a curing process by drying in a cool, dark, and well-ventilated area for several days to a week. Curing allows damaged areas to heal and the outer layer to harden, which prevents rot during storage.
Storage Conditions
After curing, store the rhizomes in a medium that provides insulation and air circulation without retaining excessive moisture. Storage mediums include dry peat moss, vermiculite, or loose paper bags that allow the rhizomes to breathe. The ideal storage location is a cool, dry space, such as a basement or unheated garage, with a consistent temperature between \(40^{\circ}\text{F}\) and \(50^{\circ}\text{F}\). Higher temperatures encourage premature sprouting, while temperatures near freezing cause damage.
Winter Monitoring
Periodically check the stored rhizomes throughout the winter for signs of mold, shriveling, or soft spots, which indicate rot from excess humidity. Immediately remove any affected areas using a sterile knife to prevent decay from spreading. Maintaining cool, dry dormancy keeps the rhizomes viable until they can be replanted outdoors in the spring after the last frost.
Treating Calla Lilies as Seasonal Annuals
For gardeners preferring a low-maintenance approach, Calla Lilies can simply be treated as seasonal annuals, similar to petunias or impatiens. This method bypasses the labor-intensive process of digging, curing, storing, and replanting the rhizomes annually. The plants are enjoyed throughout spring and summer until the first hard frost naturally kills the foliage and the underground structure.
Treating them as annuals allows maximum flexibility in garden design, as new rhizomes or potted plants are purchased each spring. This eliminates the risk of storage failure due to rot or improper temperature control during winter months. While this approach involves recurring expense, it guarantees a healthy start and a full season of vibrant blooms without needing specialized overwintering space.
True Perennial Alternatives for Zone 6
Gardeners seeking a similar architectural look without winter lifting have several true perennial options reliably hardy in Zone 6.
- Oriental Lily (Lilium species): These offer large, dramatic, trumpet-shaped flowers on tall stems. Many cultivars are hardy down to Zone 4, providing vertical elegance and blooming reliably from bulbs left in the ground.
- Jack-in-the-Pulpit (Arisaema triphyllum): Hardy down to Zone 4, this native woodland plant mimics the Calla Lily’s spathe-like structure. It produces a hooded bloom (the “pulpit”) surrounding a central spike, offering a sculptural element for shaded gardens.
- Copper Iris (Iris fulva): A strong candidate for areas with moist soil, this iris produces elegant, funnel-shaped flowers on tall stalks and is reliably perennial in Zone 6 and colder.