Calla lilies, known for their elegant, trumpet-shaped blooms, are often mistakenly called “bulbs” but actually grow from thick, fleshy underground stems called rhizomes. Understanding the proper timing for planting these rhizomes is crucial for success, especially in cooler climates. USDA Hardiness Zone 7, with average minimum winter temperatures between 0°F and 10°F, requires careful seasonal consideration to ensure these tender plants thrive.
Calla Lily Types and Zone 7 Hardiness
The colorful calla lilies found in garden centers belong to the genus Zantedeschia. Native to South Africa, these tender perennials are typically hardy only in Zones 8 through 10. Since Zone 7 regularly sees winter temperatures below the plant’s survival threshold, Zantedeschia rhizomes cannot be left in the ground year-round without significant protection.
Gardeners in Zone 7 must treat Zantedeschia as summer-blooming annuals or lift the rhizomes for indoor storage before the first hard frost each autumn. Successful growth depends entirely on planting the rhizomes outdoors only after the danger of freezing temperatures has passed. This timing ensures the delicate new growth is not damaged by cold snaps.
Determining the Ideal Planting Time
The primary factor for planting Zantedeschia rhizomes outdoors in Zone 7 is the date of the last expected spring frost. This typically occurs in mid-April to early May across most of Zone 7. Consulting a localized frost calendar is recommended to pinpoint the safest time for your specific area.
Planting should be delayed until the soil temperature consistently registers at least 60°F, with 65°F being ideal for robust root development. Cold soil inhibits growth and can cause the rhizome to rot before it sprouts. Monitoring the ground temperature is a more accurate indicator than relying solely on the calendar date.
To achieve earlier blooms, Zone 7 gardeners can start the rhizomes indoors in containers four to six weeks before the last expected frost date. Planting them in pots and keeping them in a warm, bright location gives the plant a head start before transplanting the growing plant outdoors once conditions are favorable.
Planting Depth, Spacing, and Soil Requirements
Once the optimal planting window has arrived, proper technique ensures the rhizomes establish quickly. Calla lily rhizomes should be planted two to four inches deep, with the “eyes,” or growing points, facing upward. This depth provides stability and protection for the developing roots.
When planting multiple rhizomes, spacing them approximately 12 to 18 inches apart allows for sufficient air circulation and room for the mature clump to spread. Proper spacing is particularly important for Zantedeschia, which can develop dense foliage. Immediately after planting, a thorough watering helps the soil settle around the rhizomes and begins the process of hydration.
Calla lilies perform best in soil that is rich in organic matter, consistently moist, and well-draining. Soggy conditions must be avoided, as excess moisture can cause the rhizomes to rot. The preferred soil pH tends to be slightly acidic to neutral. Selecting a site that receives full sun or partial shade is also important, though in the hotter parts of Zone 7, providing afternoon shade prevents leaf scorch and helps the blooms last longer.