The Calla Lily (Zantedeschia) is a striking plant admired for its elegant, trumpet-shaped blooms and lush foliage. Originating from southern Africa, this popular flowering plant is not a true lily but belongs to the Araceae family. Calla Lilies are perennial plants, specifically categorized as tender perennials or tropical bulbs. This classification means their ability to survive year-to-year outside depends heavily on the local climate and protection from freezing temperatures. While they naturally enter a dormant period before regrowing in their native environment, human intervention is often necessary to maintain them across seasons elsewhere.
Understanding Calla Lily Classification
The Calla Lily is botanically classified as a rhizomatous, herbaceous perennial, meaning it grows from a thick, underground stem structure called a rhizome. The term “tender perennial” is used because the plant cannot tolerate the deep or sustained freezes common in many temperate climates. Their survival outdoors is directly linked to the USDA Hardiness Zones, which map the average annual minimum winter temperature of a region.
Calla Lilies are reliably hardy and can be left in the ground year-round only in Zones 8 through 10, where winter temperatures remain above 10 to 20 degrees Fahrenheit. In these warmer zones, the plant goes dormant in the fall and resprouts in the spring, behaving like a true perennial. Gardeners in cooler climates (Zone 7 and below) typically treat Calla Lilies as annuals, accepting their loss to the first hard frost. Alternatively, they can be treated as tender perennials by digging up and storing the rhizomes indoors for the winter.
Ideal Growing Conditions
For the Calla Lily to thrive during its active growing season, specific conditions related to light, soil, and moisture must be met. The plants prefer full sun in cooler summer regions, which encourages abundant flowering. In areas experiencing intense heat or scorching afternoon sun, partial shade is beneficial to prevent leaf burn.
The rhizomes perform best in organically rich, well-drained soil that is kept consistently moist. Since they originate from swampy or seasonally damp regions in Africa, they have a high need for water, but must not sit in soggy conditions, which can lead to rhizome rot. Container-grown plants require a high-quality potting mix with excellent drainage and need more frequent watering than those planted in the ground.
During the active growth phase, Calla Lilies benefit from regular feeding to support their prolific foliage and flower production. A slow-release granular fertilizer applied at planting time in the spring is effective. Gardeners should avoid fertilizers with high nitrogen content, as this encourages leaf growth at the expense of developing the signature blooms. Container plants require more frequent feeding, often benefiting from a water-soluble fertilizer applied every few weeks.
Managing Dormancy and Overwintering
The period of dormancy for Calla Lilies begins in the late fall as temperatures drop, which signals the need to prepare for overwintering in cold climates. In Zones 8 to 10, the rhizomes can remain in the ground, but the foliage should be cut back to soil level after it has yellowed and died back naturally. Applying a layer of mulch, about two to four inches thick, over the planting area provides added insulation to the dormant rhizomes.
In colder zones, the process involves lifting the rhizomes from the soil before the first hard frost kills the plant. Once the foliage turns brown, trim it down to one or two inches above the rhizome. The rhizomes must then be allowed to cure by drying them in a warm, dry, and well-ventilated location (ideally 60 to 70 degrees Fahrenheit) for seven to ten days.
After curing, clean the rhizomes of excess soil and store them in a dark, dry, and frost-free location for the winter. A temperature range of 40 to 50 degrees Fahrenheit is ideal for storage, using a medium like dry peat moss, sawdust, or newspaper to pack the bulbs. Container-grown Calla Lilies can be overwintered by stopping all watering in the fall and moving the entire pot to a cool, dark place, allowing the plant to go dormant in its soil.
Calla Lily Safety and Toxicity
All parts of the Calla Lily plant, including the leaves, stems, and rhizomes, are toxic if ingested. The primary toxic agent is insoluble calcium oxalate crystals, which are microscopic, needle-shaped structures contained within specialized cells. When plant tissue is chewed, these crystals are released and penetrate the soft tissues of the mouth and throat.
This mechanical irritation immediately causes an intense burning sensation, oral pain, and swelling of the lips, tongue, and throat. Common signs in pets, such as dogs and cats, include excessive drooling, pawing at the mouth, vomiting, and difficulty swallowing. Handling the plant, particularly the sap, can also cause skin irritation, so wearing gloves during planting or cutting is recommended.