The genus Lilium, or true lilies, encompasses a diverse group of bulbs celebrated for their large, striking, and often fragrant flowers. While popular in many gardens, their ability to withstand cold temperatures is not uniform. Cold tolerance varies significantly depending on the specific lily variety, its growth stage, and local climate conditions. The two central challenges for gardeners are understanding the genetic hardiness of their dormant bulbs and protecting the new, vulnerable foliage that emerges in spring.
Cold Tolerance Across Lily Varieties
A lily’s cold tolerance is determined by its genetic background and classification, which relates to the USDA Hardiness Zone system. This system identifies a plant’s ability to survive the average annual extreme minimum winter temperature. Dormant lily bulbs planted in the ground are significantly more cold-resistant than their above-ground foliage.
Asiatic hybrids are among the most robust, thriving in a wide range of climates and generally hardy down to USDA Zone 3. Their dormant bulbs can often tolerate temperatures plunging as low as -30°F to -40°F without additional protection. Martagon lilies also demonstrate this high tolerance, making them excellent choices for cold woodland areas.
Oriental lilies and Trumpet hybrids, while highly prized for their fragrance and size, are generally less cold-hardy than Asiatics. Without winter protection, they are typically suited for Zones 6 through 9. However, with proper mulching, these varieties can successfully overwinter in colder climates, extending survivability down to Zone 3 or 4. The difference in hardiness highlights the importance of selecting varieties that match the garden’s specific temperature limitations.
Winterizing Dormant Bulbs
Protecting dormant lilies, from late fall through early spring, is primarily about insulating the soil and managing moisture. For in-ground beds, a deep layer of insulating mulch is beneficial, especially where snow cover is unreliable. This layer should be applied only after the ground has frozen several inches deep.
Applying mulch too early can provide shelter for rodents, which may feed on the bulbs during winter. A thick, 4- to 6-inch layer of organic material, such as straw, shredded leaves, or pine boughs, is effective insulation. The primary purpose of this insulation is to prevent the soil from repeatedly freezing and thawing, which can heave the bulbs out of the ground or damage their root systems.
For lilies grown in pots, the bulbs are more exposed to freezing temperatures because container walls conduct cold air. To prevent the bulbs from freezing solid, container lilies should be moved to a protected, unheated location, such as a garage or shed. The storage temperature should remain below 45°F (7°C) to prevent premature sprouting, but must stay above freezing.
During indoor storage, it is important not to let the bulbs dry out completely, as true lily bulbs lack a protective tunic and are susceptible to desiccation. Watering lightly once a month is usually sufficient to maintain a minimal level of moisture. Non-hardy bulbs often mistakenly referred to as lilies, such as Canna or Calla, must be dug up and stored in a cool, dark place, often packed in peat moss or sawdust for the winter.
Safeguarding Emerging Spring Shoots
The greatest risk to established, hardy lilies often occurs in early spring when new growth begins and is met by a sudden late frost. Once the new, fleshy shoots emerge from the ground, they are highly vulnerable to temperature drops. The high water content in this new foliage means that temperatures falling below 30°F (-1°C) can cause cell damage, resulting in the shoots turning black or becoming limp.
For an anticipated cold snap, immediate, temporary protection is necessary to prevent this visible damage. Gardeners can cover the emerging shoots with inverted buckets, clay pots, or row covers to trap ground heat around the tender growth. A light sheet or blanket can also be draped over the plants, ensuring the cover extends all the way to the ground to maximize the insulating effect.
A proactive tactic involves thoroughly watering the lily beds several hours before the frost is expected. Moist soil holds significantly more heat than dry soil because water has a higher specific heat capacity. By replacing air pockets in the soil with water, the ground absorbs more solar energy during the day and slowly releases that warmth through the night. This thermal buffer can raise the ambient temperature around the emerging shoots enough to mitigate frost damage.