The crape myrtle, Lagerstroemia indica, is a popular deciduous shrub and small tree, celebrated for its vibrant summer flowers and attractive exfoliating bark. While it thrives in warm, sunny climates, successful cultivation outside its traditional southern range depends on understanding its temperature limits. How much cold the plant can withstand during winter dormancy is the most important factor determining where this species can be reliably grown. Knowing these thresholds allows gardeners to select the proper variety and implement protective measures for survival and robust blooming.
The Cold Threshold: Survival and Dormancy
Crape myrtles are deciduous plants that require a period of winter dormancy to sustain their growth and flowering cycle, allowing them to survive temperatures below freezing. Established plants are generally cold hardy within USDA Hardiness Zones 7 through 9, corresponding to minimum winter temperatures of 0°F to 10°F (-18°C to -12°C). When temperatures drop, the plant enters a period of deep physiological rest that increases its resilience. However, prolonged exposure to temperatures below approximately 5°F can cause significant dieback of the above-ground wood and branches.
The survival temperature for the plant’s root crown, the area just above and below the soil line, is often lower than for the branches. Mature crape myrtles may survive temperatures as low as -5°F to -8°F, even if the entire top growth is killed back to the ground. In extremely cold regions, such as Zone 6, crape myrtles are frequently grown as herbaceous perennials, meaning the top growth dies back completely each winter but the protected roots resprout vigorously in the spring. Temperatures dropping below -13°F (-25°C) pose a severe threat, often leading to the complete failure of the root system and the death of the plant.
Selecting Cultivars Based on Hardiness
Temperature tolerance is not uniform across all crape myrtle varieties, making cultivar selection the first line of defense against cold damage. Many resilient varieties are hybrids developed by the U.S. National Arboretum, which interbred Lagerstroemia indica with the more cold-tolerant Japanese crape myrtle, Lagerstroemia fauriei. These hybrids are often identified by Native American names, exhibiting superior cold tolerance and extending hardiness into Zone 6.
- ‘Natchez’ (white)
- ‘Apalachee’ (light lavender)
- ‘Sioux’ (pink)
- ‘Tonto’ (fuchsia)
The ‘Hopi’ cultivar is also known for its exceptional cold resilience, and some specialized selections have survived even more severe freezes. For instance, ‘Velma’s Royal Delight’ was reported to have survived -18°F in trials. Gardeners at the northern edges of the crape myrtle range should prioritize these genetically hardier varieties to minimize the risk of winter injury and ensure the plant maintains its tree form. Choosing a cold-hardy cultivar mitigates the need for extensive winter protection.
Practical Strategies for Temperature Protection
When growing crape myrtles near the northern limit of their range, mitigating temperature damage requires protecting the root crown from deep freezing. This is accomplished by applying a thick, insulating layer of organic mulch in late fall. A 2-to-3-inch layer of shredded bark or leaves spread over the root zone helps stabilize soil temperatures. Care must be taken to keep the mulch several inches away from the trunk to prevent rot.
For younger trees or those in Zone 6 or lower, additional protection for the aerial parts is advisable during extreme cold snaps. Wrapping the trunk and main branches with burlap or specialized frost cloth provides a buffer against temperature drops, especially for newly established specimens. Container-grown crape myrtles must be moved to a sheltered, unheated space, such as a garage or basement, where they are shielded from lethal freezing temperatures while dormant. Deeply watering the plant before the ground freezes also helps insulate the roots and prevent desiccation during winter.
Temperature Influence on Flowering
While cold tolerance determines survival, summer heat dictates the crape myrtle’s primary function: flowering. These are heat-loving plants that require sustained high temperatures to initiate and maintain their prolific summer bloom cycle. Flower initiation in many cultivars requires a substantial number of hours exceeding 85°F, which is why they perform best in regions with long, hot summers. Cool summers or persistent temperature fluctuations can significantly delay blooming or result in a less robust and shorter flowering period.
The plant also requires a preceding cold period. Winter dormancy is a necessary physiological rest that allows the plant to accumulate energy reserves required for vigorous growth and optimal flowering in the subsequent warm season. Therefore, a climate must strike a balance, providing a cold enough winter for dormancy but a hot enough summer to fuel the vibrant floral display. If summer temperatures exceed 100°F, the plant can experience heat stress, sometimes resulting in scorching or leaf loss, necessitating attentive watering.