The Crepe Myrtle (Lagerstroemia genus) is a highly valued ornamental tree or shrub, known for its long, colorful blooming season throughout the summer. Native to China and Korea, the species most commonly grown in the United States, Lagerstroemia indica, is a deciduous plant prized for its heat tolerance. Successful cultivation relies on understanding the plant’s capacity to survive winter cold, which is determined by a standardized mapping system.
Understanding USDA Hardiness Zones
The primary tool gardeners use to determine if a perennial plant can survive the winter is the USDA Plant Hardiness Zone Map. This map divides North America into zones based on the average annual extreme minimum winter temperature. Each zone represents a 10-degree Fahrenheit range, with half-zones (‘a’ and ‘b’) representing 5-degree increments. The map uses an average taken over a 30-year period (1991–2020), not a forecast of the coldest temperature an area will ever experience. Selecting plants rated for a location’s hardiness zone is essential for successful perennial gardening.
The Ideal Crepe Myrtle Hardiness Range
The accepted hardiness range for the common Crepe Myrtle (Lagerstroemia indica) is USDA Zones 7 through 10. Within this range, the plant reliably survives winter with little protection and matures into its full potential as a multi-trunked tree or large shrub. The canopy and branches are unlikely to suffer cold dieback in these zones. In Zones 7 and 8, the plant typically reaches heights of 15 to 25 feet. Crepe Myrtles thrive in the ample heat of Zones 9 and 10, though growers must ensure consistent moisture for optimal growth, especially during dry spells.
Expanding the Range: Cold-Hardy Cultivars and Techniques
The lower limit of Crepe Myrtle cultivation has been extended through breeding programs using the cold-tolerant Lagerstroemia fauriei species from Japan. Hybrid varieties, such as the ‘Indian Summer’ series, exhibit improved cold tolerance, making reliable growth possible in USDA Zone 6.
In Zone 6, even cold-hardy varieties are often root-hardy only, meaning above-ground branches may die back to the soil line during severe winters. Gardeners must treat the Crepe Myrtle like a die-back perennial, expecting it to re-sprout from the roots in the spring. Although the plant will still flower on new growth, it will not achieve the tree-like structure seen in warmer zones.
Techniques for Marginal Zones
To improve survival, select a protected microclimate, such as a sunny spot near a south-facing wall that radiates warmth. Applying a thick layer of mulch in the fall is recommended to insulate the crown and roots from deep freezing. Some experimental cultivars have been reported to survive in warmer parts of Zone 5, but success depends heavily on location and winter protection.