The survival of the fragrant grass Cymbopogon citratus, commonly known as lemongrass, depends entirely on the climate of its location. This tropical herb is a perennial in its native environment but is highly sensitive to cold temperatures and frost. For most gardeners, keeping lemongrass alive through the winter requires intervention. Understanding the plant’s cold tolerance, determined by the USDA Hardiness Zone, is the first step in planning for its survival.
Lemongrass Hardiness and Dormancy
Lemongrass is a tender perennial, living for more than one year but easily damaged by cold weather. In USDA Hardiness Zones 10 and 11, the plant remains outdoors year-round without special protection. Outside of these zones, specific overwintering strategies are required for survival.
The plant’s root crown possesses cold tolerance that the foliage lacks. While the leaves turn brown and die back when temperatures consistently fall below 40°F (4°C), the root system may enter dormancy. This dormant root system can survive brief dips in temperature, tolerating temperatures as low as 20°F (-6°C) for short periods.
The difference between dormancy and death depends on the duration and severity of the cold. In borderline areas like Zone 8 and parts of Zone 9, the plant may survive a mild winter, but a deep freeze will be lethal to the roots. Lethal freezing occurs when the ground temperature drops low enough to destroy the plant’s core. Assume that in any zone below 9, the plant will not survive an unprotected winter.
Strategies for Outdoor Winter Survival
Gardeners in borderline climates, such as USDA Zone 9 or mild pockets of Zone 8, may keep their lemongrass in the ground with significant protection. The primary goal of outdoor winterizing is to insulate the root crown from freezing ground temperatures. This strategy relies on the plant’s ability to enter a cold-induced dormancy and survive on its stored energy.
Heavy mulching is the most effective method for in-ground protection, applying a thick layer of material like straw, shredded leaves, or wood chips over the plant’s base. This insulating layer should be 6 to 12 inches deep and extend well beyond the diameter of the root clump. Cutting the foliage back to about 6 inches before mulching helps consolidate the plant and allows the mulch to better cover the crown.
Excellent soil drainage is important, as cold, waterlogged soil is fatal to lemongrass roots. The combination of wet roots and near-freezing temperatures can lead to rot. For severe cold snaps, covering the area with a row cover or plastic sheeting can trap geothermal heat and provide a few degrees of extra warmth.
Overwintering Lemongrass Indoors
In colder regions, generally USDA Zones 7 and below, the plant must be removed from the ground and brought indoors to survive. This process should begin before the first hard frost, typically when nighttime temperatures drop below 45°F (7°C). The plant is carefully dug up, ensuring a substantial portion of the root mass is retained.
The large clump can be divided into smaller sections, each containing several stalks and a healthy root system, which are then potted in containers with drainage holes. Before potting, the foliage is trimmed back to 6 to 12 inches to reduce water needs and make the plant more manageable indoors. The potted sections are then placed in a cool, dimly lit location, such as a basement or unheated garage, where temperatures remain consistently above 50°F (10°C).
During this period of forced dormancy, the plant requires minimal care, mainly a reduced watering schedule. The soil should dry out almost completely between waterings to prevent root rot. Once spring arrives and all danger of frost has passed, the potted lemongrass can be gradually acclimated to outdoor conditions.