The plant commonly sold as “citronella” is often confused with the scented geranium, but the true source of citronella oil is the grass species, Cymbopogon nardus. This tropical, clump-forming plant is highly valued for its aromatic essential oils, which possess insect-repelling properties. As a native of tropical Asia, the plant’s survival is strictly regulated by the temperature boundaries of its environment. Understanding these temperature limits is necessary for successful cultivation outside of its native range.
Minimum Temperature Thresholds
As a tropical evergreen perennial, Cymbopogon nardus possesses a low tolerance for cold and is extremely sensitive to frost. The defining limit for this species is 32°F (0°C), a temperature it cannot withstand. Exposure to freezing conditions causes irreversible cellular damage and death. The plant is designated as a perennial only in USDA Hardiness Zones 10 through 12, where freezing temperatures are rare or absent. Even temperatures slightly above freezing can cause significant stress. When the plant is subjected to cold stress, the foliage may exhibit signs such as wilting, browning, or a dark, water-soaked appearance on the leaves, which quickly progresses to blackening as tissues die.
Maximum Temperature and Heat Stress
While Cymbopogon nardus is adapted to warm environments, its tolerance for extreme, prolonged heat is not limitless. The plant thrives best when daytime temperature remains within the range of 68°F to 86°F (20°C to 30°C). Although it can tolerate temperatures up to about 96.8°F (36°C), sustained temperatures above this can induce heat stress, especially when accompanied by low humidity and insufficient water. The plant prefers full sun to maintain vigorous growth, but intense, high-temperature afternoon sun can cause scorching. This damage often manifests as bleached or brown spots on the leaves. A lack of water during periods of high heat can quickly lead to the leaves turning yellow and the plant entering dormancy. Maintaining adequate soil moisture is important for preventing heat-induced damage and the rapid loss of moisture from the large leaf surface area.
Seasonal Protection Strategies
In regions outside of the tropical zones, particularly those below USDA Zone 10, Cymbopogon nardus must be treated as an annual or moved indoors to survive the winter. The most effective strategy is to bring the plant inside before the first forecast of frost. Moving the plant indoors when ambient nighttime temperatures consistently drop below 50°F (10°C) helps prevent cold shock and metabolic slowdown. For successful indoor overwintering, the plant should be placed in a location that receives ample sunlight, such as a sunny window, or under supplemental lighting. Managing the plant during periods of high summer heat requires focused attention on water availability. Applying a layer of mulch around the base helps retain soil moisture and reduce temperature fluctuation in the root zone. Providing temporary afternoon shade in arid, hot climates helps prevent leaf burn.