Lemongrass (Cymbopogon spp.) is the common name for several species of grasses in the genus Cymbopogon. These grasses are native to tropical parts of Asia, Africa and Australia, and they are generally frost-tender and do not survive winters in cold climates.
Annuals vs. Perennials
The term annual refers to those plants that complete their entire life cycles within a single growing season. Generally, annuals grow from a seed, produce their own flowers and seeds, and die in a single season, and new plants grow the following year from the seeds produced by the parent plant. Perennial plants, in contrast, survive through multiple growing seasons; although their reproductive cycles might be completed in one growing season, the parent plants will continue to grow and reproduce in subsequent seasons.
The year-after-year survival of perennial plants, however, is usually limited by their relative tolerance or intolerance of cold temperatures, and in climates where winters are colder than a given species will tolerate, the plant will usually die at the end of the growing season when the weather turns cold. When they're grown in climates where they're not winter-hardy, some perennials plants are treated as annuals by gardeners, who replant new plants each spring.
Sometimes when a perennial is grown in climates that fall just outside the climate in which the plant is fully winter-hardy, cold temperatures will cause the foliage and above-ground structure of the plant to die back, but the plant's root system will survive. In this case, the plant may survive the winter and begin to grow new foliage in the spring.
The Cymbopogon species most commonly grown as the garden ornamental called lemongrass (Cymbopogon citratus) is a perennial, but it is sensitive to frost and below-freezing temperatures, and in the United States, it is fully winter hardy only in U.S. Department of Agriculture plant hardiness zones 10 to 11. In these zones, the grass, which typically grows to a height of between 2 and 4 feet, will remain green throughout the winter.
In moderately cooler zones, lemongrass may survive the winter and return in the spring even though the plant's leaves die back. Lemongrass roots are typically hardy in USDA zones 8b and 9, and in these zones, the plant may return year after year.
In zones colder than USDA zone 8b, lemongrass will likely not survive the winter and must be grown in the garden as an annual. Alternatively, the plant may be grown in a container and taken indoors when temperatures begin to fall and over-wintered in a sunny room.