The question of whether citronella spreads in a garden setting is common, yet the answer is not a simple yes or no. The growth habit depends entirely on the specific plant a gardener has purchased, as the common name “citronella” is applied to two botanically distinct species. Understanding the differences between these two plants is necessary to determine their potential for spreading. The tropical grass variety and the scented geranium cultivar exhibit dramatically different root systems and vigor, which dictates how aggressively they colonize a garden bed.
The Identity Crisis of Citronella Plants
The confusion surrounding “citronella” stems from the fact that the name is applied to two distinct plants. The true source of citronella oil is Citronella Grass (Cymbopogon nardus or C. winterianus), a perennial grass native to tropical regions of Asia and Africa. The second plant, often sold as the “mosquito plant,” is a Scented Geranium (Pelargonium species). While its foliage emits a strong lemon-like fragrance, it is not the source of commercial citronella oil. The disparity in their botanical families (Poaceae versus Geraniaceae) means their mechanisms for spreading are fundamentally different.
Growth Habits of Citronella Grass (Cymbopogon Species)
Citronella Grass (Cymbopogon species) has a vigorous and aggressive growth habit, particularly in warmer climates. This perennial grass spreads primarily through robust, underground stems known as rhizomes. These rhizomes grow horizontally beneath the soil surface, sending up new shoots to form dense, expanding clumps. In suitable USDA hardiness zones (typically 10 and above), the plant can reach heights of 5 to 8 feet and widths of 3 to 5 feet, rapidly colonizing a space. This quick lateral expansion makes the grass potentially invasive if left unchecked.
Growth Habits of Scented Geranium (Pelargonium Species)
The Scented Geranium (Pelargonium species) has a significantly milder, non-running growth pattern. This plant is a perennial subshrub, but gardeners in cooler zones (below USDA Zone 9) cultivate it as an annual due to its frost-tender nature. It grows in an upright, bushy, clumping habit, generally reaching a contained size of about 2 feet tall and 12 to 18 inches wide. Unlike the grass, the scented geranium does not produce aggressive rhizomes. Its minimal spread occurs mainly through seeds or easily rooted cuttings, making the plant relatively easy to manage and contain.
Managing and Containing Aggressive Spreading
Since the Scented Geranium has limited spreading potential, management efforts should focus on the vigorous Cymbopogon grass. The most effective way to prevent the rhizomatous spread of Citronella Grass is to grow it in containers, which confines the root system. If planting the grass directly into the ground in a warm climate, installing a physical root barrier is necessary to limit lateral growth. In regions where the plant is not hardy (below Zone 10), treating the grass as an annual or bringing it indoors for winter eliminates the risk of aggressive spreading.