Plants marketed as “citronella” are popular due to their pleasant, lemon-like scent and perceived ability to repel pests. Gardeners often use these aromatic plants to enhance outdoor spaces during the warm season. However, the plant’s survival is determined by its tolerance to colder temperatures as the seasons change. Understanding the specific minimum temperature a plant can endure is the most important factor for success in growing it beyond a single season.
Clarifying the “Citronella Plant” Identity
The term “citronella plant” is often used interchangeably for two distinct species with very different cold tolerances, creating significant confusion for gardeners. The most common plant sold under this name is a scented geranium, scientifically known as Pelargonium citrosum or the ‘Mosquito Plant.’ This shrubby perennial has lacy, lobed foliage that releases a citrus fragrance when the leaves are crushed.
The true source of commercial citronella oil is citronella grass, belonging to the species Cymbopogon nardus or Cymbopogon winterianus. This is a tall, clumping, tropical grass with thin, reed-like blades. While both plants release a similar scent, the scented geranium is significantly less cold-tolerant than the true citronella grass. This distinction is crucial because appropriate winter care varies drastically depending on the species a gardener possesses.
The Hardiness Threshold
The minimum temperature a citronella plant can withstand depends entirely on its species. The scented geranium (Pelargonium citrosum) is highly sensitive to cold and is considered a perennial only in USDA Hardiness Zones 9b through 11, which are largely frost-free areas. Damage begins when nighttime temperatures consistently fall below 50°F (10°C), and the plant should be moved indoors promptly when temperatures dip to 45°F (7°C). Exposure to any hard frost (below 32°F or 0°C) is lethal to the leaves, stems, and root system of the geranium.
Citronella grass (Cymbopogon species), also a tropical perennial, thrives in warm, humid conditions. Even light frost can cause severe damage to the grass. Although its root structure may survive a very brief, light frost, the foliage will die back completely at the first sign of freezing temperatures. Both species cannot survive the sustained freezing conditions experienced in temperate climates and must be protected well before the first frost is predicted.
Strategies for Winter Survival
Gardeners outside of the warmest climates must keep their plants alive through the winter. The most reliable method for both the geranium and the grass is to transition the plant indoors once temperatures approach 50°F. Container-grown plants should be pruned back by about one-third to manage size, and any damaged foliage must be removed before the move.
Overwintering Pelargonium (Geranium)
Once inside, the Pelargonium can be kept actively growing in a bright, sunny window or under a grow light. Watering must be drastically reduced during this period, allowing the soil to dry out slightly between waterings. Excess moisture can lead to root rot as the plant’s growth slows.
Another option is to overwinter the heavily pruned Pelargonium in a state of semi-dormancy. Store the plant in a cool, dark location, such as an unheated garage or basement, where temperatures remain between 45°F and 55°F.
Overwintering Cymbopogon (Grass)
Since Cymbopogon is a tropical grass, dormancy is not a natural state for this species. It is often easier to treat it as an annual in cold zones. Alternatively, gardeners can dig up the clumps and keep them actively growing in a brightly lit, warm indoor space.