Geraniums are a popular garden addition due to the widespread belief that they repel insects, particularly mosquitoes. This idea suggests simply placing a potted plant nearby is enough to keep pests away during a summer evening. The belief is rooted in the plant’s distinct, often citrus-like fragrance, which signals the presence of natural chemical compounds. This article explores the science behind this remedy, clarifying which plants contain the repellent substances and how they work.
The Geranium and Pelargonium Distinction
The plant most people refer to as a “geranium” is, botanically, not a true Geranium at all. True Geranium species are hardy perennial plants, often called cranesbills, which are rarely associated with insect-repelling properties. The plant commonly sold as the “mosquito plant” or “citronella geranium” is actually a scented member of the genus Pelargonium.
The distinction matters because the repellent compounds are found almost exclusively in the scented Pelargonium varieties. These species, such as Pelargonium graveolens, release volatile oils when their leaves are crushed or brushed. Only the Pelargonium genus is relevant to insect repulsion, despite centuries of commercial confusion between the two genera.
How Geraniol Repels Insects
The scientific basis for the repellent effect lies in specific volatile organic compounds found within the Pelargonium essential oils. The primary active ingredients are monoterpene alcohols, notably Geraniol and, in some varieties, Citronellol. Geraniol is a colorless liquid with a pleasant floral scent, which is why it is often used in the fragrance industry.
These compounds work by interfering with an insect’s chemoreceptor system, the mechanism they use to detect potential hosts. When released into the air, Geraniol acts as a masking agent, jamming the olfactory signals that mosquitoes and other pests use to locate humans and animals. This disruption makes it difficult for the insects to find their target, causing them to move away from the area.
Geraniol is recognized by regulatory bodies as a biopesticide active ingredient. In laboratory settings, it has demonstrated efficacy against a wide variety of arthropods, including mosquitoes, ticks, and mites. Beyond its repellent function, concentrated Geraniol can also act as an insecticide by disrupting the nervous system and cellular membranes of insects, leading to dehydration and death at higher concentrations. Geraniol can exhibit greater repellent activity against mosquitoes than other common botanicals like Citronella or Linalool.
Practical Effectiveness of Plants Versus Oils
While the scented Pelargonium plant contains Geraniol, the practical effectiveness of a live, potted plant in repelling insects is significantly limited. The plant releases its essential oils at a very low concentration into the surrounding air, which is insufficient to create a wide area of protection. Placing a plant on a patio table may provide a small, localized effect, but it will not offer the reliable coverage needed to prevent bites.
To achieve meaningful insect repulsion, the active compounds require a much higher, localized concentration. Commercial repellents utilize concentrated essential oils derived from the plant for this purpose. These oils are formulated into products like lotions, sprays, candles, or diffusers, which deliver the compound at an effective dosage.
Studies using continuous-release diffusers containing concentrated Geraniol have shown a repellency rate against mosquitoes as high as 97% indoors. In contrast, a 5% Geraniol candle provided only a 50% repellency rate in the same indoor setting, illustrating that the delivery method and concentration are paramount. The US Environmental Protection Agency (EPA) treats Geraniol as a “minimum risk” ingredient, but its established presence in registered biopesticides validates its potential.