The shelves at the local nurseries are stocked with "mosquito plants," sometimes called "citrosa," that are reputed to repel mosquitoes. "Buy these plants and your deck/porch/home will be mosquito-free!" Fact or fiction?
As far back as 1996, studies showed that the citrosa plant, Pelargonium citrosum 'Van Leenii', had no discernable mosquito-repelling qualities. Still, almost a decade later the plants are still being marketed as effective controls.
According to a 1997 article in HerbalGram, a publication of the American Botanical Council, although the promotional literature claims that citrosa has been genetically engineered to produce citronella oil, chemical analysis has shown that the plant contains only a trace amount, less than 1 percent. And, based on the results of the study, "the authors conclude that citrosa should not be marketed as a mosquito repellent."
Even real citronella, a 6-foot-tall tropical plant that is the source of the citronella in citronella candles, isn't an effective repellent sitting in a pot. The leaves must be crushed to release the essential oils. Some studies have shown that crushed citrosa leaves do offer a degree of short-term mosquito-repelling qualities, on the order of 30 to 40 percent that of DEET, but there are other plants with even better numbers. For example, crushed lemon thyme (Thymus X citriodorus) offered 62 percent the protection of DEET. And lemon balm (Melissa officinalis) contains a much greater quantity of citronella, on the order of 400 times as much, as the citrosa plant. Plus, lemon thyme and lemon balm are less expensive and easier to grow.
If you want to test the repellent properties of citrosa or any of these other plants by rubbing the crushed leaves on your skin, be sure to test them out on a small spot first. Rub the crushed leaves several times a day on your inner forearm or another area of tender skin and watch for a reaction; the essential oils have been known to cause rashes on some people. If, after a day or two, the skin looks fine, feel free to use it elsewhere on your body and see if it works!