Advertised for 25 years, it sounds like a great idea
The Citrosa, or Mosquito Plant, has attractive foliage and a lemony scent.
You may have heard of a “mosquito plant,” a potted plant you can put outside in summer to repel mosquitoes. The idea seems great, if it would work. You’d set out some attractive greenery around your patio or back yard, that would not only look decorative but emit a pleasant odor that keeps mosquitoes away.
There are several plants that go by the name “mosquito plant,” but the one that was originally promoted most heavily and is still popular today is the Citrosa or Citronella Geranium, and that’s the one this article is about.
The question is, does it work? Well, according to scientific tests on mosquitoes from Canada to Florida, no, at least not the way the inventor advertised it. The leaves may have some value as a repellant if crushed and rubbed on the skin, but they don’t perform any better than any other citronella scent.
The story of the Citrosa is a classic tale of marketing hype. Its scientific name, Pelargonium citrosum ‘Van Leenii,’ reflects the name of its inventor, Dirk Van Leenen. He began developing the plant in Holland, then moved to the U.S. around 1984, according to an article in the September 9, 1985 Orlando Sentinel. In Miami, he started Horticultural DNA Products Inc. to sell the plant, and the marketing promises began. Within its first year, the company reportedly sold 25,000 plants, thanks to the enthusiasm of the inventor. “Sounding a bit like a carnival barker, Dr. Dirk van Leenen says that for only $5 he will give you a sweet smelling miracle plant which will rid your home of mosquitoes,” the Ocala Star-Banner said in 1985 .
By the early 1990s, news of the miracle plant had spread, sales were up, and the citrosa was now “produced by PhytoNova, a subsidiary of Royal Dutch/Shell that specializes in plant tissue culture. Austerica Inc., in Ontario, purchased the North American rights to the patented plant,” according to the New York Times, which ran an article on the news-worthy Citrosa in 1991. 
The high-tech names of the companies reflected the story behind the plant’s origin. Van Leenen had explained to an Ocala Star-Banner reporter in 1985 that this was no ordinary hybrid: “It’s a combination of two plants, but normally you do that by pollination… and have two compatible plants. In this case, they are not compatible. And then you take out some chromosomes or put in some chomosomes that have other characteristics.” 
The plant is still advertised as “genetically engineered.” Van Leenen claimed to have incorporated tissue culture from an Asian grass, Cymbopogon nardus, which produces mosquito-repelling citronella oil, into an English fingerbowl geranium and crossed it with an African lemon geranium, to produce an attractive geranium that emitted citronella scent. Crossing the two geraniums would be easy enough for any florist, but Van Leenen’s gene-splicing skills would be necessary to combine the dissimilar citronella grass with the geranium. 
Maybe not so high-tech
As sales increased in the early 1990s, scientists investigated the claims, and also analyzed the plant to try to confirm its origin.
Arthur O. Tucker and Michael J. Maciarello, of the Department of Agriculture & Natural Resources, Delaware State College, pointed out that “‘Citrosa’ does not have any unique compounds not found in other species of Pelargonium.” In other words, gene-splicing wasn’t necessary to create a geranium that produced citronella or related compounds. The common rose geranium (Pelargonium ‘Rose’) already contained the same related compounds in similar amounts as the Asian grass whose genes supposedly needed spliced in.
In fact, another Pelargonium variety called ‘Dr. Livingstone,’ already developed the natural way without gene splicing, contained 9% of one of the compounds, citronellal. That was a hundred times more than the mosquito plant, which contained only .09%.
Tucker and Maciarello expressed doubt that Dr. Van Leenen’s mosquito grass was different from a naturally-produced hybrid. They also quoted the opinion of Dr. Barry V. Charlwood, Director of Biotechnology at King’s College London, who was experienced in tissue culture of Pelargonium. Dr. Charlwood suggested that what Van Leenen claimed he did might not even be possible: “Although protoplast fusion is easy to carry out, selection of the required heterokaryon would be extremely difficult, and protoplast regeneration of fragrant Pelargonium is in our hands impossible.” 
But does it work?
Whether Citrosa came from a high-tech gene-splicing lab, or was the result of a florist crossing plants in the ordinary way, the important question was and still is: does it repel mosquitoes as promised?
It gives off a citronella-like odor, no doubt about that. A Long Island greenhouse owner brought “40 or 50” plants to a Fourth of July party in 1991, hoping to convince people to buy. “Historically, people are eaten alive,” he told a reporter aftewards. “This year nobody complained at all about the insects, and they bought up my supply [of Citrosa]. I’ve ordered a couple thousand more.”
The owner of a nudist resort in Kissimmee, Florida liked the plants as well, but reported something that scientists also were noticing: “They are a repellent, but sometimes a mosquito gets through the barrier. So we advise people to shake the plant, to give off more aroma, and to run their hands across the leaves and then rub their legs.” 
At the University of Guelph in Ontario, researchers tested the plants and discovered that before the plant was added to a small clear cage, a volunteer was bitten 40 times in thirty seconds. After the plant had been in the cage for half an hour, the bites dropped to 26 in thirty seconds. But when it had been there for an hour and the test was run again, the bites increased to 43 in thirty seconds. After a day, the bites were up to 61 in thirty seconds. The initial success had faded, perhaps because the plant was sitting still, after initially being jostled when moved into the cage, or perhaps because the mosquitoes had become used to it. 
Lemon thyme contains more citronella oil than Citrosa.
Crushing the leaves and rubbing them over the hands reduced bites to 12, but crushed lemon thyme, which contains more citronella, reduced bites to 6.5 on average.
The University of Guelph researchers published their findings in the March 1996 Journal of the American Mosquito Control Association, concluding that Deet, the old chemical standby, reduced bites by 90%, but “there was no significant difference between citrosa-treated and nontreated subjects.” 
About the same time, other researchers were finding similar results. Florida A&M; University scientists reported in 1994 that there were “no significant differences… in the number of mosquitoes landing on the forearms of human subjects in locations where [Citrosa] plants were present compared with areas without plants.” In laboratory cages, one species of mosquitoes actually preferred to rest on cut Citrosa leaves compared to paper cut in the same shape. 
Michigan researchers had the same result in 1993: “The ‘Citrosa’ plant was found to offer no significant repelling qualities in three different models of application used to test its reputed repellency.” They compared it to burning a permethrin coil, which they found “to offer significant repellency.” 
There was, and still is, no doubt that citronella oil repels mosquitoes, although it doesn’t last as long as Deet, but if applied to the skin, it can keep mosquitoes away for a couple of hours. 
However, there isn’t any objective evidence that just having Citrosa plants around will repel mosquitoes, even though they give off a citronella odor.
That doesn’t mean Citrosa plants are worthless. Rubbing the crushed leaves on your skin may repel mosquitoes like any citronella-based repellent for the skin. Some people are sensitive to the oil when it’s applied to their skin, though, so test a small amount and wait a day or two to make sure there’s no rash or redness, before trying more.
In addition, many people enjoy growing Citrosa plants for their ornamental foliage or because they give off a nice lemony odor, regardless of their mosquito-repelling properties. The plants have small lavender blooms in summer, but they don’t set seed, so if you want to propagate more, you need to do it from cuttings. They’re perennials, but are hardy outside only in zone 9b or warmer, so they’re best left in pots and brought indoors in the winter in colder climates. A mature plant may be three or four feet tall.
If you mainly want a potted plant with citronella oil in it, Citrosa isn’t the only option, or even the one with the most oil. Other choices are lemon thyme, hardy outside in zones 5-9, or lemon grass.
Just don’t expect Citrosa, or any plant, to repel mosquitoes without any work.
 “Cross-Hybrid Plant Gets Rid of Mosquitoes,”Ocala Star-Banner (March 11, 1985).
 “Citronella Without the Flames,”New York Times (July 25, 1991).
 “Cross-Hybrid Plant Gets Rid of Mosquitoes,” Ocala Star-Banner (March 11, 1985).
 J. E. Cilek and E. T. Schreiber, “Failure of the ‘Mosquito Plant,’ Pelargonium x citrosum ‘van leenii,’ to Repel Adult Aedes Albopictus and Culex Quinquefasciatus in Florida,” Journal of the American Mosquito Control Association, 10(4):473-476, 1994.
 Arthur O. Tucker, Michael J. Maciarello, “Is This Plant a Hoax? A Lecture in Honor of Otto Richter”
 “Citronella Without the Flames,” New York Times (July 25, 1991).
 Matsuda, et al, “Essential oil analysis and field evaluation of the citrosa plant ‘Pelargonium citrosum’ as a repellent against populations of Aedes mosquitoes,”Journal of the American Mosquito Control Association, (March 1996).
 J. E. Cilek and E. T. Schreiber, “Failure of the ‘Mosquito Plant,’ Pelargonium x citrosum ‘van leenii,’ to Repel Adult Aedes Albopictus and Culex Quinquefasciatus in Florida,”
Journal of the American Mosquito Control Association, (October 1994).
 Dr. George B. Craig, Jr., “The Citrosa Plant as a Mosquito Repellent? Failure in Field Tests in Upper Michigan,” 1993.
 C. Kongkaew et al, “Effectiveness of citronella preparations in preventing mosquito bites: systematic review of controlled laboratory experimental studies,”Tropical Medicine & International Health (July 2011).
Photos courtesy of Amazon Associates.
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