Pelargonium citronellum by melody, used with permission, thank you melody, plant provided by Beans to Blossoms of Murray, KY
The term mosquito plant is sometimes, but not always, used to mean "a plant which repels mosquitoes." The original mosquito plant and source of citronella is citronella grass, Cymbopogon nardus, a tropical grass from Ceylon (Sri Lanka). This grass is still used to produce "Ceylon" type citronella oil. A closely related species, C. winterianus, was developed in the 1800s and is the source of perfume quality "Java" type citronella oil. Both Citronella grasses, like their close relative lemon grass, are native in southeast Asia. These tropical species can be grown outdoors for the summer in most American gardens. Either citronella grass can be planted from new divisions in spring as the soil warms up. Citronella grass needs full sun and plenty of water, and will grow to a tall, grassy mound with arching, one inch wide, leaves and erect (not very showy) flower spikes. The plant will naturally emit a lemony scent as citronella oil evaporates from its leaves.
Citronella grass in Dinu's garden in India, where the grass is sometimes used for tea and medicinally
Citronella oil is not one pure chemical but actually contains more than 80 different compounds. The primary ingredients of citronella oil, giving it that distinctive, sharp, lemony scent, are geraniol, citronellal, and citronellol. These oils are also found in other plants, in varying amounts. The most widely grown, mosquito-repellant, citronella scented plant for temperate gardens is really no more exotic than familiar garden geraniums. Commonly called mosquito plant, it would be specifically labeled as a Pelargonium citrosum or a Pelargonium citronellum. Either Pelargonium mosquito plant is one of the group of scented geraniums and as easy to grow as other annual zonal geraniums. Many nurseries sell small mosquito plant geraniums in spring. They can be planted in the ground or in pots. Give this geranium a sunny location and good soil, and it and will grow into a bushy, medium sized flowering plant. For mosquito protection, you'll want to place it near where you spend time: in the garden or on the porch, perhaps. The fuzzy, coarse textured foliage is very aromatic. Like many geraniums, these plants root easily from cuttings. In late summer you may wish to take cuttings so you can bring the plant to a sunny windowsill for the winter. Like citronella grass, the geranium mosquito plant will not live through frost.
palmbob's view from the base of a Lemon Scented Gum
Looking for a whole lot more mosquito help than with a garden sized plant? The lemon scented gum tree (Corymbia citriodora) produces an oil containing a high percentage of citronellal. This tree is a member of the eucalypt group, making it a cousin to the pungent florist eucalyptus with which you may be familiar. The lemon scented gum is a large tree native to Australia and used in landscapes in mostly-frost-free areas. This handsome tree has patchy bark when young, (it brings Sycamore to mind) developing into an incredibly smooth white trunk. Despite its high citronellal content, this tree is not suggested as a mosquito repellant plant for the backyard. Maybe that is related to its preference for wamer drier zones which don't foster mosquitoes. (Or could it be that mosquitos zip right under the radar, so to speak, of citronella being produced dozens of feet up in the air?) Most mosquito sufferers have neither the correct zone nor the room for a one hundred foot mosquito repellant tree, not to mention the patience to wait for it to grow to that size. The lemon scented gum has fans who admire its unique trunk and fragrant bark and leaves. See pictures of the tree in PlantFiles at this link.
Now for the bad news: the scientific community has failed to prove that growing these plants around one's outdoor living space will provide much relief from moquito attack. Some Dave's Gardeners swear by Pelargonium mosquito plants and others grow them simply for their beauty and fresh scent. According to Mark S. Fradin, MD, in this paper, use of DEET repellants or Bite Blocker™ brand plant-based repellant, is the sure way to really keep the skeeters off. If you do buy a "mosquito plant" for its bug busting properties, you should also follow recommendations for a mosquito-unfriendly yard. The University of Kentucky Entomology Department provides an excellent guidelines here, as well as further information about mosquitoes.
Photo thanks to DG subscribers Dinu, melody, and palmbob for use of pictures in PlantFiles, and additional pictures.
Thanks to Beans to Blossoms of Murray, KY; melody says "They graciously let me use their plant for the photo"
Fradin, Mark S. M.D. "Mosquitoes and mosquito repellants: a clinician's guide" ACP-ASIM Annals of Internal Medicine, 1 June 1998, accessed 8-14-2011. http://www.fmeainfocentre.com/fences/downloads/Mosquitoes%20and%20Mosquito%20Repellents%20-%20A%20Clinician's%20Guide.PDF http://en.wikipedia.org/wiki/Eucalyptus
Potter, M. F., Lee Townsend, and F. W. Knapp, "Mosquitoes: Practical Advice for Homeowners," University of Kentucky, revised 7/03, accessed 8-16-2011. http://www.ca.uky.edu/entomology/entfacts/ef005.asp
Dave's Garden PlantFiles~ linked to text in article
Dave's Garden PlantScout ~ click here, enter a plant name and find vendors
Dave's Garden Go Gardening ~ click here to search for nurseries, gardens, and more
Bite Blocker™ http://www.biteblocker.com/intro.html
Floridata, Cymbopogon nardus, http://www.floridata.com/ref/c/cymb_nar.cfm
Wikipedia, Corymbia citriodora, http://en.wikipedia.org/wiki/Corymbia_citriodora