Growing Fragrant Rhododendrons
Rhododendron ABC's #24
As I contemplate the results of our Rhododendron ABC discussions, I find that I have emphasized collection and culture of the members of the Genus with a view directed toward stimulation and satisfaction of our visual senses. There is another facet of rhododendron collection which has not been touched upon - the stimulation of the olfactory senses. Rhododendrons, and many other flowering genera in the main, rely on attracting insects to their flowers where, in their search for nectar, the insects transmit pollen from one flower to another, and thus make cross-fertilization possible. This occurs, primary, during daylight hours where light and color is the principle attractant for the pollinating creatures.
But what about those flowers that are open at night and have taken advantage of the presence of night-flying moths and even, in some cases birds and mammalian bats? Light and color play a minor role in this nocturnal activity and the production of aromatic substances becomes very important for the cross-fertilization process.
Detection of chemical aromas can be very keen in insects. Certain moths have been shown to be able to detect, with their antennae from long distances, the presence of female moths. They do this by intercepting air borne chemicals called pheromones which are produced by the females. Although the aromatic chemicals in rhododendron flowers are not strictly pheromones, they accomplish the same purpose and moths and other night pollinators can locate flowers whether or not there is any light or color. This is probably the reason why most fragrant rhododendrons are not highly colored. There is no need to produce bright colors to attract pollinators.
I have found that fragrance is most apt to be found in rhododendrons in the Maddenia and Fortunea subsections in hybrids derived from species in these subsections. Most of the fragrant flowers (and some of them have very pleasing aromas, indeed) are white or very light pink or yellow. I don't believe I've ever encountered a red rhododendron with a pronounced scent. Some of the Maddenii group with yellow flowers are scented. The tropical vireyas, or "Malaysian" rhododendrons, may have strong fragrance combined with deeper colors. Many of these flowers have long nectar tubes and hummingbirds and even bats are suspected of being attracted by scents as well as color. They use bills and tongues to secure nectar from the long tubes, and in so doing, cause transfer of pollen from flower to flower.
Even though most fragrant rhododendrons are not highly colored, they generally are large flowered and form pleasing additions to rhododendron collections. The largest flower in the Genus is found in the species, R. nuttallii Its trumpet-shaped fragrant corollas can be five inches long and six inches wide, and although mostly white, can be pleasingly tinged with pale pink and yellow. It and its fellow Maddenii species, such as R. lindleyii and R. formosum, can only be grown outdoors in the most favored climates, but when grown indoors, their fragrance can pervade an entire room. The more hardy Fortunea rhododendrons such as R. fortunei and R. discolor are also pleasantly scented and are very welcome specimens in the garden. Probably the most well-known rhododendron hybrid group, the Loderii hybrids, are esteemed as much for their fine aromas as for their large white to pink flowers.
Perhaps in the future, hardy hybrids that combine bright colors and fragrance will be developed but I think this will not be easy. Thirty years ago, David Leach in his book, Rhododendrons of the World stated that he believed this would be difficult to accomplish and time has proven him right. In the meantime, there is a lot of satisfaction in growing and flowering the light-colored scented rhododendrons that are available. Their aromas can be as fine and pleasing as costly French perfumes!
Addendum: Some of the deciduous azaleas species, and hybrids developed from them, are also very fragrant, though not as fragrant as some of the maddeniis and vireyas. Most notable of these are the near-white colored Oregon native azalea R. occidentale, the eastern U.S. azaleas R. atlanticum and R. viscosum and their close relatives. -lgn
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