Fragrance in Roses
Scent is an incredible potential tool in the repertory of the gardener. Fragrance can tease, evoke memories and emotions, or set a mood. However, most of the time, we fail to make use of scent in even the most elemental way when planning a garden. There are some good reasons for this common neglect of one of our senses. Scent is something that enhances our lives, at this point in our evolution. It is not a human survival tool. Although our noses are considered to be 10,000 times more sensitive than our sense of taste, we humans have rarely tried to learn to identify what we are smelling, or to develop vocabulary to put our impressions into words. The vocabulary we have is very primitive, and is often based upon terms used in connection with food. Additionally, people have quite different levels of sensitivity to fragrances, and different reactions to them, based upon their past experiences – or what we call “scent memories.”
Fragrances are exuded from glands on the lower petal surfaces (and in some cases, leaf surfaces, as with R. eglanteria) and the bristly glands of the moss roses. The amount of fragrance is determined by a number of factors, including particular rose varieties and climatic conditions. Since fragrance in flowers is associated with the attraction of pollinating insects, the genetic triggers for releasing fragrance are associated with the time of day and conditions that these normal pollinators are active. Substances detected primarily by human smell are generally soluble in oil. In contrast, substances that are detected by human taste are usually soluble in water. Sunny, warm weather releases odors found in volatile plant oils. Humidity helps to prolong the smell because it reduces the rate of evaporation. The actual fragrance of roses is produced by oil-based compounds of alcohols and sugars, produced and combined in the chloroplasts, near the surface of the petals. These ingredients are surrounded with glucose, which causes the formation of scentless glucosides. Scent becomes apparent when the glucoside is hydrolized by enzymes, a process that occurs, in part, when climatic conditions are right. Since some of these compounds evaporate faster than others, the fragrance of a rose can change as the bloom opens. In addition to sun, soil, and pH, adequate water is an important factor. When additional moisture is present, the scent ingredient in the chloroplasts increases, which adds more potential fragrance.
People can educate their palate to fragrances by taking the time to notice differences, and by trying to identify dominant qualities of each – much the same as a professional wine taster does. A number of years ago, on a trip through California wine country, I was given a copy of a wine aroma wheel. The wheel divides the known aromas of wine into three levels of identifying terms. A wine example, also present in flower fragrances, is the term “fruity,” which divides into the sub-categories of citrus, berry, tree-fruit, tropical-fruit, and dried-fruit. Each of these is divided into a third level. Tree-fruit divides into cherry, apricot, peach, and apple. An amazing list of possible aromas has been accumulated on wines. This list covers a range from the fruity and floral (each with its sub-terms) to less-commonly-recognizable aromas, with sub-terms such as soapy, horsy, burnt toast, and wet cardboard. We have a long way to go, in classifying rose fragrances, to reach the level of sophistication found in the wine industry. This does, however, point out some obvious possibilities in the direction that we might go.
So far, there over thirty compounds have been involved in rose fragrance. Many of these occur in combination. Some of the most common rose scents are apple, clover, lemon, nasturtium, orris (iris root) and violet. Others include green tea leaves, cloves, raspberry, bay, spice, musk, parsley, wine, lily of the valley, linseed oil, fern, moss, hyacinth, orange, anise, honey, marigold, banana, apricot, quince, geraium, peppers, melon, and myrrh.
Additional connections have been found between rose classes and scent, and even rose colors and scents. There is a certain amount of genetic logic in these discoveries. Classes of roses, especially Old Garden Roses, are identified with particular color ranges, and also with particular fragrances. Gallica Roses come in crimsons, deep pinks, mauves, and stripes and splashes. They are most often described as having Old Rose fragrances, which can be intense and spicy. Damask Roses are white to dark pink, with unusual fragrances often associated with fruity perfumes. However, the further into complicated genetic crossings that hybridizers go, the more complex and elusive our modern rose fragrances can become. Crossing two roses, with unrelated fragrances, can produce offspring lacking in noticeable fragrance.
Many people point to the classes of OGR’s, and expound upon their abundance of fragrance by comparison to that of our modern roses. What they have forgotten is that the older roses we are familiar with are only a small percentage of the OGR varieties originally developed. Those that are commercially available today were chosen, and became commercially successful, at least partially because of their fragrances. Flower form was not the primary interest of hybridizers during the height of the OGR era. Many OGRs were developed which did not have good fragrance. Most of these roses have been forgotten.
Some years ago, James Gamble (whose name is associated with the ARS Fragrance Award) ran tests on more than 3,000 Hybrid Tea Roses. He concluded that approximately 25 percent had little or no fragrance, 20 percent were intensely fragrant, and the rest were somewhere in between. It would appear that these results are well within the bell-shaped curve of natural occurrences. Increasing the odds of producing new roses with fragrance is taking a great deal of work, since the gene for fragrance is recessive.
Probably more important than hard work will be perseverance, and a considerable amount of luck on the part of the rose hybridizers. Sam McGredy, the renowned hybridizer from New Zealand, has said (obviously, with his usual sense of humor showing) that it is simple to produce a fragrant rose. It takes about five generations of crosses and back-crosses to produce a line of fragrant roses. The roses will likely be pink in color, with poor form, and probably will be leafless, since the plant will be susceptible to most of our rose diseases. The commercial appeal would be questionable. When evaluating a new introduction today, after looking for beautiful flowers with good form, the next characteristic looked for is disease-resistance. Fragrance sometimes comes along as that special bonus.
(With thanks to “Heritage Roses,” Quarterly Newsletter of the Heritage Roses Group, Vol. XXV, No. 3, Aug., 2000, Rae Chambers, Ed. previously printed in the North Central District Newsletter, “Petals and Thorns,” and the Minnesota Rose Society Newsletter, “Roseways.”)