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Fascinating Fragrances - Three Perspectives

Karl Foord - Extension Educator, Horticulture

If someone hands you a flower, what is the first thing you do? Smell it, of course. How often have you exercised this ingrain behavior only to come up empty?

My colleague and a contributing writer to this blog, Robin Trott sells cut flowers for a living and reports, "I feel cheated when beautiful flowers lack fragrance. I insist on having fragrant flowers in the bouquets I sell. To provide fragrance, we use herbs such as basil, lavender, mint, dill, and cilantro as fillers, and grow stock (Matthiola spp.) fragrant snapdragons (Antirrhinum spp.), Sweet Williams (Dianthus barbatus) and fragrant oriental lilies (Lilium spp.)".

The Human Perspective

Fragrances have been a part of human culture for a long time. The mere presence of flowers has been shown to have positive effects on human emotions, and recent research has shown that fragrant flowers are a "positive emotion inducer". This should come as no surprise to most gardeners.

Breeding improvements in flowers have rarely included fragrance characteristics, which were considered to have a negative effect on vase life. Recent studies on fragrance suggest that this might not be the case, and researchers at David Austin Roses are making progress in improving the vase life of fragrant roses. I find it quite encouraging that researchers are working to strengthen a plant characteristic that positively effects a person's overall enjoyment of a flower.

As an avid gardener, fragrant plants are a significant addition to my landscape. Killing four-lined plant bugs or weeding in my mint patch becomes much less of a pain when accompanied by the scent of mint. Sitting on the deck when the Wisteria is in bloom is a sensory gift. Fortunately, there are quite a few plants that offer the reward of fragrance.

The Plant's Perspective

Fragrance provides a vital service for the plant. A plant's distinct aroma attracts pollinators, thereby facilitating cross-pollination and the continuation of the species. The relationship that exists between the plant and the pollinator is one of intimacy and endurance. To experience the optimal range of plant fragrances, it is important to understand the plant's pollination strategy. For example, plants pollinated by moths produce optimum fragrance in the evening as moths are nocturnal. Thus Nicotiana is more fragrant at night, when its moth pollinators are out and about. Plants pollinated by bees produce optimum fragrance during the day as bees are diurnal. Thus snapdragons are more fragrant during the day, when its bee pollinators are likely to be active.

Different but specific fragrances also help to ensure that insects visit flowers of a similar species, increasing the potential for successful cross-pollination. From a long distance, flower fragrances are more effective than visual signals in attracting a pollinator, especially to small or hidden flowers. Flower fragrances tend to be at their highs when they have sufficient nutrition.

In addition, moderate to warm temperatures and high light tend to increase fragrances. For instance, roses are picked at night when they are at their most fragrant, whereas jasmine flowers are harvested when their fragrance is at its peak just before dawn.
Every plant has its own signature scent, a complex mixture of so-called volatile organic compounds that easily turn to gases and waft through the air. Some 1,700 different compounds from 990 different plants have been identified in flower fragrances so far, according to Natalia Dudareva, a Purdue University biologist whose specialty is floral scents.

To extend your garden fragrance window, why not grow the nectar plants that pollinators on the night shift prefer. Good candidates for moths include native night bloomers such as evening primroses, yuccas, phlox, sacred datura and evening snow. To attract giant silk moths, grow the host plants of their caterpillars. Oak, sassafras, maple, birch, ash, willow and cherry are a few favored larval plants.

The Insect's Perspective

Honey bees (Apis mellifera) have 170 odorant receptors, the researchers found, compared with 62 in fruit flies (Drosophila melanogaster) and 79 in mosquitoes (Anopheles gramblae). The enhanced number of odorant receptors underlies the honey bee's remarkable olfactory abilities, including perception of pheromones, kin recognition signals, and social communication within the hive. Honey bees also use odor recognition for finding food. University of Illinois Entomology professor, Hugh Robertson, who has conducted research on the honeybee genome, states: "Foraging worker bees might encounter a bewildering number of flowers to choose from, but they can discriminate between them using subtle olfactory cues," He goes on to say that "A large number of odorant receptors allows the bees to find food and communicate its location to other bees." In striking contrast, the researchers found only 10 gustatory receptors in A. mellifera, compared with 68 in D. melanogaster and 76 in A. gramblae. Clearly taste is more important to fruit flies and mosquitoes than it is for bees.

Our olfactory apparatus

The human sense of smell is carried out by two small odor-detecting patches. Each patch is slightly less than ½ of a square inch and contains some 2.5 to 3 million receptor cells (photo 2). For comparison, a dog has 220 million of these olfactory receptors (photo 3). Nonetheless, humans can distinguish more than 10,000 different smells and are capable to detect certain substances in dilutions of less than one part in several billion parts of air.

Extend your garden's fragrance window

Grow the nectar plants that pollinators on the night shift prefer. Good candidates for moths include:

  • Night Gladiolus: (Gladiolus tristis) has creamy yellow blossoms that have an intense spicy fragrance at night.
  • Pinks: (Dianthus plumarius) The pale pink flowers have a rich clove scent.
  • Evening Primrose: (Oenothera) With sweetly-scented blossoms of soft white, pink and bright yellow that open in the evening.
  • Moonflowers: (Ipomoea alba) This night-blooming relative of the morning glory perfumes the garden as its large 5 to 6 inch white flowers open at dusk. A quick growing climber with large heart-shaped leaves.
  • Evening Stock: (Matthiola incana) Small pink or purplish flowers are not showy, but emit an intoxicating fragrance at night. Grows to one foot with lance-shaped leaves.
  • Four O'Clocks: (Mirabilis jalapa) Sweetly fragrant and colorful trumpet-shaped flowers, open in late afternoon releasing a jasmine-like perfume. Found in the gardens of Thomas Jefferson at Monticello, the bushy plants grow 24 inches tall and are an annual in our climate.
  • Daylilies: there are many daylilies that bloom at night including 'Moon Frolic' and 'Toltec Sundial'.

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