Ten Fascinating Facts about Aromatic Plants

September 04, 2017

by Linda Lane

The Arboretum’s Aroma Garden, located just across the road from the entrance to Norrie’s Gift & Garden Shop, abounds with fragrant salvias, lavenders, thymes, and many more sweet-smelling perennials. Of special note are selections from Barb Cordes, the garden’s volunteer curator. 

Aloysia triphylla
Aloysia triphylla
Salvia clevelandii brings to mind her favorite hiking trails.
Salvia clevelandii
Scented geraniums are also her preferences, in particular Pelargonium odoratissimum (right), with its smell of apples.
Pelargonium odoratissimum
And Pelargonium tomentosum, which has a mint fragrance.
Pelargonium tomentosum
*See below for photo attributions.

Lemon verbena (Aloysia triphylla) is her very favorite— it always makes her smile. Salvia clevelandii brings to mind her favorite hiking trails. Scented geraniums are also her preferences, in particular Pelargonium odoratissimum, with its smell of apples and Pelargonium tomentosum, which has a mint fragrance.

As a special feature at this year’s Fall Plant Sale, October 14, the Arboretum will offer a variety of scented plants, including Achillea millefolium ‘Lost Coast’, Aloysia triphylla, Hyptis emoryi, Prostanthera ovalifolia ‘Variegata’, Salvia apiana, Salvia spathacea, and Pelargoniums (scented geraniums).

What aromatic plants do and how they do it is one of the wonders of nature.

Aroma compunds
Aroma compunds

- The scent of plants comes from a complex mixture of low molecular weight compounds that, due to their miniscule size, vaporize into a gas when released (such compounds are called “volatiles”). The aroma of a flower may contain as few as seven to ten different oils, as in a snapdragon or petunia, or as many as 100 different chemicals, as with orchids.

- No two floral scents are exactly the same due to the diversity of compounds and their interactions.

- Research continues to identify specific plant fragrances. In 1953, chemists knew of only twenty chemicals in a rose’s scent, but, by 2006, over 400 had been distinguished. As of 2013, 1,700 different scent compounds produced by flowers have been catalogued, and the search continues.

- The creation of scent is a balancing act: a plant must generate enough smell to induce insects to fertilize its flowers but not waste energy and carbon. Thus, for many species, scent emission is not constant. For instance, snapdragons decrease scent production thirty-six hours after pollination.

- Plants with sweet scents attract bees and butterflies whereas those with spicy or fruity scents appeal more to insects such as beetles.

Yucca elata (Soaptree)
Yucca elata (Soaptree)

- Aromatic plants can be “species-specific,” i.e., their scent has developed to attract only one particular insect. In such cases, both enjoy a symbiotic relationship: the insect is guaranteed a reliable food source, and the plant is assured of pollination. As an example, the Soaptree yucca’s aroma attracts only a single species of moth, and it in turn lands only on the Soaptree flower.

Ophrys apifera
Ophrys apifera

A few aromatic plants mimic animals to achieve pollination. For example, the flower of the bee orchid (Ophrys apifera) smells and looks, to a male bee, just like a female bee. As the deceived insect attempts to mate with the orchid’s petals, it spreads pollen among the flowers.

- The part of the plant that releases fragrance depends upon the species. In some plants, the scent comes only from the flowers or only from the leaves. For many flowering plants, however, the production of odor is not confined to one area but spread throughout the outer layer of petals and other parts of the flower. And some flowers, such as orchids, have specialized scent glands called osmophores that ooze liquid scent, which evaporates on contact with the air.

Brassica rapa
Brassica rapa

In 2014, researchers found that fragrant plants have the remarkable ability of “choosing” how best to use their limited resources. When healthy, Brassica rapa, the plant used in this study, emits a strong floral scent appealing to bees, its favorite pollinator. But, when infested with an herbivore, the plant markedly reduces its sweet scent, thus discouraging the bees. Simultaneously, it emits scent signals from its leaves that appeal to wasps that can curb the infestation. After the wasps do their work, the plant produces extra flowers to again attract the bees.

Smell the flowers!
Smell the flowers!

One more reason to always stop to smell the flowers: in 2008, a team of Australian clinical neuroscientists reviewed the existing scientific literature on aromatherapy. Though they found limited support for many of the claimed benefits, certain studies showed promising effects, particularly in reducing stress.

Photo attributions:

Aloysia triphylla by H. Zell - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=9008346

Brassica rapa by TeunSpaans - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=96733

Pelargonium odoratissimum by H. Zell - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10767474

Pelargonium tomentosum by Michael Wolf - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10766458

Yucca elata by Stan Shebs, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=121845

Ophrys apifera by Bernard DUPONT from FRANCE - Bee Orchid (Ophrys apifera), CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=48715235

Ground Squirrel by Julian Rad   http://12beasts.tumblr.com/post/90852073012/nubbsgalore-the-timid-european-ground-squirrel

Salvia clevelandii by Bill Hill, UC Santa Cruz Arboretum and Botanic Garden.