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Scented plants: the super power that we still don't understand

Sweetpea 'Matucana'
Sweetpea 'Matucana' Credit: Isabel Bannerman

As so often happens with projects or research, I set out to do one thing and discovered something quite different. I started keeping notes on what I smelled in the garden about 10 years ago, and wanted to write a book about scented plants. I started reading differently, noticing references to smell, searching them out. There are some great books about scented plants, but in much garden writing there was a paucity of description, if any mention at all, of this most potent of sensual experiences.

In early writing, however, there is a childlike enthusiasm, especially when it comes to smell. Treherne, writing about lilac in the 17th century: “It made my heart to leap, almost mad with ecstasy.” In contrast, John Gerard, writing at the same time, describes the smell of lilac as “head molesting”.

Mock orange (Philadelphus serincathus) Credit: Getty Images 

In early writing, plants ambered, civited, foetored, and smeeked; they were described as breathful, em­bathed, endulced, incensial, and suffite. Scent descriptions included marechal (cherry-like), naphe (orange-like), thymiama (incense-like) and as suffiments (medicinal scents). I knew that I wanted to go in that direction, writing a book that was heartfelt and vulnerable in its subjective observations: an attempt to transmit the “feeling” of smelling plants.

Smell is the sense for which we struggle to find the right words. The problem is our difficulty in capturing the fleeting nature of the perception. William Blake wrote: “Thou perceivest the flowers put forth their precious odours, And none can tell how from so small a centre come such sweets…”

In the 18th century Horace Walpole returns often to two favourite plants of mine: philadelphus and lilac. He put off a trip to Paris rather than miss the “lilactide” in his garden, and writes about the astonishing power of the smell of philadelphus in such a way that I wondered if he were smelling something different.

When I read that a University of Virginia study shows that air pollution is destroying plant fragrance, I thought I had stumbled upon something which makes sense. The study estimates that scent molecules can travel in unpolluted air for 2,000 metres, compared to only a couple of hundred metres in modern cities. This is because the volatiles react chemically and are destroyed by ozone, hydroxyl and nitrate radicals now present in the air.

Water lilies Credit: Magdalena Bujak / Alamy 

“Air pollution destroys the aroma of flowers by as much as 90 per cent from periods before automobiles and heavy industry. The more air pollution there is in a region the greater the destruction of scents,” says the study. The knock-on effect of this for insect life is to make them travel farther and rely more on sight than smell. We do not begin to comprehend what is going on in the air, so complex is the infinitesimal balance evolved over millions of years.

Scent in plants is a complex mix of low molecular weight compounds emitted into the atmosphere; their structure and odour are generally critical in attracting pollinators. Plant “perfume” is 98 per cent water and alcohol and then a tiny bit of fat and odorous molecules. These volatile molecules can be visible to the naked eye, as in the haze above pine forests in the aptly named Blue Ridge or Smokey Mountains.

Insects distinguish between complex scent mixtures. Species pollinated by bees and flies in daytime have sweet or meat scents; those pollinated by beetles have strong musty, fruity smells. By using their “noses”, the insects increase their foraging efficiency and the plants benefit from the pollination. I was entranced to discover that the earliest insect-pollinated flowering plants, magnolias and water lilies, which evolved around 90 million years ago, have a similar fruity smell beloved by their natural pollinator, the lumbering beetle. Before that there were only wind-pollinated plants, grasses and conifers mostly. The planet was flowerless until insects developed.

Magnolia Credit: Isabel Bannerman

The theorising then continues: what we typify as the scent of flowers may well be the product of a natural selection process in which plants imitate the smell of insects. Butterflies do in fact smell of citrus, or vanilla. Without the scent of butterflies, there would be no scented flowers. The great evolutionary scheme of things seems to have ­created invertebrates as the servants of plant propagation.

And now we have to face up to a catastrophic decline in invertebrate numbers. Pesticides and loss of habitat have a lot to do with that – but scent in plants, moths, and birds are all entangled in a reproductive cycle which we are blithely destroying. A world without butterflies or flies or wildflowers or hedgehogs is becoming the “norm”. How can we miss what we never knew?

For now, I urge you to just go out and smell the world today, this afternoon, now. Smelling plants is about enjoying yourself in your garden. Enjoying autumn smells: all the smells of earth and leaves and change. Smelling is a quest that enriches, makes every experience a bigger and brighter one.

Reader offer

Scent Magic 
by Isabel Bannerman (Pimpernel Press, £30). Buy now for £25 at books.telegraph.co.uk or call 0844 871 1514.