Springing in the rain: The science behind the smells of the seasons
Spring is just around the corner, and so too is an assault on the senses.
Spring is when the meadows are ‘washed fragrant’ and freshness is breathed over the hills, writes John Clare in Spring Morning. It is a season when we are led by our noses, from the delicate rawness of the first snowdrops to the full-on hit of a newly wet pavement after an April shower.
Flowers are the most obvious scent-bearers and the earliest to perfume our hedges, from February onwards, is the almond-y blackthorn blossom. The daffodil is the most recognisable harbinger of spring, but its fragrance can divide opinion. Even delicately scented Narcissus jonquilla may quickly tend towards the overripe, its feculence provided by a compound called indole, which is also found in jasmine. Later in the season, we readily sniff out another distinctive odour in damper copses — wild garlic (Allium ursinum). Pungent yet delicious, it is one of the easiest of foraging plants to identify: simply crush a green leaf to conjure the base notes of pesto or aioli. It can also verge on overpowering. By the time the long, green leaves wilt, we’re more than ready to embrace the warm, parma-violet-and-honey scent of the bluebell, Hyacinthoides non-scripta, on our woodland walks.
Flowery fragrances are not designed to please us, although scent can have an astonishing effect on our brains, helping us to sleep (lavender), improving memory (peppermint) and calming us (roses). No, flowers have a baser motive, which is to attract pollinators. A scientist will explain that the pleasing, heady scent filling your nose is, in fact, merely a carbon-based compound. In her book Good Nature, Prof Kathy Willis, Baroness Willis of Summertown, explains that plants release these volatile organic compounds as a gaseous cloud and that scent cells are found not only in flowerheads, but in leaves, bark and sap.
The signature scent of spring comes not from beautiful blooms, but benevolent bugs. The technical term for the smell produced when rain falls on dry ground is ‘petrichor’. It’s a pleasing word and an odour that stirs us deeply. Yet it is made by a simple organism, as Paul Hoskisson, professor of molecular microbiology at the University of Strathclyde, explains: ‘Petrichor describes geosmin, one of the natural products made by bacteria, fungi and plants to influence their environments. Geosmin is made by the soil-dwelling Streptomyces bacteria to attract small invertebrates called springtails (Collembola) to distribute their spores, in the same way that plants entice animals and insects to disperse pollen and seeds. Streptomyces produce the spores when it’s dry or food is scarce — and also make antibiotics to protect their nutrients from competitors. Because geosmin is a volatile compound, it easily turns into a gas on contact with water, which is why we’re most aware of it when it rains.’
Humans are phenomenally sensitive to geosmin and can detect it at levels as minute as five parts per trillion. Although we have harnessed the Streptomyces bacteria to great effect — they account for some two-thirds of our antibiotics and antifungals and have been used in immunosuppressants, herbicides and cancer drugs — there is, as yet, no obvious commercial use for geosmin, advises Prof Hoskisson. In the meantime, we can delight in it when digging the garden, ploughing the fields or kicking through woodland mulch.
Geosmin accounts both for the earthy taste of beetroot and the muddy flavour of river fish, but, like liberally applied teenage aftershave, it can overwhelm. It is a problem if it taints our drinking water and even more so if it hangs over the surface of the UK’s largest freshwater lake, as those living near blue-green-algae-ridden Lough Neagh in Northern Ireland can attest.
If spring can be captured in florals and essence of earth, summer brings with it an evergreen favourite. Freshly cut grass is luscious and new-mown hay is more delicious still. Drying sweet vernal grass, a main ingredient of traditional meadow hay, produces a chemical known as coumarin. This closely resembles vanilla, which is deemed to have one of the most pleasing effects on our olfactory receptors. The Victorians became obsessed with it after the discovery that coumarin could be extracted from the beans of the South American tonka tree. ‘Hay’ was the fashionable scent of choice for cosmetics and perfumes and the craze continued after 1868, when chemist Sir William Henry Perkin worked out how to synthesise coumarin from coal tar.
'As frost pinches and piques our nostrils, some people are sure they can smell snow. In fact, what they are sensing is an absence of smell'
It was another delicious summer scent that so enraptured Marcel Proust. Smell can strongly stimulate memory and the sensation that famously enraptured the French writer was a madeleine dipped in tilleul, an infusion made from the delicately scented flowers of the lime tree.
When autumn comes around, the hard-working bacteria go into overtime, producing more geosmin as leaves rot down. To the rich smells of crumbling dark earth we can add bonfire smoke and bracken, as an overcome Edward Thomas did in his poem Digging: ‘Today I think/Only with scents.’ Time spent in forests is known to improve our wellbeing, but the link between nature, our incredible sense of smell and its impact on our health is still a relatively under-studied field that is now being tackled by scientists at Oxford University’s Centre for Nature Recovery.
Winter brings a different aromatic experience. As frost pinches and piques our nostrils, some people are sure they can smell snow. In fact, what they are sensing is an absence of smell. Snow transforms the landscape and disguises our usual scent references; meanwhile, cold air molecules move more slowly and plants release fewer aromas.
The rarified palette of the Nordic landscape helped Sámi-Norwegian artist Máret Ánne Sara single out the smells of a childhood spent among reindeer herders for her current exhibition at London’s Tate Modern, SE1 (running until April 6). Working with Algerian perfumer Nadjib Achaibou, she created two scents: one that of reindeer under extreme stress (‘fear’), the other based on natives such as lichen and shoegrass (‘hope’). These fragrances waft from solid polymer blocks fixed to a walk-through installation in the shape of the internal anatomy of a reindeer’s nose.
Incorporating scent within exhibitions is a growing trend. London-based food historian and artist Tasha Marks has recreated the smell of Marie Antoinette’s rouge and powder, as well as that of her dank prison cell, for ‘Marie Antoinette Style’ at the V&A Museum, SW7 (until March 22). Marks frequently draws upon the natural world in her work — take the 2021 ‘Tranquillity’ exhibition at north London’s Wellcome Collection, NW1. ‘I was asked to create the smell of happiness,’ she recalls. ‘It was a forest floor full of petrichor: damp, dark and green.’
She has most recently developed fragrances for the Ashmolean Museum Oxford’s exhibition ‘In Bloom: How Plants Changed Our World’, which runs from March 19–August 16. Worth sniffing out.
