A plant’s smell is a cloud of light chemicals drifting off its leaves, flowers, fruit and even roots. Every smell, from a rose to fresh-cut grass, is the plant releasing particular molecules into the air. These smells are not accidental. Plants use them to attract helpers, warn off enemies and send messages.1PubMed Central. Plant communication So what a plant smells like is really a readout of what it is trying to do at that moment.
The Chemical Families Behind Plant Smells
Plant scent chemicals are carbon-based chemicals that evaporate easily into the air. They fall into four main families: terpenes, aromatic compounds, chemicals made from amino acids, and chemicals made from fats.2PubMed. Current insights into plant volatile organic compound biosynthesis Terpenes give the familiar smells of pine, citrus, lavender and mint. Linalool, one terpene, is a big part of the floral smell of lavender and many other flowers. Aromatic compounds such as benzyl alcohol dominate the sweet smell of apple blossom.3PubMed. Honeybee Pollinators Use Visual and Floral Scent Cues to Find Apple (Malus domestica) Flowers
A single flower usually releases dozens of chemicals at once, so what we name as one smell is the overall mix and ratio. Changing the amount of just one ingredient can change how the whole scent reads, which is why two roses can smell quite different. Scientists describe a plant’s scent as a profile, a list of chemicals and how much of each is present, which is more precise than one chemical name.
Plants have built up huge variety in these chemicals over time. They gained new scents largely by copying genes and letting the copies change to make new chemicals.4PubMed. Evolution of flavors and scents Roses smell rich in terpenes because the gene families that make them multiplied over rose evolution.5PubMed. Evolution of the biosynthetic pathways of terpene scent compounds in roses
Where Plants Make and Store Their Scent
Many aromatic plants are covered in tiny hairs, and some of these hairs are glands that make and release terpenes.6PubMed Central. Identification of the Volatile Compounds and Observation of the Glandular Trichomes in Opisthopappus taihangensis and Four Species of Chrysanthemum The more glandular hairs a leaf has, the more terpene scent it gives off.7PubMed Central. Identification of the Volatile Compounds and Observation of the Glandular Trichomes in Opisthopappus taihangensis and Four Species of Chrysanthemum In lavender, these glands sit on leaves, stems, sepals and petals.8PubMed Central. Formation mechanism of glandular trichomes involved in the synthesis and storage of terpenoids in lavender Each gland has a head that makes the scent chemicals and a small space under its skin where the oil collects.9PubMed Central. Formation mechanism of glandular trichomes involved in the synthesis and storage of terpenoids in lavender
That oil-filled design explains why rubbing a leaf releases scent. Brushing or crushing a leaf bursts the tiny glands, which is why mint, sage and lavender smell strongest when touched.
If a herb smells faint on the plant, rub a leaf gently between your fingers. Rubbing too hard crushes the whole leaf and adds grassy damage smells, so a light touch gives a truer picture of the herb’s own aroma. Because the oil sits on and just under the surface, a leaf that has been washed, or is old and dried out, may smell weaker than a fresh one.
The same stored oils are what perfumers collect. They usually extract plant oils by steam distillation, while citrus oils are pressed from the peel, where the glands sit.
Smells also come from below ground. Roots release scent chemicals into the soil, where they interact with fungi, bacteria and small animals.10PubMed. Belowground volatiles facilitate interactions between plant roots and soil organisms
Why Flowers Smell Sweet or Rotten
Bees are drawn to flower scent and prefer it to plain air.11PubMed Central. Intraspecific variation of scent and its impact on pollinators’ preferences Honeybees find apple blossoms using both colour and smell, and their antennae detect benzyl alcohol, linalool and indole.12PubMed. Honeybee Pollinators Use Visual and Floral Scent Cues to Find Apple (Malus domestica) Flowers Some specialist bees recognise their one host flower by rare scent chemicals.13PubMed. The chemical basis of host-plant recognition in a specialized bee pollinator Bees also learn, so after feeding on a flower they search using a wider mix of its scent.14PubMed. The chemical basis of host-plant recognition in a specialized bee pollinator
Other flowers aim at a very different crowd. Flowers pollinated by flies often smell of rotting flesh or dung.15South African Journal of Botany. Convergent evolution of carrion and faecal scent mimicry in fly-pollinated angiosperm flowers and a stinkhorn fungus They release the same sulphur compounds found in carrion and the phenol, indole and p-cresol found in faeces.16South African Journal of Botany. Convergent evolution of carrion and faecal scent mimicry in fly-pollinated angiosperm flowers and a stinkhorn fungus A stinkhorn fungus evolved the same trick independently.17South African Journal of Botany. Convergent evolution of carrion and faecal scent mimicry in fly-pollinated angiosperm flowers and a stinkhorn fungus The corpse flower releases dozens of chemicals, including the fishy-smelling trimethylamine and sulphur compounds.18Biochemical Systematics and Ecology. Chemical mimicry in the corpse flower: Floral odor and phytochemical profiles of Amorphophallus titanum (Becc.) Becc.
A flower that smells of rot is not decaying or diseased. It is working normally, and the smell is how it advertises. It is aimed at flies and beetles that normally lay eggs or feed on carrion and dung, so they visit expecting a meal or nursery. Such flowers often look the part too, with dark red or mottled colours.
Indole shows how one chemical can be both nice and nasty. It turns up in jasmine and orange blossom as well as in faeces. In small amounts it gives a heady floral note, while in large amounts the animal, faecal quality comes to the front. The other chemicals present steer the impression too: indole inside a sweet blend reads as flower, and indole beside sulphur compounds reads as filth. Our brains judge the whole mix and the dose, not the chemical’s name.
The Smell of Cut Grass and Damaged Leaves
The fresh smell of cut grass comes from green leaf volatiles, chemicals made from fats inside the leaf. The enzyme that makes them stays switched off in intact cells and switches on when tissue is damaged.19PubMed. Arabidopsis lipoxygenase 2 is essential for formation of green leaf volatiles and five-carbon volatiles So a torn leaf releases a sudden burst of the cut-grass smell.20PubMed. Arabidopsis lipoxygenase 2 is essential for formation of green leaf volatiles and five-carbon volatiles In maize, separate enzymes supply this smell and the plant’s internal defence hormones.21PubMed. The maize lipoxygenase, ZmLOX10, mediates green leaf volatile, jasmonate and herbivore-induced plant volatile production for defense against insect attack
Every blade a lawnmower cuts is a wound, so the fresh smell drifting over the garden is thousands of small injuries releasing their chemicals at once. Caterpillars chewing a leaf trigger the same kind of release. To us it smells fresh, but to the plant it is an alarm.
The alarm calls in help. Plants chewed by caterpillars release smells that attract wasps that lay eggs in those caterpillars.22PubMed. Synthetic herbivore-induced plant volatiles enhance host localization and Parasitization by Bracon brevicornis A blend of three of these chemicals attracted helpful wasps from 5 metres away and raised the number of pests attacked.23PubMed. Synthetic herbivore-induced plant volatiles enhance host localization and Parasitization by Bracon brevicornis The system is not simple, though. Other wasps that prey on the helpful wasps follow the same smells.24PubMed Central. Hyperparasitoids Use Herbivore-Induced Plant Volatiles to Locate Their Parasitoid Host And one damage chemical, indole, can make caterpillars smell less appealing to the wasps that hunt them.25PubMed Central. An herbivore-induced plant volatile reduces parasitoid attraction by changing the smell of caterpillars Plants also warn their neighbours: nearby plants that smell these damage signals prepare their own defences.26PubMed Central. Plant communication
Farmers may be able to borrow the trick. Synthetic damage smells can lure pest-eating wasps into crops, and slow-release versions kept working for up to about two days.27PubMed. Synthetic herbivore-induced plant volatiles enhance host localization and Parasitization by Bracon brevicornis
How Fruit Smells Tell Animals It Is Ripe
Fruits eaten mainly by lemurs more than double their scent when ripe.28PubMed Central. Fruit scent as an evolved signal to primate seed dispersal Fruits also eaten by birds, which rely more on sight, smell about the same ripe or unripe.29PubMed Central. Fruit scent as an evolved signal to primate seed dispersal So a strong ripe smell is aimed at animals that hunt food by nose. Ripening apples keep making fruity-smelling esters as they ripen and age.30PubMed Central. Citramalate synthase yields a biosynthetic pathway for isoleucine and straight- and branched-chain ester formation in ripening apple fruit
Smells also point to nutrition. Many fruit and food flavour smells are made from nutrients such as amino acids, fats and the pigments behind carrots’ colour.31PubMed. Plant volatile compounds: sensory cues for health and nutritional value? A good smell may therefore tell people and animals that a food is nutritious.32PubMed. Plant volatile compounds: sensory cues for health and nutritional value?
You can use this at home. For melons and peaches, sniff the stem end: a sweet, fruity smell is one good sign the fruit has ripened, and little smell suggests it may not be ready, though give and colour are worth checking too. The smell builds as ripening continues, so a fruit that smells faintly sweet today often smells stronger in a day or two on the counter. A sharp or boozy smell suggests the fruit has gone past its best and is starting to ferment. Fruit picked well before it is ripe may never develop its full aroma, which is one reason supermarket fruit can smell of little.
Why Plant Smells Change With Time and Weather
Many flowers release scent on a daily schedule set by an internal clock.33PubMed Central. Circadian Rhythms in Floral Scent Emission Petunias release most of their scent at night, matching night-flying moths. A clock gene controls the scent genes, and changing it shifted petunia scent from night to afternoon.34PubMed Central. Circadian Rhythms in Floral Scent Emission
Heat and drought change the smell too. Blueberry flowers gave off more scent when warm but less when hot and dry together.35PubMed Central. Warming and Drought Stress Modify Scent and Rewards in Flowers of Highbush Blueberry Affecting Pollinator Preferences Under long drought, some grassland plants smelled stronger, some weaker, and many changed their mix.36PubMed. Drought induces moderate, diverse changes in the odour of grassland species In very hot rainforest leaves, the afternoon brought a shift from terpenes to cut-grass-type smells as heat stress set in.37PubMed Central. Green Leaf Volatile Emissions during High Temperature and Drought Stress in a Central Amazon Rainforest So a fragrant plant may smell faint in a heatwave, and the change you notice depends on the species.
Pollution and bacteria alter scent as well. Ozone in polluted air reacts with flower scent and breaks it down.38PubMed. Ozone degrades floral scent and reduces pollinator attraction to flowers The scent weakens and changes mix the farther it drifts, making flowers harder for pollinators to find.39PubMed. Ozone degrades floral scent and reduces pollinator attraction to flowers Ozone above about 60 parts per billion, common on smoggy days, already degrades flower scent and lengthens insects’ search.40Atmospheric Environment. Air pollutants degrade floral scents and increase insect foraging times Bacteria living on flowers add their own smells and consume some of the plant’s.41PubMed Central. Epiphytic Bacteria Alter Floral Scent Emissions Bacteria on bean leaves changed the plant’s smell in ways that affected mites.42PubMed. Bacteria Affect Plant-Mite Interactions Via Altered Scent Emissions
All of this suggests a few habits for smelling your garden. Smell flowers at different times of day, because those pollinated by moths often save their scent for the evening, while others peak in daytime when bees are active. If a plant seems scentless at noon, try it again at dusk. Warm, still air lets scent build up around the plant, so a calm evening after a mild day is often best. Wind scatters the smell quickly, so a sheltered spot near a wall or in a courtyard holds fragrance longer.
Do Plant Smells Affect People
People smell only a small subset of the chemicals plants release.43PubMed. Plant volatile compounds: sensory cues for health and nutritional value? Insects detect a different set, so a flower can carry a message that we cannot smell at all. In city flowers, chemicals like linalool, eugenol (the clove note) and the violet-scented beta-ionone shape what people notice.44PubMed Central. Dominant odor-active events link plant volatile signals to human perception and behavioral intentions in urban smellscapes That is why a clove or violet note can stand out in a blend of dozens.
Plant scents affect other plants too. Roots of some plants release scents that helped neighbouring plants sprout and grow.45PubMed Central. Root volatiles in plant–plant interactions I: High root sesquiterpene release is associated with increased germination and growth of plant neighbours Other plants release smells that hold back competing plants.46PubMed Central. Allelochemicals and Signaling Chemicals in Plants
Calming effects are less clear-cut. One form of linalool reduced brain activity linked to alertness after work in a small study.47PubMed Central. Influence of Fragrances on Human Psychophysiological Activity: With Special Reference to Human Electroencephalographic Response Its mirror-image form had the opposite effect.48PubMed Central. Influence of Fragrances on Human Psychophysiological Activity: With Special Reference to Human Electroencephalographic Response So claims that a scent relaxes everyone go beyond the evidence.