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WHY PLANTS SMELL
HOW WE SMELL

Herbs are leafy and green, whereas spices, such as (from left to right) allspice, nutmeg, star anise, and cinnamon, come from dried seeds and fruits, bark, roots, and woody stems.

When we visit a garden, we may bask in the perfume of blossoms. In the kitchen, we sample aromas while chopping up leaves. And countless seeds, roots, and barks impart their unique spicy fragrance to all manner of goods we enjoy, from cookies to candles. But we humans get to relish botanical fragrances by serendipity, because plants developed their scents to deter predators or attract helpers in pollination.

Some leaves, loaded with chemicals like tannins, repel insects or herbivorous animals by tasting bad, and have to be sampled to teach a potential browser that they are foul. Others, with substances such as terpenes, put their aromas to use even before they can be sampled, announcing, “We’re not good to eat.” For instance, most animals and many insects find the flavors of herbs like marjoram, oregano, rosemary, or thyme too strong, and their odors—the same chemicals that make fragrant herbs and spices appealing to us humans—serve as natural deterrents. The popular spice cinnamon comes from the bark of a tropical tree that pungent oils keep from being browsed to death. The grazing animals’ loss is our gain.

By day, color or scent will attract insects that get a reward of sugary nectar for carrying pollen from one blossom in a species to another. Thousands of flowers, however, are pollinated by nocturnal insects, and wait to release their perfumes until their animal allies are active, for why waste your scent when your pollinator isn’t around? Planting a moonlight fragrance garden is a sure way not only to enjoy some wonderful blossoms but also to meet their nocturnal pollinators. Moths, for example, which are experts at finding sweet-smelling flowers after dark, are especially fond of moonflowers and jimsonweeds and a number of desert cacti that offer their bounty after the heat of the day is past. Night-blooming flowers are often white, to reflect the light of the moon, but moths will sense VOCs floating in the air even before they see the flowers. Some of these highly fragrant blooms have long floral tubes that conceal pools of abundant nectar, and many moths coevolved long proboscises to reach the sweet reward. Watch for sphinx moths, which, like hummingbirds, can hover in place to sip nectar or fly backward to dart away.

The leaves and flower of the ginger relative Curcuma longa grow from a bright orange root that, when peeled, dried, and ground, yields turmeric.

One of many night-blooming desert cacti with large white flowers is Echinopsis subdenudata.

Although privet is related to the wonderfully fragrant common lilac, many people are sickened by the cloying scent of its blossoms. I use their odor to describe the smells of some other flowers.

Sea holly flowers, like those of Eryngium ‘Big Blue’, attract many pollinators, though they do not smell nice to us—something like creosote.

The tiny flowers of Osmanthus fragrans, sweet olive, release one of my favorite smells. It’s like apricot, with peach and Arabian jasmine.

NOT SO SWEET SOMETIMES

Most pungent smells evolved to repel predators, but some plants utilize scents we think of as “bad” to attract specific helper species in procreation. These flowers beckon pollinating beetles, wasps, and flies with odors that mimic putrefaction, and bees and wasps often go for odors that we might find sickly sweet, bleachy, or fishy. When in bloom, the Eryngium varieties in my garden are always covered with all kinds of wasps and bees. Some of these thistle-like sea hollies are iridescent steel blue, others silver or chartreuse. To me, they smell like creosote.

Some trees, including the American hawthorn, Callery pear, and various crabapples—all members of the rose family—have coevolved with flies, and have bleachy, fishy, chlorine, and ammonia odors. These plants contain trimethylamine, which is also found in fish brine. The Japanese or tree lilac, Syringa reticulata, a recently popular urban street tree, bears giant flower clusters, called panicles, that have more of a sweet, choking smell. Bees, wasps, and flies love them. Privet (Ligustrum vulgare) and similar shrubs with short white plumes, and shrubs like the summer viburnum, with flat-topped clusters of flowers, can be repulsive. Ironically, both Japanese lilac and privet are members of the olive family, which includes wonderfully fragrant plants like the common lilacs of spring and the sweet olive (Osmanthus fragrans), with tiny flowers that smell delightfully like apricots and jasmine.

I like to cut and bring flowers inside, but that may not be a good idea with all plants. For example, the tall Fritillaria varieties from bulbs are showstoppers in the garden. Crown imperials, the orange F. imperialis and yellow F. imperialis ‘Maxima Lutea’, have a coronet of green leaves above a circlet of dangling bells. The dark plum-purple F. persica and the greenish-white-flowered variety F. persica ‘Alba’ are also regal plants, and impressive in the garden, but all these Fritillaria smell like wet fur—foxy, sweaty, and skunky. As for being cut flowers? They are best left outside.

Then there are flowers that, quite simply, smell like death, such as the subtropical succulent plants in the genera Stapelia and Huernia, which live where sweet-nectar-seeking bees do not. These plants have to rely on other kinds of insects for pollination. Flies come just as the flower opens, attracted by the putrid odor of rotting flesh. These insects often lay their eggs right on the surface of the star-shaped blooms.

If you can picture a white calla lily, then you can imagine other members of the aroid family. There is a spadix at the center of the inflorescence (the flowering structure), shielded by a cowl-like spathe. In other species, the spathe may be striped or splashed pale green over dried-blood brown. Perhaps these plants are mimicking fat-streaked muscle tissue. Such patterns on the inflorescences of plants like the subtropical Sauromatum venosum, a voodoo lily that smells like decaying flesh, are irresistible to carrion feeders like flies and beetles.

A notorious stinker is the giant, ten-foot-tall tropical corpse flower, Amorphophallus titanum, the titan arum, which blooms as a crowd-pleasing novelty in some of America’s public gardens. You might be able to find a live video feed from the New York Botanical Garden leading up to the day when the gruesome flowering structure unfurls in a matter of hours. The smell sends some visitors fleeing, while it draws others in, like the flesh-gorging insects that pollinate the tiny flowers deep within the plant’s colorful ruff. The titan arum smells like a dead animal, with a piercing, strangely sweet, choking stink of sulfur dioxide, methane, and other noxious gases.

The tall late spring bulb Fritillaria persica, Persian lily, is stunning in the garden and best left there. With an odor that has been described as “foxy,” it does not make a good cut flower.

Some flowers smell bad, very bad, like the fly-attracting voodoo lily, Sauromatum venosum, which smells like carrion or manure.

Maria Perez photographed the ten-foot-tall carrion-scented corpse flower, Amorphophallus titanum, at the New York Botanical Garden.

SCENT AS COMMUNICATION

Plants also use VOCs to communicate with one another. An individual being attacked by a caterpillar may send out a signal to a related plant, which then switches on its defensive hormones. These chemicals toughen tissues or make them less palatable in order to thwart a would-be foe. Goldenrods may sense insect sex pheromones, and warn their brethren that eggs may be laid, leading to hungry larvae.

Some of these chemical compounds are employed to communicate not with other plants but with insects that may aid in defense. For example, when a giant tobacco or tomato hornworm is munching on Nicotiana, the plant signals the parasitic braconid wasp, Cotesia congregatus. The wasp swoops in to lay eggs in the caterpillar’s body; when the larvae hatch, they begin to feed.

Perhaps even stranger, another type of parasite, a plant called dodder (Cuscuta europaea), a relative of the vining morning glory, “smells” a plant it likes and travels to attack it. Orange dodder has no chlorophyll, so it cannot make its own food. A seed sprouts, and the seedling has to find a green plant within five days, after which it will have used up its nutrients. These seedlings have receptor cells that use chemosensory clues to find the tastiest host. The parasite smells its favored victim, climbs up, and pierces its stems to suck the life right out of it. I sometimes notice dodder on one plant and not on the one right next to it. I carefully collect, bag, and banish the invader.

Some Solidago spp. (goldenrod) plants “talk” to one another, releasing smells of warning to neighbors in harm’s way.

BREEDING AT THE EXPENSE OF SCENT

I’ve read in old books about the wonderful fragrance of snapdragon flowers. That was funny, because I didn’t think their blossoms had any scent at all. Then I visited the Abby Aldrich Rockefeller Garden on Mount Desert Island in Maine, where I encountered an old strain of Antirrhinum majus. A peach one and an apricot one from the ‘Double Azalea’ mix had been grown as a cut flower in the greenhouses there for years. To my surprise, the flowers were enormously fragrant—I could smell fruit punch, bubblegum, honey with green apple, and indole.

Whenever an actor on TV is handed a bunch of a roses, the first thing she does is breathe in long and deep, and smile. That’s acting. Most modern long-stemmed cut red roses have no scent. As with many “improved” modern plants, breeding for flower size, longevity, habit, or color has resulted in the loss of fragrance.

SCENTLESS ROSES

Today’s cut-flower industry grows most of its roses in Central America and Africa. These roses have to stand up to harvesting, packing, handling and shipping, repackaging, and more shipping. They are bred for durability—not for smell—and they don’t have any fragrance. Their thick, leathery petals do not disintegrate, and their VOCs do not waft into the air as scent but remain embedded and undetectable until the blossom begins to rot, by which time it smells pretty much like compost.

In old still-life paintings of cut roses, they are often shown arranged in a bowl. European rose varieties were white, pink, or red and grew in clusters with short stems. They bloomed once a season and smelled like dusting powder or rosewater. Repeat-blooming tea roses from China and Persia came in warm yellow colors, with fruit and tea fragrances. These were crossed with the old European varieties and hybrid tea roses were born. But long stems were elusive.

One of the earliest hybrid varieties to have very long stems was a fragrant dark pink rose named ‘Madame Ferdinand Jamin’. French growers gave up on it because of its susceptibility to disease, but in the 1880s growers began cultivating the rose as a cut flower in greenhouses in the United States (with plenty of pesticides). In 1886, the hybrid was introduced with a new name, ‘American Beauty’, and soon was selling for as much as six dollars each, hence the flower’s nickname, “million-dollar rose.” Over the years, the name ‘American Beauty’ was applied to any long-stemmed and, for some reason, red rose. By the 1950s, florists and competitive flower arrangers demanded more colors and repeat blooming, but most of all, long stems. Many of those mid-century-modern hybrids probably smelled more of pesticides than of perfume.

Today’s sophisticated garden market wants fragrant roses, and some contemporary hybridizers are breeding to get them back. For instance, the David Austin English roses are bred for scent (this page). Tom Carruth in Los Angeles introduced several perfumed varieties, including the red-and-white-striped ‘Scentimental’, smelling of old rose and spice, in 1997, and the anise-scented, egg yolk–yellow ‘Julia Child’ in 2006.

Few of the newest, overbred snapdragon varieties have a scent. I came across an older variety, Antirrhinum majus ‘Double Azalea Apricot’, that smelled like fruit punch, with bubblegum, honey, green apple, and a trace of indole.

The deep pink hybrid perpetual rose known as ‘American Beauty’ originated in France as ‘Madame Ferdinand Jamin’ (1875), with buds as large as goose eggs, up to fifty petals, and a strong centifolia rose scent. Rechristened, it became a best seller. This is the climbing version.

Some rose breeders are working to get scent back, for instance, with ‘Julia Child’ (2006), a floribunda bred by Tom Carruth with a sweet licorice fragrance. The plant was renamed ‘Absolutely Fabulous’ in the UK and ‘Soul Mate’ in Australia.

Pépinières et Roseraies Georges Delbard, the largest commercial grower and breeder of roses in France, hybridizes for scent. Like the perfumers, Delbard categorizes fragrance in a pyramid. The head of the pyramid is the spirit of the fragrance: for example, citrus, anise seed, or lavender. The heart of the scent would come from the notes in the middle of the pyramid: floral/sweet, herbal/green (like mown grass), fruity, or spicy. The bottom of the pyramid contains the base notes, which could be woody, balsamic, vanilla, or, perhaps, heliotrope. Every rose entry in the Delbard catalog has its own perfume pyramid, allowing customers to buy roses by their fragranceS.

How to Smell

When first encountered, a strong smell can be overwhelming. You’ve probably entered a friend’s house while dinner is being prepared and been greeted by all sorts of cooking odors. You soon become accustomed to the smells and may even forget they are there. Some people think that our scent receptors become anesthetized. This might be a kind of olfactory habituation, analogous to the way we tune out any constant stimulus—the feel of clothes against our skin, for example. Not every expert agrees that “nose fatigue” exists, but we all meet people who wear too much cologne and seem oblivious to the smell. We lose the ability to register the quality of a smell when we are overwhelmed by the quantity.

To sample a flower’s scent to the fullest, take many quick sniffs. When you sniff, you create currents that force air into the upper chambers of the nose, which greatly increases perception. Short whiffs keep the floral fragrance fresh by exposing our scent receptors to little bursts of molecules—a deep inhale smothers them and they soon become indiscernible. It’s like sampling wine: The first sip is different from subsequent mouthfuls. But, unlike wine tasting, which may eventually get you drunk, sampling flower smells is something you can do all day.

For testing perfumes at a fancy department store, the often-cited recommendation is to “reset” your ability to enjoy the individual brands by carrying a jar of coffee beans as a kind of “palate cleanser,” and to sniff them between fragrances. This isn’t really necessary. If you smell a lot of flowers of the same kind and want to take a break, do what pros do and just smell the crook of your elbow so as to be ready for the next, fresh scent. Since you are always accustomed to your own skin, it is the best thing to smell to reset your nose to the baseline.

When sampling the smell of a blossom, concentrate; try to ignore distractions. Close your eyes. Don’t wear perfume or cologne or allow someone who does to stand nearby. Don’t carry a strongly flavored drink or chew gum.

Again: Take sniffs. Focus on the fragrance. List the things it reminds you of and try to characterize these associations in detail.

Cananga odorata is tropical ylang-ylang. The heavy, somewhat fruity and floral scent, described as musk with lime rind, is in perfumes such as Chanel N° 5 and Jean Patou’s Joy.