Why do flowers smell?

Flowers you can plant to feed the bees.

From the unsavoury stink of the skunk cabbage to the seductive fragrance of sweet peas, some flowers discharge a cornucopia of scent molecules into the air. But while many of these fragrant compounds may be pleasing to the human nose, they're not actually for the benefit of humans.

Floral scent signals have predominantly evolved to attract pollinating animals, and the type of pollinator relates to the type of fragrance emitted.

Plants that amplify their fragrance output during the day are primarily pollinated by bees or butterflies. Those that release their scent at night are pollinated by moths and bats.

In the bush short-tailed bats play a role in the pollination of matai, rewarewa, pohutukawa, koromiko, nikau and hebe, but you're unlikely to find them in your backyard.

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Moths, however, are rampant. "The main flower-visiting moths people are likely to see are the noctuids, the chunky cream, gold or green moths that look like they are wearing a fur cape, and the geometrids, the slender pale moths with darker geometric markings," says University of Otago plant evolutionary biologist Dr Janice Lord. "Moths lose their colour when they die, so you can only see how beautiful they are when they are alive."

Moths are attracted to pale, night-scented flowers with abundant nectar. "Garden plants such as nicotiana, jasmine, honeysuckle, night-scented stock and evening primrose are visited by moths overseas so should also be attractive to moths here," reasons Janice.

"A student of mine has shown that native pimelea, manuka, cassinia (aka ozothamnus) and ericaceae (gaultheria and dracophyllum) are attractive to native moths."

Sweet smell of success
As gardeners know, not all flowers smell alike… for good reason. "Hoverflies, butterflies, moths and bees are most attracted to sweet smells," says Janice, "whereas bats and flower-visiting mammals are more attracted to musky smells."

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Flowers use these sensory signals to advertise their rewards – nectar and pollen – ensuring the transfer of pollen from flower to flower.

Bees then remember the specific plant scents associated with recent experiences of good food rewards and will return to that same plant as a reliable food source.

Some flowers are generalists and release scent signals that attract a number of different pollinator species; others are more specific, targeting only certain pollinators. The soaptree yucca (Yucca elata) releases scent signals that attract only the yucca moth. Hence, floral scent signals are essential in allowing pollinators to discriminate among plant species, and even individual flowers of a species.

The time of day scents are released can vary too. "The blue chicory has nectar in the morning. Red clover is best after lunch. The four o'clock flower opens in late afternoon.

A collection of fragrant sweet pea flowers.
JANE USSHER

A collection of fragrant sweet pea flowers.

The evening primrose follows the four o'clock," writes Sharman Apt Russell in Anatomy of a Rose, an absorbing book which unveils the inner life of flowers.

But a flower must 'want' to smell. Newly opened flowers that are not yet ready to release pollen have fewer scent signals.

Likewise, when an older flower has been sufficiently pollinated, its scent changes or shuts down to help direct pollinators to unpollinated flowers. Snapdragons lower scent production about 48 hours after pollination – it takes that long for the pollen tubes to reach the ovary, which triggers the shutdown. Petunias shut down scent production within 36 hours of pollination.

Each flower varies, but the result is the same. Scent signals change or shut down after pollination so that the plant doesn't have to waste energy producing them.

The owlet moth is a noctuid, one of main types of flower-visiting moths.
MGUNTOW/123RF

The owlet moth is a noctuid, one of main types of flower-visiting moths.

Why some plants smell bad
Beautiful fragrances are not the only lure for pollinators.

Certain plants specialise in emitting odiferous scents to disguise their true nature and intent. "Some plants trick animals into pollinating them by mimicking the smell of something completely different," says Janice. "For example, the dead horse arum, Helicodiceros muscivorus, is pollinated by carrion flies which think it is rotting meat."

Certain plants specialise in smells to encourage pseudocopulation. "Some species of orchids, for example Ophrys apifera, produce volatiles that mimic the sex pheromones of female insects," she adds.

These attract the males to land on the flower and inadvertently pollinate them.

But timing is still everything
As in most markets, supply and demand fluctuates at certain times of the year. A study published in Scientific Reports in 2013 reported that "scent advertisement" was higher in plants that bloomed early in the season when pollinators are scarce, compared to flowers that bloomed later when there are more pollinators. It was also found that plants with fewer flowers compete with dominant species for pollinators early in the flowering period by sending out more of a chemical signal called beta-ocimene.

For example, rosemary and thyme were two dominant flower species in the study. They accounted for most of the nectar and pollen produced. However, less abundant flower species flowering at the same time and which shared pollinators emitted a similar scent but with a much higher proportion of beta-ocimene. These included Iris lutescens, Euphorbia flavicoma, Muscari neglectum and Ranunculus gramineus.

Red admiral butterfly.
ALLAN WALLBERG/123RF

Red admiral butterfly.

Why some plants don't smell of anything
Birds, with the exception of vultures and kiwi, have no sense of smell, so it follows that flowers that are pollinated by birds seldom have a scent, says Janice.

Likewise, wind-pollinated flowers tend to be dull in colour and have no nectar or scent to attract pollinators.

In the case of grasses, there are not even petals. Sweet corn, a grass, is one example. Its pollen is windblown, and thus is not sticky, which might otherwise cause it to stick to leaves.

 

Mecaela Lynch 123/RF SUPPLIED Tony Wills, CC BY-SA 3.0, Wikicommons Pjt56, Wikicommons

Stocking up to feed their larvae is why bumblebees are interested in pollen. The way they gather it is remarkable: the foraging female bumblebees grab a flower and then vibrate their wings at an almighty pace to dislodge the pollen from the anthers. Clouds of pollen swirls around the flower and some falls right on top of the target, kicking off the fertilisation routine. Tomatoes and other solanaceous crops simply love this kind of vibratory attention – that’s why bumblebees are commercially available to gardeners and commercial tomato growers alike.

The fact that butterflies regularly visit flowers (for a quick feed of nectar) is a great indicator that their actions are beneficial for the distribution of pollen. The unfurled proboscis constantly probes to find the nectar source, gathering the grains by accident, while the feet pick up a fair amount of pollen too. Curiously, those butterfly feet are also able to "smell" or "taste" the flowers and nectar. Different butterfly species prefer different types of flowers.

Our native bees tend to sip nectar too, usually from flowers that have an open nectar reservoir (manuka, pohutukawa and such species), but their real intention is gathering pollen to provide for their larvae, deep inside their tunnels in the soil. Judging from the numbers of native bees in late spring and summer, their pollination potential must be considerable in the New Zealand landscape. We have a couple of dozen species in just about all the major native habitats, right into the alpine zones of our mountains, and some have adapted themselves well to exotic plant life too.

Most people don't recognise hoverflies as flies. They hover here, there and everywhere, usually near or above pollen-rich flowers. The cool thing is that some of these hoverfly species lay their eggs among aphid colonies, where the fly’s larvae (let’s call them maggots) will feast on these small sucking insects. Blowflies are also implicated in pollination services. The brown variety is particularly active in urban gardens, especially when the resident dog is let out at regular times of the day. Handy things, those blowflies, and very proficient at multi-tasking.

The narcissus-bulb fly maggot can do some damage, but the elegant, adult fly is a pollinator that resembles a small bumblebee, with similar colouring and fuzzy hair. Experienced gardeners know that adult flies differ from adult bumblebees in a few important ways: the flies only have one pair of wings (most other insects have two pairs) and they don’t sting, which allows you to gently catch them with your hands and impress the kids with your daring trick!

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How pesticides affect scents
It's also worth noting that spraying your plants can affect some pollinators' ability to find those plants again, in particular, bees. The global decline in bees has been linked, among other things to pesticide use. "One of the main issues to do with memory is that the use of neonicotinoid pesticides – said to be safe for beneficial insects – have been shown to affect bees' ability to navigate," Janice explains, "so they might not get back to the hive or might not remember how to find the flowers again."

A 2017 study pointed to neonicotinoids causing a reduction in bee numbers. In the year following exposure to the seed coatings of winter-sown oilseed rape that contained neonicotinoids, the bees established smaller colonies in the following spring (24 per cent decline).

"Avoid spraying bee plants with products containing thiamethoxam, clothianidin, thiacloprid or imidacloprid [all chemical names for neonicotinoids which will be listed as an active ingredient] or – my preference – just not use these products at all," says Janice.

Sign NZ Gardener's petition against the sale of pesticides containing neonicotinoids.

 

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Honeybee (Apis mellifera).

Large garden bumblebee (Bombus ruderatus).

Wool carder bee (Anthidium manicatum).

Native bee (Leioproctus fulvescens).

Native bees (Lasioglossum sordidum).

Hover fly (Melangyna novaezelandiae).

Common wasp (Vespula vulgaris).

German wasp (Vespula germanica).

Drone fly (Eristalis tenax).

Buff-tailed bumblebee (Bombus terrestris).

Short-haired bumblebee (Bombus subterraneus).

Small garden bumblebee (Bombus hortorum).

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Why florists' bouquets don't smell of much
You've probably noticed that flowers at florists shops do not always have great scents. That's because cut flowers are typically bred to improve appearance and focus on colour, stem length and flower quality.

Scent doesn't usually get a look in, which has led to a reduction in fragrant blooms from the florist.

Cut flowers also often travel long distances, so toughness is deemed more important than scent. And as cultivated plants have no use for scent – at least not in the sense of reproduction – the lack of fragrance is simply tough luck for the consumer.

In comparison, if a plant in the wild was to lose its scent – think wild roses – the result would be devastating. Wild flowers could not survive without their aromatic lures to entice pollinators.

So next time you stop to smell the roses, remember, that fragrance is not actually created to benefit you. But you can enjoy it anyway. As British writer Beverley Nichols said, "To be overcome by the fragrance of flowers is a delectable form of defeat." 

Seen any moths on your flowers at night? Dr Janice Lord is putting together a list of plants for a nighttime scented moth garden and invites you to contact her with your observations. Email janice.lord@otago.ac.nz.

 - NZ Gardener

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