Why Do Flowers Smell? The Chemistry and Purpose of Floral Scents

The perfume of a rose, the heavy sweetness of jasmine on a summer evening, the fresh scent of a lime tree in bloom: flower scents are among the most pleasant things in nature. Humans have used them for thousands of years in perfumes, teas and rituals. But flowers do not smell for us. Their scents are chemical messages, aimed mainly at insects and other animals, and they are the result of millions of years of evolution.

A scent is an advertisement

Plants cannot move, so to reproduce, many need animals to carry pollen from one flower to another. Flowers offer a reward, usually nectar or pollen, and advertise it with colour, shape and scent.

Scent has some advantages over colour. It travels on the wind, works in the dark and can be detected from a distance. A bee can follow a trail of scent molecules to a patch of flowers it cannot yet see. Studies show that bees learn and remember floral scents very well, returning to flowers that smelled rewarding before.

According to a widely cited review by ecologist Robert Raguso, floral scent serves several purposes:

• attracting pollinators from a distance,
• helping them recognise the right flower up close,
• filtering visitors, so the flower attracts animals that pollinate it well,
• defending the plant against herbivores, fungi or nectar thieves.

Different perfumes for different visitors

Flowers have evolved scents that match their pollinators’ tastes, a pattern known as pollination syndromes:

Timing matters too. Many flowers release their scent only when their pollinators are active. A petunia pollinated by night moths emits most of its scent in the evening, a rhythm controlled by the plant’s internal circadian clock.

How a petal makes perfume

Scent molecules belong to a group called volatile organic compounds: small, light molecules that evaporate easily at normal temperatures. According to a 2013 review by Natalia Dudareva and colleagues, plants make more than 1,700 such compounds, falling into a few main chemical families:

• Terpenes, such as linalool (lavender, many flowers) and limonene (citrus),
• Benzenoids and phenylpropanoids, such as methyl benzoate (snapdragon, petunia) and eugenol (clove-like scents),
• Fatty acid derivatives, which often give fresh, “green” notes.

The raw material is sugar made by photosynthesis. Enzymes in the petal cells convert it step by step into these molecules, which then pass out through the petal’s surface into the air. A single flower’s scent can be a blend of just a few compounds or of over 100. The exact recipe gives each species its signature smell.

Not all flowers smell sweet

Some flowers rely on flies and beetles that feed on dead animals or dung. These flowers imitate the smell of decay, using sulfur compounds similar to those released by rotting meat. The most famous is the titan arum (Amorphophallus titanum) from Sumatra, which can take years to bloom. When it finally does, it heats itself up to spread its stench further, attracting carrion insects that pollinate it.

At the other extreme, flowers pollinated by the wind, such as grasses, cereals and many trees, have no need to attract animals. Their flowers are usually small, plain and odourless.

Why many roses lost their scent

If you have ever bought a beautiful rose and found it smelled of almost nothing, you are not imagining it. Many modern roses were bred above all for colour, shape and long vase life. Making scent costs the plant energy and depends on many genes. During breeding, scent was often weakened or lost. Researchers are now studying the genes behind rose scent, in part to bring it back.

Scent and us

Humans have long harvested floral scents. Rose oil, from Bulgaria and Isparta in Turkey, is among the most precious perfume ingredients: several tonnes of petals are needed for a single kilogram of oil. Many of today’s perfumes also use synthetic copies of floral molecules, such as linalool, which smells of lavender.

In short

• Flowers smell mainly to attract pollinators such as bees, moths, flies and bats.
• Scents travel on the air, work in the dark and help insects find and recognise flowers.
• Each species has its own blend, matched to its pollinators and released at the right time.
• Petals make scent from sugar using enzymes; one scent can contain over 100 compounds.
• Some flowers, like the titan arum, smell of rotting meat to attract flies.
• Wind-pollinated flowers are usually scentless, and some modern roses lost scent through breeding.