Take stock of the flowers that bloom on summer nights and smell their fragrance to cool off

Produced by: Science Popularization China

Author: Yue Lang ( Wuhan Botanical Garden, Chinese Academy of Sciences )

Producer: China Science Expo

This summer may be as hot as ever, and the heat wave in the morning makes it almost impossible to go outdoors. It is only in the evening after the sun sets that it becomes much more comfortable. Everyone knows that you can see colorful, varied textures and visually interesting flowers during the day, but have you ever thought about what the garden looks like at night? Before the widespread use of modern landscape lighting, gardeners would use the moonlight of summer nights to make plants come alive in the dim night. When night falls, plants under the moonlight will show their best side, showing off flowers, fragrance, color, texture, shadow and outline.

Summer garden after nightfall

(Photo source: veer photo gallery)

Why do some flowers bloom at night?

Many people have the impression of summer when Cnidium monnieri, Trifolium repens, Lagerstroemia indica, Telosma cordata and Nerium oleander bloom in the summer fields, ridges and roadsides, swaying in the summer breeze. All the colors spread to the sky like a huge canvas. We take it for granted that flowers bloom during the day, just like the warmth and growth of summer. However, some flowers violate the life law of "working at sunrise and resting at sunset" and are true "night owls". They become active when night falls. How did this strange flowering habit come about?

There are actually good reasons to explain their peculiar habits .

The main purpose of flowers blooming is to allow the plant to reproduce. Flowers that bloom during the day attract bees, who act as pollinators to complete the pollination process. However, bees are not the only pollinators, moths, mosquitoes, beetles, and even ants can help with pollination. Most pollinators only roam during the day, which is why most flowers bloom when the sun is out to accommodate the pollinators' schedules. Butterflies, in particular, are famous pollinators during the summer daytime, but when night falls, it is the moths that dominate, wandering from flower to flower in the dark to transport pollen. Since pollinators like moths only come out at night, the flowers that need them to pollinate must adjust their schedules to complete the pollination process, so these flowers choose to bloom at night.

These night-flying pollinators tend to visit white, fragrant flowers, such as Jasminum sambac and Hosta plantaginea. In addition, moths tend to visit flowers with corolla lengths that are compatible with their proboscis lengths, meaning that moths with proboscis of a certain length prefer flowers of certain species. The most famous story of nocturnal moth pollination is the pollinator hypothesis proposed by naturalist and biologist Charles Darwin for the great comet orchid Angraecum sesquipedale, which has an exceptionally long spur and requires the help of moths with equally long proboscis to complete pollination. Darwin was initially ridiculed for his hypothesis, but it was proven correct when scientists discovered that the African long-beaked hawkmoth Xanthopan morganii used its 25 cm long proboscis to suck nectar from the spur.

Moths from the families Noctuidae, Geometridae, and Pterophoridae are common nocturnal pollinators. Flowers pollinated by moths often share a common feature, especially those pollinated by the long-beaked hawk moth: they are often tubular flowers with a trumpet-shaped opening, somewhat like a trombone, bell, or the trumpet of an old phonograph.

Flower visiting behavior of nocturnal mothsA: Noctuidae B: Pterophoridae C: Torodorinae D, E: Crambidae F: Geometridae

(Image source: Reference 1)

Types of flowers pollinated by moths

(Image source: Reference 6)

What are the night-flowering plants?

Some night-flowering plants bloom for just one night, but new flowers open every evening to lead the way throughout the summer nights.

Ipomoea alba is a plant of the Convolvulaceae family. It is known for its large, full-moon-shaped flowers and is a close relative of the daytime morning glory. Ipomoea alba has heart-shaped leaves and large, pure white flowers that open at dusk, emitting a pleasant fragrance, and close in the morning sun. The subtle fragrance of Ipomoea alba attracts the giant white-browed hawk moth, Hyles lineata, for pollination.

Moonflower and its night-pollinating hawkmoth

(Image source: woodrome.com & Garden Guides.com)

Mirabilis jalapa is probably a night-flowering plant that everyone is familiar with. Its flowers usually open from the evening or dusk (between 4pm and 8pm), so foreigners use "four o'clock flower" to refer to this flower. Its flower colors are most common in white, yellow and pink, and it has the same color change phenomenon as the summer sunset. Its yellow variety gradually turns into deep pink as the flowers mature. Similarly, white flowers can also turn into light purple. Mirabilis jalapa's pleasant vanilla and citrus mixed aroma is very strong throughout the night, and its sucrose-rich nectar attracts long-beaked moths for pollination. At night, the appearance of flowers in the eyes of insects is crucial to their pollination. The structure of Mirabilis jalapa flowers has a light filtering effect. One of the yellow pigments (dopaxanthin) emits visible fluorescence at night, which can be absorbed by another anthocyanin (gomphreninⅠ) that shows red, forming a contrasting fluorescent pattern on the petals to guide pollinators to dance close to the flowers.

(a) A Mirabilis jalapa flower with red or yellow areas under white light; (b) only the yellow petal area emits green fluorescence when excited by blue light; (c, d) Optical micrographs of red and yellow petal sections under bright field (c) and fluorescence (d; excitation wavelength, 450-490nm). The green fluorescence is caused by dopaxanthin, and the dark area in the figure corresponds to the orange area in the petals, and the emitted fluorescence is absorbed by betacyanin.

(Image source: Reference 3)

Evening primrose Oenothera biennis blooms soft yellow flowers at nightfall, adding a sweet, clean scent to the summer evening air, with petals shimmering in the moonlight. Evening primrose has a sweet lemon scent that attracts night pollinators, moths, to the garden. It is also a host plant for the beautiful pink-coated primrose moth, Schinia florida, and the white-browed hawk moth, Hyles lineata, whose larvae, caterpillars, feed exclusively on evening primrose.

Evening Primrose and Spodoptera

(Photo credit: bryanpfeiffer.com & Lac Bonin, Quebec)

What are the night-scented plants?

The flowers blooming during the day in summer may not give people time to appreciate them carefully, but the flowers blooming at night in summer are often fascinating. Many plants will emit fragrance at dusk, filling the surrounding air with a different fragrant scent, which makes people want to explore carefully.

Instinctively, when we see a beautiful flower, we lean over and sniff it, hoping to smell a sweet aroma. The natural scent of a plant is a property that botanists have long sought. There are two types of scented plants in the plant kingdom : those that release their scent directly into the air , and those that release their distinctive scent only when touched, bent, or crushed .

Flowers usually release scents to attract pollinators to visit, and many night-blooming plants have strong scents to attract moths and other night-flying insects. In addition to having excellent vision to accurately find blooming flowers in the dark, most pollinators also rely on their excellent sense of smell, so they can detect fragrant flowers from a certain distance. Many plants that need to be pollinated by nocturnal insects and bats are most active when they are ready for pollination, and the scents emitted by plants during this period will be more intense.

Polianthese tuberosa adds sweet jasmine and creamy scents to daytime gardens, but the brilliant white flowers of tuberose are even more fragrant on late summer nights. Unlike ordinary petals, the stomata on tuberose's tepals open wider when the temperature drops and the air humidity increases. There is no sunlight on summer nights, and the air humidity is much higher than during the day, so the stomata of tuberose open wider at night and the fragrance is stronger. Since the 17th century, tuberose has been widely used in perfume making for its absolute advantage. Marie Antoinette, Queen of Louis XVI of France, used a perfume made by it called Sillage de la Reine, which has been enduring and is still very popular today.

The tepals of tuberose are dyed with Sudan red. The volatile oil substances that emit fragrance are mainly distributed on the stomata of the petals. As the stomata expand at night, the fragrance of the flowers will become stronger.

(Image source: Reference 4)

Nicotiana sylvestris , with its large leaves, is one of the most important parents of modern tobacco. It has 7 cm long tubular white flowers, borne in racemes above the leaves, which look like falling fireworks at night. Like most tobaccos, its rough leaves are covered with glandular hairs that feel sticky to the touch, and each flower eventually produces a large number of fine dusty seeds that are scattered throughout the garden. In Victorian gardens, it was planted on walkways and paths so that those strolling on summer nights could enjoy the fragrance of the flowers. Nicotiana sylvestris's strong, sweet, jasmine-like fragrance is strongest at night to attract the tobacco moth Manduca sexta for pollination. Different lengths of tobacco correspond to different attractiveness of the tobacco moth, and the length of the corolla tube of Nicotiana sylvestris best matches the length of the tobacco moth's proboscis, which can better suck nectar and pollinate it while flying and hovering.

Leaves and flowers of forest tobacco

(Photo credit: Photo taken by the author)

Different tobacco and long-beaked hawkmoth

(Image source: Reference 5)

The trumpet-shaped flowers of Brugmansia suaveolens can reach up to 30 cm in length and range in color from white to soft orange and red. The Brugmansia suaveolens is a plant native to South America and is used as an ornamental plant all over the world. These evergreen plants can be cultivated as large shrubs or trees, reaching up to 10 meters in height in some areas. The large trumpet-like flowers are like horns, they are long, pendulous, trumpet-shaped, and hang down like a large skirt. The flowers of Brugmansia suaveolens usually bloom at night, and a single bloom can last from one day to several days. They emit a fragrance at night, and the fragrance contains some acyclic terpene alcohols and their hydrocarbons, as well as benzene alcohols and esters and some nitrogen compounds, which can be more intense at night to attract large numbers of moths to come for pollination.

Datura flowers and fruits

(Photo credit: Photo taken by the author)

Gardenia jasminoides has been cultivated in China for 1,500 years, and its stunning creamy white flowers gradually open in a rotation, emitting fragrance in the streets and alleys from June. Gardenias do not necessarily open at night, but their fragrance is enhanced at night. Like most plants, they have a circadian rhythm, making them an excellent olfactory feast for night gardens.

Gardenia

(Photo credit: Photo taken by the author)

Telosma cordata is one of the most common night-scented plants in southern China, Myanmar, Vietnam and India. As its name suggests, its fragrance reaches its peak at night. As the temperature drops at night, the stomata open and the volatile oils in the petals escape. The main components are geraniol, nerol, farnesol, eugenol and methyl o-aminobenzoate. Clusters of golden five-pointed star-shaped flowers exude fragrance along the vines on summer nights, attracting moths. In China, Thailand, Vietnam, Cambodia and Laos, people often eat the unopened flowers and young leaves as vegetables and cook delicious dishes with eggs and meat.

Tuberose

(Image source: kraeuter-und-duftpflanzen.de)

Hesperis matronalis has the same sweet scent as Matthiola incana. The Greek word for Hesperis means "evening" or "evensong flower" because the scent of Hesperis is at its most intense at night, when its elegant flowers can bloom all summer long. Hesperis populations are polymorphic in flower color, with purple, white, and pink intermediate forms. Purple petals contain high levels of anthocyanins, while white petals have little or no anthocyanins. The different flower colors of Hesperis do not differ in size, shape, pollen, or nutritional characteristics, but they do differ in the composition of their scent. The scent emission rate of Hesperis at dusk is twice as high as at dawn (0.041 μg vs. 0.019 μg of scent component/flower/hour), and the composition of the flower scent also changes significantly, with mushroom compounds released during both day and night, and aromatic compounds that are most attractive to pollinators starting to be released at night.

Hoverflies visiting flowers

(Image source: Reference 7)

Night-scented Phlox Zaluzianskya capensis is an evergreen plant from South Africa. During the day, its flowers remain tightly curled like little lollipops, but at night, their curled petals stretch out and open into white pinwheels, exuding a delicious scent mixed with vanilla, almond candy and honey. The wonderful mixed scent attracts long-beaked hawk moths and flies to suck nectar.

The flowers of Phlox truncatum close during the day and begin to open at dusk.

(Image source: anniesannuals)

Nocturnal flower-visiting behavior of long-nosed hawk moths and flies

(Image source: Reference 8)

Many plants bathed in moonlight at night will bloom beautiful but short-lived flowers that only bloom for one night and quickly wither with the morning light. But the short-lived and magical appearance and fragrance of these flowers blooming on summer nights bring joy and enjoyment to us humans and many wonderful pollinating creatures. Let us forget the troubles of the heat on summer nights, explore these blooming flowers, and share the charm and pleasure of summer nights.

References:

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(Note: Latin text should be italicized.)