If you have ever taken a stroll through a garden late at night, you may have noticed that certain flowers have retired for the night, tightly rolled petals hiding the inside of blooms that were wide open just a few hours earlier. Sometimes leaves fold or curl upwards too. No, they aren‘t tired and recuperating from a hard day’s work, and for some reason not all flowers do it.
The way they close up at night is a natural behaviour called ‘nyctinasty’, the name coming from the Greek word ‘nux’ which means night, and ‘nastos’, meaning pressed down. Experts know the mechanism of ‘how’, but it’s not clear if they know ‘why’ – why some do and why some don’t. The bottom petals of certain flowers grow at a faster rate than the upper petals, forcing the flowers shut. Some examples of flowers that ‘do’ are the lovely orange gazanias, tulips, poppies and the delicate blooms of hibiscus, together with the exotic lotus flower.
Even the humble dandelion does it – they have the added complication of being something called ‘apomictic’: their seeds are not fertilized.
Dandelion flowers open each morning and close each night, but sometimes they don’t open at all on cloudy days! When the flower is completely mature, it closes one night and simply doesn’t open again until the seeds are ripe.
But scientists are not quite sure why some flowers evolved this way, but there are several theories though. Charles Darwin believed that plants close up at night to reduce their risk of freezing, and I can understand their need to protect against night time chills. Another theory suggests that nyctinastic plants are conserving energy — and perhaps their smell — for the daytime, when pollinating insects are most active, which could also be true.
And one idea holds that nyctinasty is a highly evolved defence mechanism against a plant’s nocturnal predators. By closing up tight, the flowers create a clearer view of the ground for night time hunters, like owls, who kill off flower-munching herbivores out looking for a midnight snack, but I am not sure I buy into this theory, but they could be protecting themselves against night time nectar thieves, such as bats or moths.
A technical explanation is that when light hits outer flower petals it triggers a chemical called auxin that causes cells to grow and expand. This causes the flower to open. But because its inner petals are less exposed to light, those cells remain the same and cause the flower to close once light is gone.
Some scientists believe that this night time closing behaviour prevents pollen from becoming wet and heavy with dew or rain. Insects can more easily transfer dry pollen, improving a nyctinastic plant’s likelihood of successful reproduction. I think this theory is most likely to be true, but who am I to judge?
On the other side of the coin are flowers that only bloom at night!
Night-blooming flowers can reflect the moonlight and are often much more fragrant than their day-blooming counterparts. One of the reasons flowers that bloom at night are often perfumed is that they need to attract nocturnal pollinators that must find them in the dark. This is also why most night bloomers have white flowers that reflect light and are more visible at night. Moonflower, night-blooming jasmine, evening primrose, angel’s trumpet, night phlox, and night-blooming cereus open only at dusk or at night. Some plants are extremely clever too - they react to touch and close up their leaves and ‘play dead’ if a hand or twig brushes against them. For example, if you touch a mimosa plant, it will fold its leaves and the stalk will droop.
So, whenever you see a flower closed up for the night, remember it will be open for business in the morning - and that one opening out as the sun sets is just setting up its stall for night time trade!