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Why do some flowers close up at night but others stay open?

Leslie Wilson
Reading, Berkshire, UK

The timing of the opening of flowers often relates to the pollinators they hope to attract.

Bees and hoverflies largely go to bed early, in my garden at least. However, some flowers are set up to attract night-flying moths. They are often pale in colour and if I stroll in the garden at twilight, I see that they look more noticeable at that time of day.

In particular, I am thinking about dame’s violet or night-scented stocks, both of which release scent at night but give off very little of it in the daytime. I often see moths at these flowers. Evening primroses only open at dusk, and are similarly attractive to moths. They have a luminescent quality in the half dark.

Roses don’t shut at night, but they tend to produce the most scent in warm sunlight when bees are most active. I have recently noticed that cultivated water lilies not only close at night but also prefer not to open on dull days.

Heather Whitney
Reader in plant interactions, University of Bristol, UK

There are several reasons why flowers are thought to close at night. One is to protect nectar from nighttime thieves, and potentially to keep the rest of the floral tissue safer from florivores. Another reason could be to prevent pollen from getting wet, due to rain or dew forming on the petals.

Closing petals at night also helps to control the temperature, keeping the pollen, stigma and overall flower warmer. Insect pollinators prefer warmer flowers.

There are also flowers that open at night rather than during the day. A classic example is the giant water lily, which opens in the evening and actively warms itself up to attract pollinating beetles. It then closes in the morning, trapping its pollinators inside before releasing them, covered in pollen, the following evening.

Guy Cox
Sydney, Australia

Plants have different pollination strategies. Many are pollinated by bees, which are active by day. Others are pollinated by moths, which are mostly active at night. In fact, some flowers only open at night.

However, some flowers have adapted so that they don’t need to expend energy in opening and closing. Many moth-pollinated flowers remain open all the time, but only produce their perfume by night, which can make a summer evening pretty magical.

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