plants falling asleep

White Oxalis purpurea closing up for the evening
White Oxalis purpurea closing up for the evening.
Detail of white Oxalis purpurea thinking about some shut-eye.
Purple-leaved Oxalis purpurea closing up in the late afternoon shade.

A lot of the flowering plants in the garden don’t bother opening their petals until the sun’s up and then shut their flowers as soon as the light begins to fade and temperatures drop in the afternoon. Over the weekend I was noticing this going on with my oxalis plants and, less dramatically, with my arctotis.

There must be a name for this behavior, I thought, and so off I went looking for an answer. Before long up pop three interesting words: photonasty, thermonasty and nyctinasty.

According to one of the sources, the Textbook of Botany by Chhatwal and Singh, photonasty happens when a plant senses light and reacts to it by opening or closing its flowers. Because of this, morning glories open in the…well, morning. Then there’s thermonasty, where flowers react primarily to temperature. Tulips will open with a rise of 2-3 degrees Celsius, while a crocus will zip open when the temperature rises just a half degree.

And then there’s the more complex phenomenon of nyctinasty, which “is influenced by the intensity of light and also temperature differentials, the former stimulus being more powerful and effective. The foliage leaves and also the floral leaves in many species of plants…attain different positions at day time and at night viz during the day, the leaflets remain open or spread up in case of Oxalis, clever beans, alfalfa, etc., while by the onset of darkness they close down. This is also known as sleep movement.”

Yesterday afternoon was pretty bright, but cool. The oxalis barely opened before shutting back up. So it requires both heat and warmth to open fully. So nyctinasty makes sense. The arctotis seemed to open more fully, earlier in the day. My guess is that they respond more simply, mainly to light, which would mean that they exhibit photonasty. (What’s truly going on could be lots more complex than this and really might only be solved by experimentation, a point made in an article, “Flower opening and closure: a review” by Wouter G. van Doorn and Uulke van Meeteren in the Journal of Experimental Botany. Read the interesting text [ here ].)

Next I need to find out what “clever beans” are.

In my web trawl it turns out I’m not the only garden blogger looking at this phenomenon this week. Tilthy Rich took a quick spin around nyctinasty [ here ]. Maybe he has the same plants blooming, making him ask the same questions…

Flowers of Arctotis acaulis 'Big Magenta' beginning to fold up for the night.

Another clone of Artotis acaulis closing up in the afternoon: Photonasty? Thermonasty? Nyctinasty?

13 thoughts on “plants falling asleep”

  1. Thank you for such an informative and interesting post. I do enjoy my flowers that do close at night, but didn’t realize that there may be different stimuli that they respond to – opening with a half-degree difference in temperature? Amazing…

  2. I might have to start using the word nyctinasty more often. When we did the garden show a few years ago, a couple of the natives wouldn’t open their flowers and petals all the way because the light wasn’t strong enough inside the building. Though there was also a time of day or internal clock thing happening, because they would open part way by mid-morning, even though the light levels hadn’t changed for several hours.

  3. And one day, if you come to Namaqualand in the Northern Cape (South Africa) to see those flowers at home in the spring – you go out to look at flowers between 10 and 4. They keep very civilized hours. If it is overcast, or windy – then they don’t get out of bed. But when it all comes toghether …

  4. James, I love it that when you notice something interesting happening, you actually take the time to research it. I must confess that unless the information comes to be in an absolutely effortless way, I am much too lazy to find out more.

    Which of course, is why I visit here.

  5. Wonderful post James! It is so interesting to observe plants and to learn about their inner clocks and intelligence. Botany is a fascinating subject. ;>)

  6. Noelle, I was surprised myself. I thought it was just daylight. But then on bright but cold mornings I have a hard time getting out of bed, so it makes sense that there’s more going on than just sunlight.

    Ryan, I hadn’t thought of this in the context of garden shows or garden shows where the displays are all shaded. It’d be catastrophic to do an amazing garden design where the flowers failed to open!

    Sunny, thank you!

    EE, what you say about the wind reminds me of the one trip I took to the California Poppy Preserve. After traveling 4 hours from my home on a cold, stormy day, I learned that California poppies are one of the plants that keeps their flowers closed in inclement weather–like some of the plants in your Namaqualand displays. What a disappointment that was.

    Tina, I agree. The more I find out about plants the more interesting they become.

    Susan, I have a bit of a lazy streak too. It took me how many decades to finally research this one? But I hope you’ll indulge me the occasional post with pretty pictures but with zero information content…

    Carol, thanks! That first is my favorite of the bunch as well. The little tubular buds on the oxalis are pretty cool how they shade from white to yellow to green.

  7. I view these plants like people. Some people are full of energy and can go with little sleep, while others need more rest to be able to live up to their full potential when awake. :-). In some countries they have long lunch hours so people can take a nap. These plants just don’t wake up from their nap until the next day…

  8. I love botanical language. But I think photnasty is when I forget to clean my lens and there’s a big blob on all the images from a shoot!

    BTW our blog was tagged with an “Honest Scrap” award by Christine at Idora Design – I did the post for us mice and tagged you (and six others) in turn. Don’t feel obligated – but have fun if you like.

  9. Skeeter, I like your take on this. Some folks (not me!) are major night owls, and there are plants like datura, night-blooming jasmine or four o’clocks that don’t open up until near dusk but stay open late into the evening or the next morning.

    CM, I love the language too, and the fact that people have observed a behavior closely enough that they decided it needed a name. And thanks for the tag! I’ve sworn off memes, but I might assemble something if time and inspiration cooperate!

  10. I’m doing this for a school project.
    Why don’t you add extra pictures or something. The info is good. I love the purple oxalis.

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