Four O'Clock Plants and Other Clock Flowers

Take a closer look at the schedules flowers are on.

| April/May 1998

  • Illustration by Susan Strawn Bailey
  • The cheerful blossoms of chicory welcome pollinators between 7 a.m. and noon.
    Photographs by Joseph G. Strauch, Jr.
  • On sunny mornings, the yellow petals of mullein unfurl.
  • The soft light of dusk dances on evening primrose petals.

Have you ever noticed how flowers seem to be on a schedule? Some wake up and show their faces to the sun the first thing in the morning, while others wait until later in the day or even dusk. Carolus ­Linnaeus (1707–1778) noticed. The father of modern plant classification even devised a floral clock based on the time at which the component plants open. You might want to try this fascinating idea in your herb garden. After all, flowers are a lot more fun to watch than a clock.

Timing’s the Thing

What determines the time of day a flower opens?

Light seems to trigger the opening of poppies early in the morning of a sunny day. On a plant-collecting expedition to southernmost Sweden in June 1749, Linnaeus observed that scarlet pimpernel (Anagallis arvensis) opened at 8 a.m. and closed at midday. This little wildflower is also known as poor-man’s-weatherglass because flowers close or don’t even open in cloudy weather. Flowers of California poppy (Eschscholzia californica) also may not open at all on overcast days.

To maximize cross-pollination, many kinds of plants have evolved so that their flowers are open while their pollinators are most active; meanwhile, pollinators have evolved so that they’re busiest when pollen or nectar is most available.

Nectar in chicory, for example, is produced only between 7 a.m. and noon on sunny days, the hours when its bee pollinators are most ­likely to stop in. The pollen of mullein, red ­poppies, and bindweeds (three of Linnaeus’s clock plants) is released only during the hours when bees visit them. Bee visits to wild mustard and some dandelions have been shown to peak about 9 a.m.; to blue cornflowers, 11 a.m.; to red clover, fireweed, and marjoram, about 1 p.m.; and to viper’s ­bugloss, about 3 p.m.—an indication of these flowers’ peak availability of pollen or nectar.

Dandelions, daisies, and some cacti appear to have a built-in clock that makes them open at daybreak and close late in the afternoon; even when the plants are kept in total darkness or constant light, they open and close on schedule.

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