Linnaeus’s Flower Clock

I’ve always been fascinated by morning glories. I know that some people regard them as unruly, invasive weeds—the kudzu of ornamental flowers. But I adore the blankets of blossoms they produce as they cover a fence or hedgerow. And I never tire of the way they open and close each day—not merely spreading their petals wide each morning and drawing them in at night, but actually curling and uncurling them, like fingers opening and closing into a fist.

So imagine my delight when I learn that the pink blossoms on my purslane also bloom by day and close up late afternoon. On workdays, I rarely see them in their full glory, as I leave for the office too early and return home too late. But on the weekends, I revel in the midday profusion of flowers.

It’s all the more remarkable, because, I just learned, each blossom lasts only a day. (So that’s why there are so many fallen buds littering my stoop every morning!)

Most flowers open once and remain that way until they wilt. But in those with daily rhythms, these patterns are so predictable that an astute observer could use them to form a rough sense of time. In 1751 the Swedish botanist Carolus Linnaeus took the idea to its logical conclusion and proposed building a flower clock, using a full array of plants that open and close on a set schedule—morning glories starting at 10 am, water lilies at 11, and so on through the aptly named four o’clock flowers. And unlike a sundial, which can only function during broad daylight, the flower clock could theoretically continue ticking well past dusk, as evening primroses and moonflowers begin their nocturnal watch.

Granted, this is no precision timepiece. A few years ago, James Duke, PhD, a former USDA researcher and author of The Green Pharmacy Guide to Healing Foods, set one up at his Maryland home. “I can tell you to within an hour what time it is,” he says, noting that the degree of heat or cloud cover can affect timing, and some plants are more dependable than others. “The spiderwort opens punctually at 6 am. Scarlet pimpernel is good by 10 am. And evening primrose blooms promptly at 8:30 pm,” he says. “But my four o’clocks don’t open at 4, so they’re a failure. It’s a lot easier just to buy a cheap watch.”

But was a watch ever so much fun? Check out this uber-cool video of morning glories opening.