Robert Krulwich, NPR
Before we get to today’s topic (flower blooming), let’s take a Sloth Break. I know this isn’t usual, but hey, I think everybody should see this adorable baby sloth named Matty giving his human caretaker, Claire, a flower. If you’ve already seen it, jump ahead to my essay. But if you haven’t? Well, your day is about to get a wee bit lovelier.
OK, I just had to. Now we can start.
My subject today is flowers and how we get them to do exactly what we want. Even when it’s cold out (like now) and flowers are totally out of season.
For a long time I’ve been puzzled by this. Maybe you have been, too.
If you sell flowers for a living, February is one of the big sales months of the year — because of Valentine’s Day which, in the Northern Hemisphere, is a winter holiday …
… when there are no flowers growing outside. You can, of course, build warm nurseries, and grow the plants inside, but here’s my puzzler: How do you get roses and tulips and daisies and mums to burst into blossom exactly when they’re the most valuable — on Valentine’s Day week, Feb. 10 through the 14?
The answer, I figured, has something to do with artificial lights. There must be a way to trick flowers to blossom. Which is true. But the trick is so different than I thought, I want to share it with you. It took about a hundred years to perfect. So here is my short primer:
How To Force A Plant To Celebrate Valentine’s Day
Almost 100 years ago, two botanists, Wightman Garner and Harry Allard of the U.S. Department of Agriculture, got some tobacco plants and put them in pots. Half the pots were left outdoors. The other half got moved, every afternoon, into a dark shed.
It was summer, so the outdoor plants got a lot of light. The other plants had their light sharply reduced. And guess what?
The plants that experienced a loss of light, began to blossom! Apparently, plants know how to measure how much light they get. When the amount of light goes down, autumn bloomers think, “Oh, it’s time!” And so, they blossom — which is how botanists learned that, for mums and fall bloomers, you can trick them by reducing the light.
What About Springtime Blossoms?
But now comes the tricky question. The flowers we prize, like irises, daffodils, daisies, roses and tulips — they blossom in the spring or summer, when there is more light. So how do you trick them?
The short answer is: Don’t think about days. Think about nights.
Imagine that your plant isn’t measuring how long the days are, but rather, how long the nights are. Plants, it seems, are very sensitive to the length of continuous periods of darkness. (The key word here being “continuous.”)
So if a plant senses no light for 14 consecutive hours, that’s a long unbroken darkness, which the plant associates with the colder seasons of fall, winter and early spring. So it has evolved to avoid blossoming. Its hormones signal, “Not Yet!”
The equation is very simple: Lots Of Continuous Dark = Don’t Blossom.
But if you interrupt the dark — even just for one brief flash — turn on the lights for only one minute, the plant thinks, “Oh, my continuous dark period is shorter! That means the days must be getting longer.” It’s time to blossom.
Plants don’t have eyes, so they can’t inspect the sky. They don’t have our ability to say, “Hey, that light went on for only 60 seconds, so really, it’s still night time.” All they know is that when there is less continuous dark, a trigger goes off that says, “Blossom Time!” In a sense, the plant has been “counting” time.
And so, if you flash daffodils, roses, lilies or tulips just the right way — to trigger their “blossom!” hormones, that’s what they do:
… they blossom. And if you get your flashes right, you can make all kinds of flowers blossom exactly when you want them to. And that’s how flower farmers do it. They don’t keep lights on all winter. Or lights off. They flash. That’s how they get blossoms.
Here’s a fascinating postscript from Israeli botanist Daniel Chamovitz, whose book, What a Plant Knows: A Field Guide to the Senses, has a vivid section on flower farming and light flashing. He points out that you can’t trigger a blossom with any light. Mostly, farmers used red light. Blue lights, green lights, other colored lights, don’t work. He writes:
[In] the early 1950’s, Harry Borthwick and his colleagues in the U.S. Department of Agriculture … made the amazing discovery that far-red light — light that has wavelengths that are a bit longer than bright red and is most often seen, just barely, at dusk — could cancel the effect of the red light on plants.
Let me spell this out more clearly: if you take irises, which mostly don’t flower in long nights, and give them a shot of red light in the middle of the night, they’ll make flowers as bright and as beautiful as any iris in a nature preserve.
But if you shine a far-red light on them right after the pulse of red, it’s as if they never saw the red light to begin with. They won’t flower. If you then shine red light on them after the far-red, they will. Hit them again with far-red light, and they won’t. And so on.
We’re also not talking about lots of light; a few seconds of either color is enough.
It’s like a light-activated switch. The red light turns on flowering; the far-red light turns it off. If you flip the switch back and forth fast enough, nothing happens.
On a more philosophical level, we can say that the plant remembers the last color it saw.
Chamovitz is, of course, speaking metaphorically. Plants don’t “remember” the way humans do, but still, in their botanical way, they detect light, know one color from another, “remember” how long a light or a darkness lasts which, I guess is a form of time keeping, primitive counting — and they do all this without a brain. Plus, they’re gorgeous.