On the Clock

Tapping in to your garden’s circadian rhythms reveals the secret to knowing the best times to plant.

By Michele Owens

Photography by Matthew Benson

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all plants have an internal clock that regulates their lifeLike comedy, vegetable gardening is all about timing. Even beginning gardeners realize that the last and first frost dates that bracket a gardening season mean that it's possible to plant the tomatoes too early–and harvest the basil too late.

Ask longtime vegetable gardeners for the keys to their success, and after they finish bragging about the quality of their soil, they will invariably say, "timing."

I'm as shrewd about timing as any passionate 20-year gardener, but a newfound taste for glazed turnips a few years ago convinced me that I was missing some key information. Though turnips are described as a spring crop, for the life of me, I could not grow them in spring. They inevitably bolted and produced unappetizing, woody roots. But when I planted turnips for a fall crop, I'd pull beauties out of the soil. The two plantings should have been equivalent–55 days to maturity, plenty of cool weather–but in my garden they were not.

What I did not understand was that my turnips, too, had a sense of timing.

They were not merely reacting to the conditions thrown at them by the weather and the gardener, such as temperature and moisture levels. They were also carefully measuring the progress of the gardening season, so they could avoid being tricked by the weather and reproduce at a moment favorable to the survival of the species–if not to my cooking.

Plants keep track of the shifting seasons the same way you would if all clocks were taken away: by being acutely attuned to the waxing and waning of daylight as the year progresses. In many plants, this sensitivity to day length, known as photoperiodism, determines when they bolt, fruit, or produce storage organs as potatoes do. By understanding a bit about a crop's sense of seasonality, gardeners can improve their chances of success.

"To distinguish changing day length, a time-keeping mechanism is required," explains biologist Takato Imaizumi of the University of Washington, who is uncovering the mechanisms of the seasonal calendar in plants. In other words, it takes a clock to understand a summer.

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