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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 University of Washington biologist Takato Imaizumi, who is uncovering the mechanisms of the seasonal calendar in plants. In other words, it takes a clock to understand a summer.
Almost all living creatures—from blue-green algae to gardeners—possess a powerful clock, known as the circadian clock, that controls many of life's processes. Though it mimics Earth's 24-hour rotation, it is internally generated within every cell. Its molecular biology is still being unraveled, but at the simplest level, the hands move around the clock because genes are activated to rhythmically produce proteins, which then degrade, activating the genes again 24 hours later. Studies with subjects that range from blind animals to plants locked in the dark prove that the clock is innate and that plants and animals don't actually have to experience day and night in order to have regular wake/sleep or stiff-leaf/relaxed-leaf cycles—as proven by the Flowers That Are More Beautiful At Night.
Sunlight is still crucial to our biological sense of time as it resets our internal clocks, harmonizing our daily rhythms with the environment. Otherwise we might live in a state of perpetual jet lag because our internal clock runs slightly longer or shorter than that of a solar day. As light lingering on a winter's evening tells a gardener that spring is near, this light also helps plants determine the season. "The [circadian] clock is not constantly sensitive to light," Imaizumi explains. "It has a light-sensitive window at the end of the day that acts as a gating mechanism."
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In what are called long-day plants, light striking during this window begins the molecular process that produces flowering. These plants want to flower as we move toward the longest days of the year. In short-day plants, light striking during this window inhibits flowering. These plants want to flower earlier in spring or in fall. In day-neutral plants, it is not the photoperiod that prompts flowering but another factor such as temperature or stage of maturity.
In my northern climate—where the snow often lingers on the ground into late April—long-day plants like mustards, lettuces, and the 6 Radishes Worth Seeking Out are generally difficult in spring. Because I have to plant them relatively late—as we race towards the summer solstice—they often bolt before they produce anything worth eating.
For other long-day plants, even my late planting date is too early in terms of temperature. Some long-day plants flower only after experiencing vernalization—a period of prolonged cold. This is what keeps delicious biennial root and stem crops like turnips, carrots, parsnips, and beets from going to seed in their first summer and using up their stored food before we can harvest them, but we have a plan for how to plant carrots. But, for example, too much chill in early spring can fool turnips into thinking that they have already lived through a winter and now have permission to flower and set seed.
My answer? Avoid the summer solstice entirely for fast-maturing long-day crops like turnips, radishes, beets, and mustards. Planted in midsummer as fall crops—they are crisp and creamy, with long, luxuriant leaves.
Short-day crops can also cause gardeners grief, particularly if they demand such short days that they can flower only in the frosty spring or fall. It was this very problem—soybeans that flowered too late to be productive in Minnesota—that led USDA scientists in 1920 to the breakthrough discovery that plants use sunlight for timekeeping as well as energy.
Essentially, the problem with soybeans was one of latitude. Day length at the equator is a constant 12 hours in all seasons, while at 50 degrees latitude it can swing from about 8 hours in January to 16 hours in June. Soybeans are tropical in origin and their flowering response was inhibited by Minnesota's long summer days. Biologist C. Robertson McClung, Ph.D., of Dartmouth College, an expert in the genetics of the circadian clock, explains why, unlike in 1920, soybeans can now be grown all over the country: "Breeders found the rare variants that lost the short-day requirement."
Onions are another crop whose latitude of origin is important. All onion varieties respond positively to lengthening days—producing a bulb when light strikes the light-sensitive window—but window placement is very different in different varieties. There are 3 Ways To Plant Onions and a variety that requires a full 15 hours of daylight for bulb production would be appropriate for Maine but not for Florida, and a variety appropriate for the shorter summer days of Florida would bulb up too early in its life cycle in Maine and produce something too small to bother with.
For the poor vegetable gardener simply hoping for perfection from a wide range of crops, there are a lot of variables to keep track of: the different light requirements of different vegetables and even of different varieties of the same vegetable, the radical difference in day length at different latitudes, and the fact that seasonal responses in plants are not light-determined alone but can be adjusted by weather-related factors like temperature and moisture. Still, it's good to be aware of the seasonal preferences of your plants and to pay attention to the ways these preferences play out on your particular piece of property over the course of several years—and even, if you are willing to put down your shovel in favor of a journal, to take a few notes.
Keep in mind that with photoperiod-sensitive plants you may have more success if you choose varieties proven to work not just in climates similar to yours but at latitudes similar to yours. For example, I was astonished at how well seeds imported from northern Italy did in my much more frigid garden, until I realized that Bergamo and my part of upstate New York are at nearly the same latitude.
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Gardeners will likely get more help in the near future from plant breeders as the radically new science of circadian clocks reveals new avenues for research. "It is attractive to hypothesize that tweaking the clock might optimize plants for growth in the photoperiodic environments of Florida versus North Dakota," McClung says. Both McClung and Imaizumi believe that clock science will be used to increase yield in crop plants in the future.