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


Strawberries can be short-day or day-neutral, depending on type. 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. 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.

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.

In the meantime, I am grateful that this emerging science has helped me and my turnips to cease being so frustrated with each other and to arrive at a new state of peace and understanding.