Midnight: time to plow the north 40.

Wise farmers aiming for healthy, weed-free crops should ignore the adage "early to bed, early to rise," according to a ground-breaking study of a new weed-control strategy. Instead, the study suggests, farmers should consider cultivating their fields after dusk in order to prevent weed seeds that are briefly churned up during plowing from soaking up the sun they need to sprout.

A team led by Ana L. Scopel of Oregon State University in Corvallis reports that test plots plowed only at night grew roughly half as many weeds as plots plowed only during daylight hours. But this doesn't mean farmers should plow completely in the dark. Plots plowed at night by tractors with up to eight headlights had similar reductions in weed growth, the researchers found.

Scopel estimates that daytime plowing exposes buried weed seeds to sunlight for roughly one-quarter of a second before recovering them with freshly turned earth. This is just long enough, she reports, to activate photoreceptors in the seeds that normally prompt germination. Burial for a few weeks makes these receptors, called phytochromes, especially sensitive to light, she says.

"Cultivation during the night has a dramatic effect" in reducing weed growth, says Scopel, because it prevents the activation of phytochromes in weed seeds. She predicts that some farmers could drastically reduce or even eliminate their need for herbicides by plowing at night.

Simply placing a tarpaulin or a cardboard box over a tractor's plowshares during daytime plowing can reduce the sprouting of broad-leaf weeds by 40 percent, Scopel reports. However, she cautions, such sunshades would have no effect on weedy grasses.