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Many millennia before the invention of herbicides, farmers simply plowed their fields to control weeds. Even today, plowing can constitute a valuable part of an integrated weed-management 5 program. Although plowing kills standing weeds, farmers have long known that it often leads to the emergence of new weed seedlings in a few weeks.Ecologists have shown that a farmer's field can have 50,000 or more weed seeds per square meter 10 buried beneath the soil surface. Plant physiologists have shown that seeds buried more than about one centimeter below the soil surface do not receive enough light to germinate. Do the blades of a plow, which can reach more than a foot beneath the soil15 surface, bring some of these buried seeds to the surface where their germination is induced by exposure to sunlight?Two ecologists, Jonathan Sauer and Gwendolyn Struik, began to study this question in the 1960s. In a 20 relatively simple experiment, they went to ten different habitats in Wisconsin during the night and collected pairs of soil samples. They stirred up the soil in one sample of each pair in the light and stirred up the other sample of each pair in the dark. They then25 exposed all ten pairs to natural sunlight in a greenhouse. For nine of the ten pairs of soil samples, weed growth was greater in the samples stirred up in light. They concluded that soil disturbance gives weed seeds a "light break," and this stimulates their 30 germination.More recently, Karl Hartmann of Erlangen University in Germany reasoned that when farmers plowed their fields during the day, the buried weed seeds are briefly exposed to sunlight as the soil is35 turned over, and that this stimulates their germination. Although the light exposures from plowing may be less than one millisecond, that can be enough to induce seed germination. Thus the germination of weed seeds would be minimized if40 farmers simply plowed their fields during the night, when the photon fluence rate [the rate at which photons hit the surface] is below 10^15 photons per square meter per second. Although even under these conditions hundreds of millions of photons strike 45 each square millimeter of ground each second, this illumination is below the threshold needed to stimulate the germination of most seeds.Hartmann says that he was very skeptical when he first came up with this idea because he assumed 50 that such a simple method of weed control as plowing at nighttime must be ineffective or it would have been discovered long ago. But the subsequent experiments, first presented at a 1989 scientific meeting in Freiburg, Germany, clearly demonstrated 55 that the method can be effective.Hartmann tested his idea by plowing two agricultural strips near Altershausen, Germany. The farmer Karl Seydel cultivated one strip, repeated threefold, at around midday and the other strip60 at night. No crops were planted in these pilot experiments, to avoid possible competition with the emerging weeds. The results were dramatic. More than 80 percent of the surface of the field plowed in daylight was covered by weeds, whereas 65 only about 2 percent of the field plowed at night was covered by weeds.This method of weed control is currently being used by several farmers in Germany. Because many of the same weed species that invade farmers' fields70 in Germany also invade fields elsewhere in the world, this method should be successful elsewhere. In fact, recent studies at universities in Nebraska, Oregon, Minnesota, Denmark, Sweden, and Argentina support this idea. Number of Emerged Seedlings in Soil Samples One Month after Soil Was Disturbed 2*Sample Source of soil Number of emerged seedlings in soil disturbed in 3 - 4 light darkness A deciduous woods 4 0 B deciduous woods 2 1 C deciduous woods 6 2 D conifer plantation 8 3 E conifer plantation 2 1 F tall-grass prairie 1 G old pasture 0 2 H old pasture 2 1 I muck field 14 2 J muck field 5 3 Adapted from Jonathan Sauer and Gwendolyn Struik, "A Possible Ecological Relation between Soil Disturbance, Light-Flash, and Seed Germination." @1964 by Jonathan Sauer and Gwendolyn Struik.