When German researchers reported that fewer weeds grew in fields tilled at night, Doug Buhler at the National Soil Tillage Research Lab, Ames, Iowa, decided to find out for himself. Tracking the growth of 13 common annual weeds, he compared a field cultivated at midnight to a field cultivated in the middle of the afternoon. Buhler reports that nighttime tilling was effective in controlling some weeds.
In Buhler's study, two night cultivations (about 2 weeks apart) produced a 70 percent reduction in lamb's-quarter and an 80 percent reduction in smartweed. Ragweed and black nightshade were reduced by about half, pigweed and mustard by about 40 percent. Night tillage had no effect on many other species, including giant ragweed, velvetleaf, and most of the grasses tested. Buhler says weed species that need light to break dormancy are the ones affected by nocturnal cultivation. That's because tilling allows light to penetrate deeper in the soil. Each time you cultivate during the day, you kill some weeds that have sprouted but wake up others.
Does this mean we all should get night-vision goggles and start hoeing by the light of the moon? Not really, thank goodness. But it does remind us that minimal tillage, plus mulch, is often a better way to prevent weeds than churning up the ground. Buhler theorizes that even very short bursts of light--from a spotlight or even a full moon--could be enough to stimulate some of weed seeds to grow.
Photography by the National Gardening Association.