Recent media reports stating that tillage during darkness reduces weed populations may have oversimplified the matter. It is true that there are strong indications that tillage in the absence of light reduces the emergence of some annual weed species. The light requirement for germination found in many small-seeded species is believed to be an evolutionary strategy for survival. The majority of seedlings developing from small seeds found more than 0.5 inch deep in the soil would fail to survive due to insufficient energy reserves to reach the soil surface. Light-sensitive seeds buried deep in the soil remain dormant until soil disturbance, such as tillage, exposes them to light.

Research in Iowa and Minnesota indicates a wide range of responses to tillage in darkness among important annual weed species. Also, total darkness is required for a response to occur. For example, tractor lights provide more than enough light to negate a response in susceptible species.

Research was conducted at Rosemount, Minnesota in 1994 and 1995 to determine the effect of tillage in the dark on the emergence of 13 annual weed species common to our region. Some of the results of this research are summarized in the accompanying table. Tillage treatments included: two passes with a tandem disk in the light, one pass in the light followed by the second pass in the dark, two passes in the dark, and no tillage. Tillage operations in the light were conducted between 2:00 and 4:00 pm and dark operations were conducted between 11:00 pm and midnight. Weed emergence was evaluated 14, 30, and 50 days after tillage.

Emergence of the annual grass species (barnyardgrass, green foxtail, giant foxtail, and yellow foxtail) was not affected by the time of tillage. Emergence of large-seeded broadleaf species (common cocklebur, giant ragweed, and velvetleaf) was also similar following tillage in the light or dark. Emergence of small-seeded broadleaf species (common lambsquarters, common ragweed, eastern black nightshade, pigweed species, Pennsylvania smartweed, and wild mustard) was affected by the time of secondary tillage. The level of reduction in emergence when tillage was conducted in the dark rather than during the day varied by species and date of tillage. Emergence reduction with small-seeded broadleaf species ranged from 30 to 70 percent.

Based on the results of this experiment and research conducted at other sites in the U.S. and Europe, it appears tillage in the dark may have potential as a component of integrated weed management. It may also be possible to design equipment that excludes light from the soil while working during daylight. However, several important weed species (annual grasses and large-seeded broadleaf species) were insensitive to tillage in the dark. Additional research is needed to determine the light sensitivity of various weed species, as well as the effects of tillage depth, tillage implements, and agronomic factors.

Emergence of annual weeds following tillage in the light (2-4 p.m.) and dark (11 p.m.-midnight) at Rosemount, MN in 1994 and 1995.