The use of cover crops is an effective technique to control weeds, which are one of the most serious problems for crop production without using herbicides. This study investigated the characteristics of cover crops for weed suppression at an organic farming field in a snowy-cold region, Hokkaido. Nine, three and two species of cover crops comprising both Poaceae and Leguminosae were grown in 2003, 2004 and 2005, respectively, at different sowing densities from 50 to approximately 4000 seeds m -2. The relationships between weed dry matter production and characteristics of cover crops, such as plant height and coverage, were investigated at 4 and 10 weeks after cover crop sowing (WAS). Correlation analysis of the weed dry weight with characteristics of cover crops revealed that the cover crop coverage at 4 WAS had a strong and stable effect on weed suppression. The cover crop coverage at 4 WAS was affected primarily by their seed weight when cover crops with a large variation in seed weight were used, and by the sowing density when cover crops with a small variation in seed weight were used. These results suggest that to achieve high weed suppression it is important to obtain higher coverage at the early growth stage of the cover crops with a heavy seed weight and high sowing density.
