The influence of weeds on yield and quality were studied in sugar beet stands kept mechanically weedless for different periods in small-plot. Plots which were weeded early in the season (1^st May or 15^th May) and were kept clean to the end of the growing period (30^th September) gave significantly higher and better quality yield than plots where weed killing started later (1^st June) or where the weeds were allowed to reappear after 15^th May or 1^st June. Trials proved that weed control in sugar beet has to be started early and that the stand has to be kept weedless to the end of the growing period. The late spread of weeds is an important yield-reducing factor.