Increasing variability in the start and end of the season, with climate change, is presenting a dilemma to farmers on the optimum planting dates for bush beans in semi-arid areas. A study to investigate the effect of planting date on bean yield and weed pressure was conducted at Nyagatare, Rwanda, during the 2009/10 and 2011/12 seasons. The highest seed yield and weed density were recorded at the second planting date in the wet 2009/10 season. Seed yield and weed density were significantly (P<0.001) lower at the first and third planting dates in the wet 2009/10 season. In the dry 2011/12 season, seed yield was highest at the earliest planting date and significantly (P<0.001) decreased ten times with each 15 day delay in planting. There was a significant linear correlation between days delay in planting and weed density (r = 0.99, P<0.001), bean plant dry weight (r = -0.97, P<0.01) and monthly rainfall (r = -0.97, P<0.01) in the dry 2011/12 season. These results suggest that early planting may be detrimental to bean seed yield in long wet seasons, most likely due to intermittent water logging and pre-disposition of the beans to higher disease and pest pressure. Bean seed yield declined as planting was delayed in a short dry season because the beans increasingly matured during dry hot conditions when the rainy season had ended and weed pressure increased as planting was delayed.