Green gram (Vigna radiata (L.) R. Wilczek) is one of the most economically important grain legumes of the traditional farming systems of Sri Lanka because it is a cheap source of protein and animal feed, and sustains soil fertility by fixing atmospheric nitrogen. Weeds are one of the major problems in green gram cultivation, reducing the yield through competition, interference with harvest and harboring pests and diseases. Controlling of weeds by applying herbicides would definitely cause unexpected damage to human health and the abundant biodiversity of Sri Lanka. Therefore, an investigation was planned to evaluate the yield loss due to weeds and to determine the optimum weed free period to minimize the yield losses. Two experiments were performed. The first experiment was conducted to determine the effects of different weed functional groups on the yield of green gram. In the second experiment, weeds were continuously hand weeded and areas kept weed free. In the third, weeds were allowed to compete with green gram until 2, 3, 4, 5 or 6 weeks after cultivation. All the treatments were conducted in randomized complete block design with three replicates. The data collected on types of weed, number of pods and pod weight at 3–6 weeks after planting (WAP) were analyzed using the SAS 9.4 statistical package, and DMRT was performed to determine the best treatment combination. The results from the first experiment showed that average yield loss due to total weed populations was 54.77%. Yield loss due to grasses alone was 46.56%, far worse than broad leaves (16.49%) and sedges (18.01%) at p < 0.05. Crop stand count at 3–4 WAP was not significantly different among treatments. However, biomass weight of 50 plants, number of pods in 50 plants and grain weight of 10 plants were found to be significantly different after 3–4 WAP in weed free conditions at p < 0.05%. When the weed free period increased, the yield was increased until 3 WAP; thereafter, not significant yield increment was observed. In contrast, yield steeply declined in plots that had weeds until 3 WAP. According to the results of the present study, it can be concluded that the critical weed free period from the planting of green gram is 3 WAP. Maintaining a weed free period for 3–4 weeks is recommended to minimize the yield loss of green gram at minimal weed management cost.