In the period 2010–2013 in the region of Serres, Greece a field experiment was carried out with the cotton variety Carmen, including 8 variants weeded with the tuberous sour grass Cyperus rotundus L. 0, 1, 2, 4, 8, 16, 32, 64 pcs./m2 respectively. The experiment was set by the block method in four replications with the size of the trial plot – 1 m2. It was found that when weeding cotton with Cyperus rotundus L. (16 or more pcs./m2) a decrease of the cotton plants was observed in the crop, which, when particularly strongly weeded (64 pcs./m2) could reach 50%. Dispersive analysis of the data showed that only with weed densities of 32 and 64 pcs./m2 the differences in the number of cotton plants were lower. The yield of cotton decreased significantly at a density of Cyperus rotundus L. of 16 pcs./m2. This value can be regarded as the economic threshold of harmfulness of weeds. At higher levels of weed infestation the yield was reduced by more than 50%. The highest yield in the experiment was reported with a single weed per m2, аnd provenly lower yield was obtained with a weed density of 32 to 64 pcs./m2.