Weeds pose the greatest danger to vegetables and melons in the second half of May, when their number reaches a maximum. The purpose of the research is to assess the degree of harmfulness of weed plants in crops of vegetables and melons in irrigated plots in the Lower Volga region. The studies were conducted in accordance with the "Guidelines for the study of economic thresholds and critical periods of harmfulness of weeds in agricultural crops". Assessment of harmfulness of weed plants was performed by the method of the model plots, where was created a weed infestation of soil of varying degrees by one type of weed by removing excess plants as they appear within the whole period of vegetation. As the result of the research were studied the dynamics of germination, intraspecific and interspecific competitive abilities. Were identified critical periods of weed infestation for non-seedlings of cucumber and tomato – 20 days after germination, seedlings of watermelon and tomato – 50 days from germination of watermelon or transplanting. Were determined the economic thresholds of harmfulness in the planting of tomato: barnyard grass – 4±2; amaranth upturned – 3±1; nightshade black – 5±2,5 PCs./m². In watermelon crops: barnyard grass – 3,5 pieces/m²; amaranth upturned - 3.8 pieces/m²; black nightshade – 4,2 PCs./m².