Weeds are unwanted plants in crop that can be controlled by different methods among them use of aqueous extracts of crops is an imperative method. In this method allelochemicals present in crop plant help in suppressing or inhibiting weeds. Therefore, this study was planned to investigate the allopathic potential of aqueous extract of winter crop Papaver somniferum (opium) on summer weed Echinochloa cruss-galli (barnyard grass. The experiment comprised of seven concentrations (0, 0.25, 0.50, 1, 2, 4, and 8%) of different plant parts i.e., leaves, stem and flower of P. somniferumwere. All the tested concentrations and plant parts of P. somniferum significantly reduced mean emergence time, germination index, germination percentage, time to 50% germination as well as well growth of E. cruss-galli weed. However, maximum mean emergence time (9.07 days), time to 50% germination (3.67 days) was noted by applying leaves and stem extract, respectively. Application of aqueous extract of stem at 8% concentration resulted in greatest time to complete 50% germination (5.42 days) and lowest fresh weight (6.28 g), dry weight (1.00 g) and root length (0.33 cm) of E. cruss-galli. Whereas leaf extract at 8% concentration produces less shoot length (1.13 cm) and more emergence time (9.18 days). Lowest emergence percentage (6.67%) and germination index (0.89) were produced by aqueous extract of fruit at 8% concentration. On the basis of this experiment, it can be concluded that higher concentration (8%) of stem extract of P. somniferum was used to biologically control the infestation of E. cruss-galli weed.