Crop canopy characteristics and management factors that affect the canopy structure can influence crop competitive ability against weeds. The goal of the current study was to understand the interaction between nitrogen (N) fertilizer rate and redroot pigweed (Amaranthus retroflexus) density on common bean (Phaseolus vulgaris L.) canopy characteristics and N assimilation. Experiments were conducted to determine the growth responses of two bean genotypes with contrasting growth habits to different N fertilizer rates under no, low and high redroot pigweed pressure. Nitrogen was applied at rates of 0, 50, 100 and 200 kg/ha for a semi-erect variety and 0, 35, 70 and 140 kg/ha for an erect bean variety. Leaf area index (LAI), leaf area duration (LAD), leaf chlorophyll, N assimilation and N assimilation rate for both bean growth habits increased with N application rate at no and low weed density. The negative effect of redroot pigweed competition on LAI, LAD, N assimilation and N assimilation rate increased with redroot pigweed density, and with N application rate for the high weed density. Stepwise multiple regression analysis showed that N assimilation and chlorophyll concentration accounted for 0·73–0·87, overall, of the variation in yield loss due to weed competition. It is concluded that greater N assimilation in the semi-erect growth habit compared with the erect growth habit is the cause of its higher competitive ability against the weed, as it improves the effective canopy traits that confer potential for resource pre-emption.