Red/far-red (R/FR) ratio, a signal of impending competition from neighbouring plants, fluctuates in plant canopies because of selective absorption of red light and transmission and reflection of far-red light by leaves. The differential response of a crop and associated weeds to changes in R/FR ratio could therefore influence competitive interaction among them. In a growth chamber study, growth, morphological, and allometric responses of common lamb’s-quarters (Chenopodium album L.), redroot pigweed (Amaranthus retroflexus L.), and tomato (Solanum lycopersicum L.) seedlings to changes in R/FR ratio were investigated. Low R/FR ratio increased plant height, stem length below the first node, stem diameter, leaf length, and stem/shoot weight ratio, and decreased leaf greenness, leaf/shoot weight, and leaf/stem weight ratios in some or all of these species; the species differed significantly in this regard. Total length and surface area of tomato root were greater under low R/FR ratio. R/FR ratio did not affect root parameters of lamb’s-quarters and pigweed. The observed differential response to R/FR treatment suggests that fluctuations in R/FR ratio in a canopy of mixed populations could impact interactions among these species.