Herbicide drift is the movement of herbicide away from its intended target. The effect of drift on non-target plants is considered in environmental risk assessments, where the goal is to protect plant populations and communities. The aim of this study was to evaluate the assumption that the single species tests used in risk assessments are sufficient for protecting plants, as these do not account for interspecific interactions. In a greenhouse two-species competition experiment, it was found that the competitive effects of the two model species, Centaurea cyanus and Silene noctiflora, were affected by low doses of glyphosate representing drift. As interspecific competition is an important determinant of plant community structure, competitive interactions may need to be included in risk assessment to make more credible predictions on the effects of herbicide drift on non-target plants.