Biodiversity–ecosystem function (BDEF) experiments commonly group species into arbitrary a priori functional groups, e.g. the grass/forb/legume (GFL) classification. As a result, the causes of functional group diversity effects are often poorly understood. This paper presents a new process that uses functional trait data to create customized plant functional groups that can be tailored to address specific questions. This method is illustrated throughout with an example taken from a temperate mesotrophic grassland in southern England.