Use of growth retardants enables post-planting optimization of vegetative growth, which is particularly important with the on-going climate change. Mepiquat chloride is an economical and safe retardant widely applied in cotton, but is not uniformly effective. Here identification of its molecular target as the ent-copalyl diphosphate synthase that initiates gibberellin biosynthesis enabled introduction of selective agrochemical inhibition, leaving intact more specialized metabolism important for resistance to biotic and abiotic stresses.