The way species avoid each other in a community by using resources differently across space and time is one of the main drivers of species coexistence in nature(1,2). This mechanism, known as niche differentiation, has been widely examined theoretically but still lacks thorough experimental validation in plants. To shape niche differences over time, species within communities can reduce the overlap between their niches or find unexploited environmental space(3). Selection and phenotypic plasticity have been advanced as two candidate processes driving niche differentiation(4,5), but their respective role remains to be quantified(6). Here, we tracked changes in plant height, as a candidate trait for light capture(7), in 5-year multispecies sown grasslands. We found increasing among-species height differences over time. Phenotypic plasticity promotes this change, which explains the rapid setting of differentiation in our system. Through the inspection of changes in genetic structure, we also highlighted the contribution of selection. Altogether, we experimentally demonstrated the occurrence of species niche differentiation within artificial grassland communities over a short time scale through the joined action of both plasticity and selection. Niche differentiation has been theorized to occur in plants, but experimental validation of this mechanism of species coexistence is lacking. This study finds selection and phenotypic plasticity to be the main drivers of differentiation.