River corridors are hotspots of biodiversity and harbour plant species which are predominantly distributed in this dynamic ecosystem (river corridor plants). During the last centuries, floodplain meadows became increasingly threatened due to dike constructions and land use changes. An additional threat from future climate change might be posed by the increased probability of drought during summer due to reduced precipitation and water levels in the rivers. Our aim was to test how the characteristic floodplain meadow plant Cnidium dubium responds to water level changes, and to develop implications for its management in the course of climate change. Two mesocosm experiments were set up with plants originating from floodplain meadows along two tributaries of the Elbe River, Germany. In the first experiment, we investigated growth characteristics and biomass of juvenile C. dubium individuals in response to groundwater level (20, 40, and 60 cm below soil surface) and origin. In the second experiment, competitors were included to analyse interactive effects of competition and the water level on mature plants of C. dubium. The growth of C. dubium was affected by the water level, with a physiological optimum at water levels of 40–60 cm below soil surface. C. dubium showed adaptations to the sites of seed origin and evidence for phenotypic plasticity in relation to competition, which suggests that this species might possess adaptation potential. Nevertheless, we propose raising the groundwater level to 20–60 cm below soil surface as a possible management strategy, when drought (as a consequence of climate change) is perceivable.