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Hydrological alteration due to climate change events such as floods and drought is a significant threat to globally important wetlands, including floodplain wet grasslands. This research incorporated two field experiments with the aim to assess immediate and longer-term functional responses of floodplain plants to hydrological change. Plant introductions and transplants between a wetter riparian and a drier site in southern England were used to simulate hydrological change. Species showed immediate and differential responses to contrasting hydrologies. Rhinanthus minor, a hemi-parasitic annual species with ruderal traits, was lost from the riparian grassland within four weeks. The survival and production of a leguminous perennial, Lathyrus pratensis, in high groundwater levels soon decreased. However, the perennial Primula veris mostly functioned well in contrasting hydrological regimes, possibly because it can tolerate stress. The perennial wetland species Caltha palustris showed lag effects, over three years, when its hydrology was altered to a sub-optimal drier scenario by transplantation, with declining survival and a sustained reduction in leaf production and flowering. Disturbance caused by transplantation and weather conditions also affected its performance. Thus, this study shows that some functionally important floodplain species may succumb within weeks to a hydrological event facilitated by climate change, unless they are able to tolerate the challenging conditions, while the performance of other characteristic species could decline and continue to show constrained performance for years as a consequence of altered hydrology.