This dataset represents a greenhouse experiment investigating the effects of fluctuating soil hydrology on plant growth and flowering, including root diameter and root length in raingardens. Plants in raingardens are important for evapotranspiration and maintaining infiltration properties. However, hydrological conditions, including cycles of dry and saturated conditions, influence plant performance and therefore affect plant selection. This study includes an experiment to assess whether fluctuating hydrology impacts growth, flowering, and root morphology in two species: the drought-tolerant Knautia arvensis and the wet-tolerant Lythrum salicaria. The results show that Knautia reduced growth under flooding cycles, while Lythrum reduced growth and flowering under drought cycles. Repeated consecutive drought and flooding cycles negatively influenced the growth of both species in the same way, with flooding affecting Knautia and drought affecting Lythrum. Lythrum showed more plasticity than Knautia, allocating more biomass to roots during drought cycles. Flooding cycles reduced root length in Knautia, while the drought cycles and consecutive Drought+Flooding cycles increased root diameter and decreased root length in Lythrum. Both species had coarse roots (> 2 mm) to support infiltration through biopores. If we assume that raingardens have free drainage and only rarely flood, drought-tolerant traits are more important. However, wet-tolerant species can benefit the system, especially through higher water use.