Plants in raingardens are important for evapotranspiration and maintaining infiltration properties. Hydrological conditions, however, fluctuate between dry and saturated, strongly affecting plant performance and limiting plant selection in design. This study experimentally assessed whether fluctuating hydrology impacts growth, flowering, and root morphology in the drought-tolerant Knautia arvensis and the wet-tolerant Lythrum salicaria. The hydrological regimes included repeated cycles of dry and wet conditions: Drought regime with cycles of a drought period followed by a single watering; Flooding regime with cycles of 72-hour flooding and a 48-hour drained period; and a subsequent Drought+Flooding regime with cycles of 72-hour flooding followed by a drought period. 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. This research contributes to the ecological understanding of plant responses to fluctuating hydrology, enabling a better plant selection for effective raingardens.
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