Impact of fluctuating hydrology during summer and autumn on winter frost hardiness and salt tolerance of selected Raingarden species

Vegetated infiltration systems such as raingardens and bioswales are challenging for plant growth and survival due to fluctuating hydrological conditions and further subsequent stresses. Here, we investigated the effect of fluctuation hydrology on growth and flowering and subsequent winter frost hardiness or spring salt tolerance for two common raingarden plant species, Filipendula ulmaria, and Calamagrostis ×acutiflora ‘Karl Foerster’, under controlled conditions. During summer, plants were exposed to four hydrological regimes, each with a different combination of repeating dry and wet cycles. Then, after natural winter acclimation and storage, plants went through standardized freezing tests to determine LT 50 and regrowth potential or were exposed to four levels of salt treatments (Control, 28 mM, 56 mM, and 84 mM NaCl) in the following spring. We found that fluctuating hydrology reduced the growth of Filipendula ulmaria, experiencing cycles of 72 hours of flooding and 264 hours of drained conditions, followed by a reduction of growth and flowering after salt exposure. Calamagrostis xacutiflora was less responsive to both fluctuating hydrology and salinity. Cycles with the longest dry conditions (Wet-dry cycles) showed the strongest negative effect on the performance of tested species. The hydrological regimes did not influence freezing tolerance in either species. Moderate hydrological fluctuations did not cause damage to vegetation in vegetated infiltration systems, at least under shaded conditions. At the same time, drought tolerance is an important trait for species and cultivars in raingardens during hydrological fluctuations. Our prediction that hydrological conditions that negatively affected plant growth would reduce subsequent frost and salinity tolerance was only partially supported. • Marginal effects of different hydrological regimes on plant performance during summer. • No effect of hydrological regimes on frost tolerance and only partial effects on spring salt tolerance for the two plant species. • Fluctuating hydrology reduced the growth of Filipendula ulmaria (72-h flooding and 264-h drought), followed by a reduction of growth and flowering after salt exposure. • Calamagrostis xacutiflora was less responsive to both fluctuating hydrology and salinity. • Drought tolerance is an important trait for species and cultivars in raingardens.