Hydrological efficiency of established rain gardens and plant adaptability to their soil water conditions

In the last few decades, rain gardens (RGs) and other bioretention systems were proposed to restore the natural water cycle improving infiltration of rainwater runoff from paved surfaces. In spring 2011, at the Agripolis Campus of the University of Padova, a system of two RGs was designed to receive and manage the stormwater runoff from a building roof. The two rain gardens have an area of 10.5 and 21 m2, corresponding respectively to 10 and 20% of the roof drainage area. In spring 2018 the plants were changed, and RGs were both vegetated with five herbaceous perennial ornamental species: Echinacea purpurea ‘Magnus’, Iris spuria, Panicum virgatum ‘Cardinal’, Rudbeckia fulgida ‘Goldsturm’, Salvia ×sylvestris ‘Mainacht’. Plants were transplanted in different positions to assess their suitability to the different soil water conditions. The efficiency of the two rain gardens to manage stormwater runoff and the aesthetical and the growth response were evaluated. The results showed that both RGs were able to manage 99.9 and 98.0% of the stormwater runoff (large and small RG, respectively), almost all of which by infiltration and a small part (2.1 and 3.9%, respectively) by evapotranspiration. As for the vegetation, all the species showed good aesthetic values over a long period of time. The growth of the plants was influenced by their position in the rain gardens (i.e. different soil water content). In particular, E. purpurea, I. spuria, R. fulgida, and P. virgatum, showed better performances in the inner wetter position, while S. ×sylvestris in the outer, drier locations of the rain garden