Waterlogging tolerance of Rhododendron × pulchrum ‘Oomurasaki’ as a candidate shrub for roadside stormwater management in Japan

Source abstract

Rain gardens are increasingly recognized as effective nature-based infrastructure for urban stormwater management, but their application in Japan remains limited. In Japanese cities, roadside planting strips are widespread and are commonly planted with ornamental shrubs, suggesting potential opportunities for introducing stormwater management functions through appropriate structural redesign. Plant performance is a key criterion in rain garden design, as species must tolerate temporary ponding and dry cycles while maintaining ornamental and functional value. However, the flooding tolerance of commonly used ornamental shrubs has received little attention. This this study investigated the morphological and physiological responses of R. × pulchrum ‘Oomurasaki’ four flooding regimes (control, 1-day, 3-day, and 5-day flooding) under greenhouse conditions. R. × pulchrum ‘Oomurasaki’ is one of the most commonly used roadside shrubs in Japan, because of its evergreen foliage, conspicuous flowers, tolerance of pruning, and suitability for managed urban landscapes. Plants subjected to 5-day flooding died, indicating that this duration exceeded the tolerance of plant species. Three-day flooding significantly reduced biomass, canopy spread, visual quality and stomatal conductance, whereas 1-day flooding caused only minor effects. These findings suggest that R. × pulchrum ‘Oomurasaki’ may be considered as a candidate shrub for roadside stormwater management where prolonged ponding is avoided and drainage occurs within approximately one day.