Urbanisation and climate change have intensified the challenges of stormwater management, with increasing risks of flooding, drought, and water quality degradation. Rain gardens are widely promoted as nature-based solutions, providing temporary storage, infiltration, and pollutant removal while offering ecological and aesthetic benefits. However, their long-term effectiveness depends critically on plant performance under fluctuating hydrological regimes. Existing design guidelines often rely on anecdotal species lists, with limited mechanistic understanding of how plant traits govern survival, transpiration, and infiltration.
This thesis addresses this gap through three controlled experiments that systematically evaluate the performance of herbaceous species in rain garden contexts. The first experiment examined species survival and morphological adaptation under prolonged drought and waterlogging, identifying tolerance thresholds and revealing that ecological background alone does not reliably predict persistence. The second experiment investigated species-level variation in transpiration, testing responses to waterlogging, canopy reduction, and temperature variation. Results demonstrated that functional contributions to stormwater regulation are strongly mediated by canopy size and water-use strategies, with trade-offs between persistence and evapotranspiration capacity. The third experiment focused on root–substrate interactions, showing that fibrous-rooted species enhanced lateral infiltration and soil stabilisation, taprooted species promoted vertical drainage, and rhizomatous species provided storage and resilience under fluctuating conditions. Substrate amendments, such as compost–grit mixes, were found to modulate trait expression and performance.
By integrating findings across survival, transpiration, and infiltration, this thesis develops a trait-based framework for rain garden design that assigns plants to functional roles within micro-topographic zones: hygrophytic fibrous-rooted species for wet depressions, fibrous stabilisers for slopes, and drought-tolerant rhizomatous taxa for dry edges. International case studies further validate this framework, highlighting its applicability across climates and design scales.
Overall, the research demonstrates that rain garden performance cannot be fully understood through species identity alone but must be conceptualised through trait–function linkages across above- and below-ground dimensions. The trait-based framework proposed here advances predictive, evidence-driven guidance for plant selection, offering both theoretical insight and practical strategies for enhancing the resilience and multifunctionality of rain gardens in urban environments.
