Definition
Rain water gardens are shallow, vegetated basins that capture, slow, and filter stormwater runoff from impervious surfaces, allowing water to infiltrate through planted soils.
What this strategy does
Manages stormwater at source while supporting vegetation, soil processes, and limited aquatic habitat. Avoids hard-engineered, single-function drainage solutions.
Context
In urban Aotearoa New Zealand, rain water gardens support compliance with water-sensitive design objectives, reduce downstream flooding risk, and improve receiving water quality where impervious cover is high.
Technical considerations
Design considerations
Plant selection and layout
Use diverse, predominantly native plant assemblages arranged along moisture gradients (ponding, saturated, free-draining zones) to support survival under fluctuating hydrological conditions.
Species resilience
Select species tolerant of periodic inundation and drought to maintain function during variable rainfall patterns.
Planting structure
Prioritise polyculture planting over monocultures to improve ecological resilience and pollutant processing capacity.
Soil media performance
Design engineered soil media to balance infiltration, pollutant retention, and support for microbial activity critical to nutrient and contaminant breakdown.
Substrate depth and composition
Provide sufficient soil depth and organic content to support root development and stable microbial communities.
Surface treatments
Incorporate gravel or coarse mulch where appropriate to improve infiltration and support soil invertebrates.
Landscape integration
Locate rain water gardens as part of connected green–blue networks rather than isolated elements to maximise biodiversity and system performance.
Implementation considerations
Design priority
Integrate rain water gardens early with site drainage and landscape design to ensure correct sizing, placement, and overflow management.
Key constraint
Performance is highly site-specific; inappropriate soil design or placement can compromise infiltration, vegetation health, and pollutant removal.
Issues and barriers
Regulatory classification
Rain water gardens are often treated as civil infrastructure, limiting ecological design flexibility and biodiversity outcomes.
Connectivity limitations
Urban planning frameworks frequently fail to integrate rain water gardens into wider habitat networks.
Pollutant exposure
Urban runoff may carry particulates, heavy metals, and emerging contaminants which can clog soils, impair plant growth, and disrupt soil microbial and invertebrate communities.
Long-term soil degradation
Accumulation of pollutants can reduce microbial diversity and compromise soil function over time.
Maintenance dependency
Lack of ongoing maintenance can lead to invasive species dominance, sediment clogging, and functional failure.
Synergies and opportunities
Climate change
Rain water gardens contribute to urban cooling, carbon storage, and adaptation to increased rainfall intensity and drought variability.
Human wellbeing
Well-designed rain water gardens enhance visual amenity and are associated with reduced stress and improved social outcomes.
Disaster risk reduction
By reducing runoff volume and delaying peak flows, rain water gardens lower flood risk in urban catchments.
Freshwater security
Filtering nutrients and metals from stormwater improves receiving water quality and supports groundwater recharge.
Financial case
Ecosystem services and performance value
Reduced stormwater costs
On-site runoff retention reduces demand on downstream drainage infrastructure and flood mitigation systems.
Avoided treatment costs
Improved stormwater quality reduces the need for downstream water treatment and remediation.
Cost-effectiveness
Long-term landscape value
Biodiverse, well-functioning systems can reduce replacement planting and maintenance costs over time.
Monitoring and evaluation metrics
Core metric
Runoff retention, infiltration rate, and overflow frequency can be measured through rainfall, water level, and soil moisture monitoring.
Water quality metric
Changes in suspended solids, nutrients, and heavy metals between inflow and outflow.
Ecological metric
Plant survival, species richness, and invertebrate diversity can be assessed using standard ecological indices.
Soil health metric
Soil organic carbon and microbial community structure can be tracked over time.
Case study
Wellington Rain Gardens
Related design strategies
Additional resources or tools
Bioretention Planting Guide (Auckland Transport)
https://at.govt.nz/media/uy5jmrfk/auckland-transport-bioretention-planting-guide-version-1.pdf
Manaaki Whenua – Bioretention Devices
Landcare Research
Wellington Water – Stormwater Treatment Device Guideline
Wellington Water