A rain garden is a planted depression designed to collect and temporarily hold rainwater running off impervious surfaces — roofs, driveways, paths, and compacted ground. Water infiltrates into the soil over a period of hours rather than immediately entering the drainage network. The approach reduces peak runoff volumes, filters some surface pollutants through the soil and root zone, and supports urban biodiversity where conventional lawns would otherwise dominate.
Site assessment before design begins
The first task is establishing whether a given site can support a rain garden at all. The key variables are:
- Underlying soil permeability. Sandy loam and loam soils typical of much of the Mazovian Lowland and parts of the Silesian Upland drain adequately. Heavy clay soils common in parts of the Łódź region and Lower Silesia require either deeper media replacement or an underdrain to prevent waterlogging.
- Depth to groundwater. The base of the rain garden should sit at least 600 mm above the seasonally high groundwater table to prevent the planted media from becoming permanently saturated.
- Distance from buildings. A minimum of 3 metres from a building foundation is generally observed to limit subsidence risk. Polish guidelines from COBRTI Instal recommend a minimum of 5 metres where foundation walls have no waterproofing.
- Slope. A slope exceeding 12 percent makes construction of a level-bottomed basin difficult and increases the risk of scour at the inlet.
If the above conditions cannot be met, a raised planter bed with an underdrain connected to an existing soakaway or surface water drain may be a workable alternative that achieves similar hydrological results.
Sizing the garden to the catchment
The most widely referenced sizing method for small residential rain gardens uses the relationship between the contributing impervious area, a target draw-down time, and the infiltration rate of the amended media. A simplified version:
For a 50 m² roof draining to a garden in sandy loam, with a 25 mm design storm and a 24-hour draw-down target, a rain garden of approximately 5–8 m² is typically sufficient.
Poland's design rainfall data is published by the Institute of Meteorology and Water Management (IMGW-PIB) in the form of intensity-duration-frequency (IDF) curves for individual meteorological stations. For urban drainage design, the c = 2-year return period storm is commonly used as a starting point, though municipalities with known flooding histories sometimes specify c = 5 or c = 10.
Media layers
Standard rain garden cross-sections consist of three layers:
| Layer | Depth | Composition |
|---|---|---|
| Mulch surface | 50–75 mm | Coarse wood chip or bark mulch; retains moisture, suppresses weeds |
| Planting medium | 400–600 mm | Sandy loam (60%), compost (30%), sharp sand (10%); adjusted for local soil |
| Transition/drainage layer | 150–300 mm | Coarse gravel or crushed stone; only included where underdrain is used |
Where native soil permeability is adequate — measured at field capacity by a simple percolation test — the native subsoil can be loosened rather than replaced, reducing material costs considerably.
Plant selection for Polish conditions
Plants in a rain garden must tolerate two opposite conditions: temporary flooding (up to 24–48 hours) and dry periods between storms. Native Polish species that meet both criteria include:
- Iris sibirica (Siberian iris) — tolerant of wet feet and periodic drought; effective at the lower zone
- Carex acutiformis (lesser pond sedge) — suited to the deepest central zone; suppresses weeds
- Filipendula ulmaria (meadowsweet) — for the mid-zone; flowers attract pollinators
- Molinia caerulea (purple moor grass) — structural grass for the upper transition zone
- Cornus sanguinea (common dogwood) — shrub suitable for larger gardens at the outer edge
Non-native plants are not excluded, but species known to be invasive in Polish conditions — particularly Reynoutria japonica (Japanese knotweed) and Fallopia sachalinensis — must not be used.
Inlet and overflow design
The inlet — where water enters the garden — is the point most likely to suffer erosion. A rock apron or dense stone pad at least 300 mm long, placed at the entry point, dissipates flow energy. For larger catchments, a shallow concrete check dam or curbstone inlet with a 50 mm depression can stabilise the flow path.
An overflow outlet is required. It should be set at the rim elevation of the basin and routed to a downslope drainage point — an existing surface water drain, a perforated soakaway, or a permeable surface. Without an overflow, a blocked or saturated garden can direct water toward a building foundation.
Maintenance expectations
Established rain gardens are low maintenance compared to conventional irrigated beds, but they are not maintenance-free. In the first two growing seasons, hand weeding is necessary to prevent aggressive species from outcompeting slower-establishing natives. Mulch should be topped up annually. The inlet stone apron should be inspected after significant storm events and reset if displaced. Sediment buildup in the deepest zone may need removal after several years if infiltration rates noticeably decrease.
Further technical guidance on green infrastructure design in urban Poland is available from the European Environment Agency's stormwater resource pages.