Decide whether a rain garden fits your site

Choose the right kind of water space

Consider a rain garden when a defined source of runoff can be managed on suitable ground with a lawful, safe overflow route. Consider a pond when the objective is persistent open water and the site can support that safely. Investigate drainage or groundwater professionally when the ground remains waterlogged or flooding threatens buildings.

The RHS describes rain gardens as temporary storage and notes that their ground must be sufficiently draining. A permanently wet hollow is not automatically a suitable location. Nor is a flower bed labelled “rain garden” proof that a property is protected from flooding.

Make a water-route sketch before a planting plan

Draw the contributing roof or hard surface, downpipes, existing drains, observed low points, buildings, tree-root areas and known utilities. Mark where excess water goes now and where it would go if the proposed basin were full. Observe actual rainfall events from a safe position and record duration of standing water.

Do not disconnect a downpipe or excavate a test pit until ownership, services, contamination and the appropriate drainage permissions have been resolved. Obtain site-specific advice for nearby foundations, unstable slopes, high groundwater, poor infiltration or uncertain overflow. The RHS specifically calls for geotechnical advice when proposing infiltration close to a house; its advice is UK-context guidance, not a universal setback certification.

Understand volume without mistaking it for a design

An illustrative 30 m² contributing surface receiving 10 mm of rain has a gross rainfall volume of 300 litres: 30 × 0.01 = 0.3 m³. This is a geometric upper-bound input before losses, not a measured runoff volume. A 3 × 2 m rectangle with 5 cm of empty storage also represents 300 litres before accounting for slopes, soil, vegetation and infiltration.

Those equal numbers do not make a functioning rain-garden specification. A storm’s intensity, earlier saturation, soil infiltration, storage shape, safety and overflow determine performance. Do not build the example from this arithmetic. It demonstrates why a labelled bed area alone cannot prove capacity.

The RHS offers a roof-area rule for typical UK summer rainfall. Applying that proportion everywhere would conceal the very climate and drainage assumptions the design needs. Ask the local drainage adviser which design storm and infiltration evidence are appropriate.

Plan planting by wetness zones

After feasibility is established, divide the planting into the lowest temporary-water zone, the intermediate slope and the drier rim. Select exact species for both the wet phase and the dry intervals, then check local native range, invasive status and mature spread. A plant that tolerates moist soil may not tolerate inundation; a pond plant may not tolerate the dry interval.

Do not copy an overseas rain-garden list wholesale. Several plants sold for wet places can be invasive outside their home region. The moist clay border is a useful comparison of mature spacing, but it is not certified for the base of a rain basin. Its plants require a separate inundation check before transfer.

Budget for establishment and inspection

Record the agreed inlet, outlet, overflow and inspection access in the site brief. Plant when local conditions permit establishment without damaging wet soil. Temporary wetness after a storm does not remove the need to check new plants during dry weather.

After rain, inspect for unexpected ponding, erosion, blocked inlets and overflow towards buildings or neighbours. Stop and obtain appropriate advice if water behaves differently from the assessed design. Keep maintenance access open as plants mature. If feasibility is not demonstrated, retain the existing drainage and choose a conventional condition-matched border instead.