Why a rain garden earns its place

British weather rarely arrives politely. We get a fortnight of rain in a weekend, then three weeks of nothing, and our gardens are expected to cope with both. A rain garden is simply a shallow, planted hollow that catches water running off a roof, drive or patio and lets it seep slowly into the ground rather than racing into the drains. It is one of the few garden features that does two jobs at once: it looks lovely, and it works.

That work matters. Every hard surface we add — a new driveway, a paved seating area, a shed roof — sends water somewhere, and in heavy clay districts it usually ends up in a neighbour's garden or over a road. A well-placed rain garden slows that flow, filters it through soil and roots, and tops up the groundwater instead. In older built-up areas it is one of the quietest acts of flood prevention a householder can make.

Siting and sizing the hollow

Put your rain garden where water naturally wants to go: at the end of a downpipe, below a sloped path, or in the low corner of a lawn. Keep it at least three metres from house foundations and away from the base of boundary walls, and check where services run before you dig. Avoid the spot where the ground already sits waterlogged for weeks in winter — that is a sign of poor drainage rather than an opportunity.

As a rough rule, allow around 20 per cent of the roof or paving area that drains to it, and rather less if your soil is sandy and free-draining. A front garden rain garden of two by three metres will comfortably take the runoff from a typical semi-detached roof through one downpipe. Deeper is not better; shallow and wide soaks away more efficiently.

Building it: layers, levels and overflow

Dig out 150–200mm of soil across the whole shape, with gently sloping sides so the water spreads rather than pools in one spot. Break up the base with a fork if the subsoil is compacted, then refill with a mix of roughly two parts garden soil to one part grit or coarse sand, with a little well-rotted compost. This blend drains freely while still holding enough moisture for plants.

The critical detail is the overflow. Shape the rim so that once the water reaches about 100mm deep, it spills out through a shallow channel towards a border, a soakaway or a drain. That way an exceptional storm passes through rather than turning your planting into a swamp. If you can, intercept the downpipe with a diverter feeding a water butt, and let the butt's overflow run into the rain garden — you get stored water in summer and controlled delivery in winter.

Plants that shrug off a soaking

The centre of the rain garden floods briefly; the edges only get damp. Plant accordingly, with the thirstiest subjects in the middle.

• Bog perennials: rodgersia, ligularia, astilbe, meadow rue and Filipendula ulmaria (meadowsweet).
• British natives: Caltha palustris (marsh marigold), Lychnis flos-cuculi (ragged robin) on the drier margins, and Iris pseudacorus if you have room for its vigour.
• Grasses and sedges: Deschampsia cespitosa, Molinia caerulea and Carex elata 'Aurea', which look good well into winter.
• Ferns: Osmunda regalis and Matteuccia struthiopteris for ferny structure in shade.
• Bulbs for spring: Camassia, Fritillaria meleagris and Leucolium, planted in autumn where they will not rot.
• Shrubs for structure: Cornus alba and its coloured-stemmed cousins, which tolerate wet roots and give winter colour once cut back in March.

Steer clear of thugs such as Houttuynia cordata and creeping Lysimachia, which will colonise the whole hollow and march into your borders.

Keeping it working through the year

Maintenance is light but not zero. In early spring, cut back last year's growth, tease out fallen leaves and check that the overflow channel is clear — a blocked outlet is the main reason rain gardens fail. Top up the mulch around plants but keep a bare, gravelly strip at the inlet so silt drops out before it reaches the planting. After two or three seasons, fork over the surface if it has crusted, and divide crowded perennials, replanting the strongest pieces around the rim. Water new plantings through their first dry summer; after that they should fend for themselves.

Wildlife that finds its way in

Because the water drains within a day or so, you will not be breeding mosquitoes. You will, however, be offering something rarer: reliable damp ground. Expect common frogs and smooth newts sheltering under stones at the margins, damselflies and dragonflies hunting over the foliage in high summer, and hoverflies and bees working the marsh marigold in early spring when little else is open. Damp soil is a rich larder for blackbirds and thrushes, while the seed heads of sedges and grasses feed finches into the winter months.

Plant thickly, leave a few stones and a small log at the edge, and let the rain garden be slightly untidy. It will repay you with colour, with birdsong, and with the quiet satisfaction of knowing that the next downpour is soaking in rather than running off down the street.