What a rain garden actually does

A rain garden is a shallow planted depression that catches the rain running off your roof, path or drive, holds it briefly, and lets it soak away slowly into the ground. It is not a pond, and in most gardens it should be dry within a day or two. Think of it as a sponge with roots in it: the soil and gravel layers do the drainage, while the plants slow the water, take up nutrients and keep the surface open.

The benefits add up quietly. You reduce the volume hitting the drains during a downpour, which matters more each year as heavy summer storms become more common. You keep water on site for the plants rather than sending it down the road. And you gain a planting area that thrives on the damp, awkward corner most gardens have somewhere.

Siting it and getting the size right

Position is everything. A rain garden needs to be where water naturally wants to arrive, not where you would prefer it aesthetically. Look at your downpipes, any gully outside a back door, and the low corner where puddles already form after rain.

• Keep it clear of buildings. Aim for at least 3 metres from house walls and foundations, and further on clay.
• Check what lies beneath. Avoid drains, soakaways, septic tanks, cables and services. A quick look at your deeds and a cautious dig will tell you a lot.
• Choose a sunny or partly shaded spot. Most wet-loving perennials flower best with some sun, though sedges and ferns will cope with more shade.
• Do not site it where the ground is already waterlogged year-round or where the water table sits high. Rain gardens are for occasional, moving water.

For sizing, a rough guide is to make the rain garden about 10 to 20 per cent of the roof area draining into it, and to make it deeper rather than wider if space is tight. A typical domestic size is 2 by 3 metres, dug 20 to 30 centimetres below the surrounding ground level, with a gentle ramp or short swale leading the downpipe water in.

Digging and building the layers

Mark the shape with a hose, then lift the turf and set it aside. Dig out to your chosen depth, keeping the sides gently sloping rather than vertical, and use the spoil to build a low berm along the downhill edge if the ground falls away. That berm is what holds a shallow pool after heavy rain.

If your soil is heavy clay, break up the base with a fork to about 30 centimetres and work in coarse grit or sharp sand. Then refill with a planting mix: roughly 50 to 60 per cent coarse sand or grit, 20 to 25 per cent topsoil, and 20 per cent well-rotted compost. This blend drains freely but still holds enough moisture and nutrients for plants to establish. Top it off so the finished surface sits about 15 to 20 centimetres below the surrounding ground, giving you space for the water to gather.

Mulch the whole area with 5 centimetres of gravel or coarse compost. Avoid bark chip, which floats during a downpour and ends up blocking your inlet.

Planting the three zones

Treat your rain garden as three bands: the wet centre that floods most often, the damp margin, and the drier rim. Match the plant to the band and you will have very little failure.

The wet centre wants British natives and tough characters that shrug off being submerged for a day:

• Pendulous sedge (Carex pendula) and soft rush (Juncus effusus) for structure
• Marsh marigold (Caltha palustris) for early gold
• Purple loosestrife (Lythrum salicaria) and meadowsweet (Filipendula ulmaria) for height and late nectar
• Royal fern (Osmunda regalis) where there is some shade

The damp margin takes the plants that like moisture but will not sit in standing water:

• Tufted hair grass (Deschampsia cespitosa) and purple moor grass (Molinia caerulea)
• Siberian iris (Iris sibirica) and ragged robin (Lychnis flos-cuculi)
• Bistort (Persicaria amplexicaulis), masterwort (Astrantia) and water avens (Geum rivale)

The drier rim softens the edge and hides the berm. Use blue fescue or Sesleria, hardy geraniums, catmint, yarrow and a few tall verbenas for late summer. Alder buckthorn and dogwood work well if you want a shrub at the back.

One caution: yellow flag iris (Iris pseudacorus) is a fine British native but vigorous, and it seeds freely into watercourses. Keep it in a sunken pot if you use it at all.

Keeping it working through the year

Maintenance is lighter than you might expect. Check the inlet and any swale after autumn leaf fall and clear debris so water can still get in. In late winter, cut stems back to 10 to 15 centimetres and rake out the cuttings so they do not smother the crown or clog the surface — this is also when you divide crowded clumps.

Top up the mulch every couple of years, and watch the ponding time after a heavy storm. Water should vanish within 24 to 48 hours. If it lingers longer, the base has compacted or silted up; fork it over and add grit. That single check tells you more about the health of your rain garden than anything else.

Common problems and how to solve them

Most complaints come down to three things. If water never reaches the plants, the inlet is too high or too flat — regrade the swale so it falls gently towards the garden. If plants drown, the soil mix is too fine or the base was not broken up; lift the plants into fresh gritty mix. And if the garden looks tired by August, you have probably planted too many wet-zone species on the rim. Move them inwards, add dry-tolerant grasses at the edges, and the whole thing will settle into itself.

Get it right and a rain garden earns its place: an attractive border, a wildlife pause, and a quiet answer to the water arriving off your roof.