A rain garden is a shallow, planted dip in the ground that catches rainwater from a roof, a drive or a path, and lets it soak slowly into the soil instead of running off into drains. It is a simple idea, and it solves several problems at once: it stops puddles and flooding, it waters plants for free, it filters rainwater as it soaks through soil and roots, and it creates a beautiful planted feature that attracts birds, bees and butterflies.
Rain gardens are becoming popular in towns and cities, where paved surfaces send rainwater straight into drains that can overflow in heavy rain. A rain garden slows that water down, holds it for a few hours, and lets it soak away. It is not a pond — it should drain within a day or two — and it is not a bog garden, though some of the same plants suit both. If you are unsure whether a rain garden suits your soil and site, check your local extension service before you dig.
How a rain garden works
A rain garden works by holding rainwater in a shallow basin, usually 10–20 centimetres deep, and letting it soak into the soil over hours rather than minutes. The plants help: their roots keep the soil open so water can soak in, they take up water as they grow, and they filter pollutants as the water passes through. The soil helps too: a mix of topsoil, compost and grit drains well but holds moisture, and the mulch on top stops the surface from crusting. A rain garden is usually fed by a downpipe, a drive or a path, with the water directed into it by a shallow channel or a pipe. It should empty within 24–48 hours after rain — if it holds water longer, it is not draining, and it needs attention. A well-built rain garden needs little maintenance, looks good all year, and handles far more water than a lawn or a paved area of the same size. Our soil drainage improvement guide explains the soil side, and soaker hoses covers a related way of using water slowly.
Siting a rain garden
Siting is the most important decision. Put the rain garden where water already collects, or where you can direct water to it easily from a downpipe or a slope — but not right against the house, where water could affect foundations. Keep it at least 3 metres from the house, and further if your soil drains slowly. Choose a sunny or part-shaded spot, because most rain garden plants like sun, and avoid a spot under large trees, where roots will compete for water and make digging hard. Check the soil: dig a test hole about 30 centimetres deep, fill it with water, and see how fast it drains. If it drains within a day, the site will work. If it is still full after two days, the soil drains too slowly for a rain garden, and you may need a different solution — ask your local extension service. Avoid a site over drains, cables or a septic tank, and check with your local authority if you plan to disconnect a downpipe. A rain garden in the right place works with almost no effort; in the wrong place, it becomes a permanent puddle.
How to build a rain garden
Building a rain garden is straightforward, and a small one can be done in a weekend. Mark out the shape — a natural oval or kidney shape looks best — and dig out the basin to a depth of about 15–20 centimetres, piling the soil on the downhill edge to make a low ridge, or berm, that holds water in. Break up the bottom with a fork so it is not compacted, and mix in compost and grit to improve drainage and fertility. If your soil is heavy clay, add more grit and compost, and consider a deeper basin with a layer of gravel at the bottom. Shape the inlet so water from the downpipe or channel flows in gently, and add stones at the inlet to stop the soil washing away. Plant up, mulch, and water the plants in. Use the soil volume calculator to work out how much compost and grit you need. Do not line the basin with plastic — the whole point is that water soaks into the ground — and do not make it too deep, or it will hold water too long and become a mosquito problem.
Best plants for a rain garden
Rain garden plants must cope with two extremes: wet feet after rain, and dry soil between rains. The centre of the basin, which is wettest, suits plants that like moisture, such as asters, daylilies, irises, sedges and rushes. The edges, which are drier, suit plants that like ordinary soil, such as coneflowers, lavender, salvias, ornamental grasses and hardy geraniums. See growing asters and growing lavender. Native plants are often the best choice, because they are adapted to local wet and dry cycles — ask your local extension service for a list. Plant in groups, with the wettest plants in the middle and the driest at the edge, and mulch with bark or gravel to hold moisture and suppress weeds. Avoid plants that hate wet soil, such as rosemary and Mediterranean herbs, in the centre of the basin — they belong on the dry edge, or not at all. A rain garden planted well will flower for months and need little watering, because the rain does it for you.
Maintaining a rain garden
Maintenance is light but regular. Weed in the first year, while plants establish, and mulch each spring to suppress weeds and hold moisture. Check the inlet after heavy rain, and clear leaves and silt that block it. Check that the basin drains within a day or two after rain: if it does not, the soil may be compacted, and you may need to fork it, add compost, or improve the drainage. Cut back dead growth in spring, not autumn, so that stems shelter insects over winter. Divide plants that outgrow their space, and replace any that fail — a plant that dies in the centre is probably too dry-loving, and one that dies on the edge is probably too wet-loving, so swap them. Do not use fertiliser in a rain garden, because it will wash into the soil and then into drains; compost is enough. And do not let the rain garden become a dumping ground for leaves and grass clippings, which will block it and make it smell.
Rain gardens and the wider garden
A rain garden is not just a drainage feature — it is a garden in its own right, and it can be beautiful. Plant it with flowers that attract bees and butterflies, and it becomes a pollinator garden too — see our pollinator garden guide. Add a few stones, a log and a shallow dish of water, and birds will visit. Use it to water the garden for free, by directing roof water into it instead of into a drain. In a community or school garden, a rain garden is a wonderful teaching tool, showing children where rainwater goes and how plants use it — see our school gardening guide. If you have a large roof or drive, you may need more than one rain garden, or a rain garden plus a rain barrel — see our rain barrels guide. Together, they can handle most of the rain that falls on a house, keep the garden watered, and reduce the load on drains, which helps the whole street in heavy rain.
Common mistakes to avoid
The biggest mistake is siting a rain garden too close to the house, where water can affect foundations — keep it at least 3 metres away. The second is building it on soil that does not drain, so it becomes a permanent pond that breeds mosquitoes. The third is making it too deep, so it holds water for days. The fourth is lining it with plastic, which stops water soaking away and defeats the purpose. The fifth is planting dry-loving plants in the wet centre, where they will rot, or wet-loving plants on the dry edge, where they will dry out. The sixth is forgetting the inlet, so water never reaches the basin, or letting it block with leaves and silt. Finally, do not use fertiliser in a rain garden, and do not let it become a tip for leaves and clippings. If the basin does not drain within two days after rain, something is wrong — fork the soil, add compost and grit, or ask your local extension service for help.
Frequently asked questions
How deep should a rain garden be?
About 15–20 centimetres deep is right for most gardens. Deeper than that, and it will hold water too long and may breed mosquitoes. Shallower, and it will not hold enough water to be useful. The basin should drain within 24–48 hours after rain.
Will a rain garden breed mosquitoes?
Not if it drains properly. Mosquitoes need standing water for several days to breed, and a well-built rain garden empties within a day or two. If yours holds water longer, the drainage needs improving — fork the soil, add compost and grit, or check the inlet and outlet.
Can I build a rain garden on clay soil?
Yes, with care. Add plenty of compost and grit to improve drainage, make the basin a little deeper with a gravel layer at the bottom, and choose plants that tolerate wet soil. Test the drainage first with a test hole: if water is still standing after two days, the site may not suit a rain garden.
What plants are best for a rain garden?
Plants that cope with wet and dry: asters, daylilies, irises, sedges and rushes in the wet centre, and coneflowers, lavender, salvias, ornamental grasses and hardy geraniums on the drier edges. Native plants are often best — ask your local extension service for a list suited to your area.
Do I need permission to build a rain garden?
Usually not for a small garden rain garden, but check local rules if you plan to disconnect a downpipe from the drain, or if you live in a listed building or a conservation area. Avoid building over drains, cables or a septic tank, and check with your local authority if you are unsure.