Why a slope changes the design

Water on a sloped plot behaves differently from water on the flat. It arrives faster, gathers momentum and carries silt with it. A single sunken basin halfway up a gradient — the sort of rain garden usually described for level gardens — will often disappoint: it fills quickly, spills over the downhill lip and scours a channel through the planting on its way out. On a slope, the aim is not to hold water in one place but to slow it down repeatedly, giving it time and space to soak away as it travels down the garden.

Done well, a terraced rain garden does three jobs at once. It takes the worst of the runoff off your patio, path or roof downpipe, it reduces erosion on the slope itself, and it creates a ribbon of damp, varied habitat that suits a surprising range of British native plants. It is one of the more satisfying pieces of domestic green infrastructure you can build with a spade, a level and a few tonnes of gravel.

Survey the site before you dig

Spend an afternoon understanding what you are working with. The design decisions that follow all depend on it.

• Gradient. Measure it with a spirit level and a straight edge, or a length of string and a line level. Gentle slopes of around 1:20 to 1:10 are straightforward. Anything steeper than about 1:5 needs shorter terraces, more robust retaining and a proper overflow route.
• Soil. Dig a 300mm test hole, fill it with water and see how long it takes to drain. If it is still full after 24 hours, you are on heavy clay and should plan for a shallow, wide rain garden with amended soil, or treat it as a detention feature with a controlled outlet rather than relying on infiltration.
• Catchment. Work out how much water you are dealing with. As a rough guide, allow between 10 and 20 per cent of the contributing area — roof, patio, driveway — in rain garden surface, with a ponding depth of 150–200mm. Choose the larger figure on clay.
• Constraints. Keep the feature at least 3m from your house and from neighbouring foundations (further on shrinkable clay), away from retaining walls, and clear of any services. Check where the water will go if the garden is overwhelmed.

Building a staircase of terraces

On a gradient, replace one basin with a series of three to five shallow terraces stepping down the slope. Each terrace has a level base, a berm at its downhill edge and a drop of 150–300mm to the next one. Use a string line and a spirit level — or a laser level if you have one — to set the base truly flat; a base that merely looks level will pool at one end and dry out at the other.

Retain each terrace with timber sleepers, local stone or a planted earth berm. Earth berms are cheapest and settle into the planting best, but they need a good 300mm of width and should be keyed into the slope rather than simply piled on top of it. If you are retaining more than a metre, or building close to a boundary, check with your local planning authority before you start.

Between terraces, place a short gravel-filled trench or a stone-lined channel so water can move down in a controlled way rather than pouring over the lip and cutting a gully.

Spillways and erosion control

Every terrace needs a designated overflow. A shallow, stone-filled spillway set into the berm gives excess water a safe route to the next level and protects the structure from being undermined. Line spillways with a butyl offcut or heavy-duty geotextile beneath the stone to stop water washing out the subsoil.

Bare soil on a slope is vulnerable, so cover it quickly. Jute mesh or coir matting pinned across freshly shaped banks will hold everything together for the first season while plants establish. Mulch the terrace bases with coarse bark or gravel, and set a few larger stones at the toe of the slope to break the flow of any water that does escape.

Planting for wet feet and dry crowns

Think in zones: wettest at the base of each terrace, transitional on the sloping sides, drier on the berms and the top of the bank. Choose plants that tolerate root moisture in winter and drying out in summer, and favour British natives and near-natives for wildlife value.

• Wettest zones: yellow flag iris (Iris pseudacorus), marsh marigold, meadowsweet, purple loosestrife, soft rush and sedges such as Carex pendula.
• Transitional slopes: tufted hair grass, astrantia, hardy geraniums, lady's mantle and ferns.
• Drier berms and tops: sedums, thyme, lamb's ears, ornamental grasses and low shrubs such as dogwood and guelder rose, whose roots also help knit the bank together.

Plant densely — closer than you would in a border — and avoid fertilisers and pesticides. A rain garden should be a small, functioning habitat, not a manicured bed.

Maintenance through the year

In the first summer, water new planting during dry spells and keep on top of weeds while the canopy closes. After every heavy storm, walk the garden and check that spillways are clear and that no terrace is holding water for more than 48 hours. Rake out accumulated silt each autumn, cut back spent stems in late winter, and leave some seed heads standing for insects and birds. Little else is needed — and the reward is a slope that holds its shape, sheds water gracefully and looks better every year.