Video: How to Create a Rain Garden. This guide was informed by a rain garden tutorial featuring garden designers from the RHS Chelsea Flower Show. Worth watching for the visual explanation of how water flows through the system.
1. Find the right position
The ideal spot is where water already naturally collects, or close to a downpipe that currently discharges onto hard standing or lawn. The hollow needs to be at least three metres away from any house foundations or boundary walls, and not directly over drains or inspection chambers.
Check the soil drains at all. Dig a hole about 30cm deep and fill it with water. If it has not drained within 24 hours, the ground is poorly draining clay and you will need to improve the sub-base before the garden will work.
2. Work out the size
As a rough rule, the rain garden should be about 20 to 30 percent of the area that drains into it. So if you have a 30 square metre roof draining to one downpipe, aim for a hollow of roughly 6 to 9 square metres. Smaller gardens can get away with a compact version - even a 2 or 3 metre oval makes a noticeable difference.
The depth matters too. A shallow hollow of 15 to 30cm is usually enough. Deep hollows are harder to plant attractively and are only needed where drainage is very slow.
3. Mark out and dig the hollow
Use sand or a hose to mark out the shape. Kidney and teardrop shapes work well, as they look more natural than a square. Dig the hollow to your target depth and keep the sides gently sloped rather than vertical - this helps water spread across the whole area rather than pooling in the centre.
Keep the excavated soil to one side. You will use some of it to build the berm (see step 5), and the rest can be used elsewhere in the garden or composted.
4. Improve drainage beneath
On clay soils, loosen the base of the hollow with a fork and mix in horticultural grit or sharp sand to a depth of 15 to 20cm. This breaks up the clay and helps water move through faster.
On sandy or loamy soil, you may not need to do much here. Test drainage again after loosening - fill the hollow with water and time how long it takes to drain. It should clear within a few hours at most.
5. Build the berm
The berm is a low raised edge on the downhill side of the hollow. It holds water in long enough for it to soak in, rather than flowing straight through. Use the excavated soil to build a gentle mound roughly 15 to 20cm high and compact it gently.
An inlet channel on the uphill side directs water from the downpipe into the hollow. Keep it broad and shallow so water spreads across the full surface rather than scouring a single channel.
6. Choose your plants
Rain garden plants need to cope with both wet feet during heavy rain and dry spells in summer. Native and near-native species are the most reliable. Good choices for UK rain gardens include marsh marigold, ragged robin, purple loosestrife, meadowsweet, and yellow flag iris for the wettest zone. Use tougher plants like native grasses, day lilies, and astilbes on the edges and berm where it stays drier.
Avoid plants that need consistently dry or well-drained conditions - lavender and rosemary, for example, will struggle in a rain garden hollow. And avoid invasive species like Japanese knotweed or giant hogweed, however they end up there.
7. Plant up and mulch
Plant in autumn or spring when conditions are mild and moisture is reliable. Space plants generously - they will fill in. After planting, lay a 5cm layer of bark or gravel mulch over the whole area to suppress weeds and slow moisture loss between rain events.
Water the plants in well after planting and keep them watered through their first dry spell. Once established, a rain garden largely manages itself, refilling naturally and draining between showers. That said, you will want to check the inlet and outlet channels occasionally and clear any blockages.
When to call a handyman
Serious drainage problems - waterlogged foundations, broken drains, or large areas of standing water - are beyond what a rain garden can fix and need proper drainage assessment. Richard can help with garden clearance, edging, and general prep work before you start, but drainage surveys and drain repairs need a specialist.
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Richard covers garden clearances, raised bed building, and general outdoor improvements in Sandwich and East Kent.
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