• Any garden, back or front, where there are roofs or outbuildings from which water can be collected
• Any garden with a slope of less than 10%
• A reasonably well-drained garden

The idea is to make the most of rain in the most creative ways you can.

Put simply, a rain garden is a shallow area of ground or dip which receives run-off from roofs and other hard surfaces. It is planted with plants that can stand waterlogging for up to 48 hours at a time. More drought-tolerant plants are used towards the edges. Storm water fills the depression and then drains.

A rill or channel of some kind will connect the roof downpipe to the dip. You can include other linking elements such as a water butt, rain chain, stormwater planter, or pond.

In this way, the whole garden could be considered a rain garden. However, more commonly, the low-lying area or dip itself, is defined as the rain garden.

Benefits of rain gardens

• Low maintenance garden – no watering once plants have established
• Can absorb up to 30% more water than a lawn
• Offers opportunities to plant a wide range of perennials
• Reduces erosion by slowing heavy rainfall
• Increased planting attracts insects and birds
• Avoids the need to sink a soakaway

Where to site a rain garden

Be aware of underground services, such as gas and electricity, and dig carefully if you are not sure of their exact position. A rain garden should be located;

• In full sun or partial shade
• In a well-drained area, on a very gentle incline (10% or less)
• If siting your rain garden closer than 5m (16ft) to your house, seek advice from a Geotechnical Adviser or Registered Ground Engineering Professional to avoid any damage to foundations by infiltrating water
• Where it is practical to install a pipe leading to the main drainage system below the rain garden in case of excessively heavy storms

How to decide if the ground is suitable

Assess your soil texture, so that you know how quickly the soil drains.

1. Excavate a hole about 25cm (10in) deep and fill it with water.
2. Once empty, refill and observe the time it takes to drain.

The ideal rain garden site is one that drains at a rate of a minimum of 1.25cm (½in) per hour, but will still work with absorption rates of up to 5cm (2in) per hour. If it takes longer, the site is not suitable. This might be true of a very heavy clay soil or where there is a high water table.

What size should the rain garden be?

This depends on the type of soil you have, the size of the roof and the drainage rate of your soil. This will determine how deep the rain garden should be. The space available will also govern the size. Even a small rain garden will control run off; it will just overflow more often.

How to calculate the area of the rain garden

1. Allow the rain garden to constitute 20% of the roof area.
2. Measure the surface area of the roof that will be receiving the rain (which will then run off the roof via the downpipe).
3. Multiply the area of the roof  by 0.2 (20%) to obtain the area of the rain garden to accommodate typical UK summer rainfall.

Example

A roof 10m by 10m has an area of 100 sq m
Area of the rain garden is 100 x 0.2 = 20 sq m

Depth

On a soil that drains at 5cm (2in) per hour, the rain garden would need to be 15cm (6in) deep, with an extra 5cm-10cm (2-5in) below the lowest point where excess water would leave. A less well-drained soil would necessitate a deeper rain garden.

The basics of how to make a rain garden

Use garden hose, or stakes and string to create the shape you want. Kidney-shaped can work well.

• Remove any vegetation
• As you dig, aim for a saucer shape, with a flat base and ensure levels are correct
• The soil you dig out can be used to make the berm (the lip or bund on three sides)
• The berm will keep the water in the rain garden, so has to be well-compacted, around 30cm (1ft) wide and 10cm (6in) in height
• Leave a notch in the berm, with a gravel-filled channel for the water to exit into a conventional drainage system
• If the area is sloping, the top end will require more digging out then the lower end. The spoil form the top end can be used to fill in at the lower end to make the rain garden level
• Incorporate organic matter into the excavated soil to improve soil structure and then backfill to the original level. Leaf mould, homemade compost, soil conditioner or well-rotted manure are all suitable media
• The soil should be not be compacted

How water enters the rain garden

• Make a rill or channel of bricks or setts to carry the water the few metres from the downpipe to the rain garden
• Install a downpipe diverter or ‘elbow’ to the downpipe leading from the roof gutter
• At the point where the water enters the rain garden, cobbles or gravel will help prevent soil washing away

Planning and planting the rain garden

• Design the rain garden to be attractive from all angles
• Concentrate the more wet-tolerant plants near the inlet pipe and towards the base
• Use plants that attract pollinators and have seasonal interest. Plant in drifts e.g. 7 plants per square metre
• Grasses provide welcome cover for insects, especially over winter
• Lay the plants out, still in their containers, according to a plan, so you can move them around if necessary
• Take care to plant the plants at the same height they were in the containers
• Mulch the rain garden with organic material such as recycled green waste
• Water well, even though it is a rain garden, for the first two summers if it is dry

Avoiding problems is better than trying to solve them after they have happened.

The following points are worth bearing in mind:

• Make sure the rain garden area is lower than where the water is being collected from
• Check whether water, gas electricity services run through the proposed site
• Avoid planning a rain garden in the vicinity of tree roots 
• For the rain garden to collect the optimum volume of rain, the longer side should face up the slope
• If the soil is less well draining, incorporate gravel or grit
• Ensure that the lowest point of the rain garden allows overflow water to access a drain, or other rain garden element
• Make sure the bottom of the rain garden is level
• Allow enough room for water to pool, after you have added the organic matter and topped with mulch
• To avoid weed growth between new plants, consider sowing a flower seed mix suited to the conditions to infill while plants establish
• Test the rain garden by using varying water pressures to see how water flows, in, overflows and whether the soil washes away. If soil washes away, add some more stones at inflow and outflow points

The choice of plants will be governed by regional rainfall and speed of drainage. On a less well-drained soil (clay) or areas with regular heavy rain, plants tolerant of wetter soil may be needed. This is not a bog garden, however (i.e. it cannot be guaranteed to be damp all year round).

For the base and inlet on moisture-retentive soils

Herbaceous perennials

Iris pseudocorus
Juncus effusus
Carex pendula
Lobelia cardinalis
Zantedeschia aethiopica

Plants tolerate temporarily wet soil as well as dry

Shrubs

Sambucus nigra cultivars
Cornus sanguinea ‘Midwinter Fire’
Hydrangea ‘Annabelle’ AGM
Rosa rugosa

Herbaceous perennials

Ajuga reptans
Campanula glomerata
Crocosmia 'Lucifer' AGM
Geranium 'Rozanne' AGM = 'Gerwat' (PBR)
Hemerocallis
Iris sibirica

Grasses

Calamagrostis brachytricha AGM
Deschampsia cespitosa
Miscanthus sinensis cultivars

For a full list, please see: Wet and dry soils: plants for

Further reading

Rain Gardens – Managing water sustainably in the garden and landscape by Nigel Dunnett and Andy Clayden (Timber Press 2007, ISBN 978-0-88192-826-6)

Creating Rain Gardens – Capturing the Rain for your own Water-Efficient Garden by
Cleo Woelfe-Erskine and Apryl Uncapher (Timber Press 2012, ISBN 978-1-60469-240-2)

The above books are made available through RHS Libraries