Harness the water that flows from your gutters by creating a rain garden.

As urban density increases, rain gardens are making an important contribution to the management of stormwater run-off. They’re also a way of ensuring rainwater stays on-site longer, providing water for irrigation.

A rain garden is a garden bed or planted area that has been especially designed to accommodate water run-off, generally from a house roof or other buildings, after it has been raining. It captures the stormwater from hard surfaces and filters it through plantings and a soil medium, after which it dissipates into the ground. The idea is to minimise stormwater entering the stormwater mains.

Creating a rainwater garden is a simple way of making a positive contribution to your local environment, as they:

• Slow the flow of water into stormwater outlets, thereby reducing the chances of flooding
• Help with the removal of pollutants before the water enters the stormwater system, which in turn protects the local plants and aquatic life that rely on clean and healthy waterways for their survival
• Increase the volume of water that filters into the ground, which recharges local aquifers
• Provide valuable habitat for birds and beneficial insects in the garden

Rain garden design

The following is a guide only and would need to be modified to suit the specifics of your site and to comply with the requirements of local authorities. But, whatever the design specifics of your rain garden, getting the right location is the first step — and it’s one of the most vital if you want to maximise its effectiveness.

Choose an area where the water can be captured from a roof or hard-surface run-off, such as near a driveway, house or garage. To prevent the water seeping into foundations, the garden bed should be placed a suitable distance from the house. Three metres is a good distance but it may depend on the depth of your house footings. An ideal location is a sunny or partly sunny position on a slight slope. Just check first that it is not anywhere near a septic system.

The layered approach

Once you have selected the location, mark out the perimeter of the garden bed. The size of the bed depends on the amount of water that will be captured, but it should be 20 per cent of the run-off area and twice as long as it is wide, with the longer side upslope. Cover the area with black builder’s plastic or newspaper to kill the grass beneath.

Generally, there are three layers in a rain garden:

1. The base layer of gravel, which should be about 200mm deep. Agriculture pipe will be placed into this layer and connected to a grated discharge pit, which in turn is connected to the main stormwater drain.

2. Next is a 100mm layer of white-washed sand.

3. The final layer is a 350mm layer of sand and topsoil mixed at the ratio of four parts sand to one part topsoil. This should be a level surface and about 100–200mm from the top of the bed. The plants are planted into this and covered with a light layer of mulch.

Other considerations

As you excavate the soil, it can be used to form the berm (a low, narrow mound) that surrounds the rain garden. The top of the downhill side of the berm should be the same level as the entry point at the uphill end. The berm should have gently sloping sides, which you need to compact to minimise erosion. Instead of forming a berm, you can also use timber boards to form the edging; like a raised garden bed.

Before you start digging, though, check to see of there are any underground electricity or phone cables or pipes you could dig into. You can contact Dial Before You Dig (www.1100.com.au), a referral service designed to help you locate underground utilities anywhere in Australia. Also keep in mind that a licensed plumber is required for any modification or reconnection to the stormwater system and you should consult your local water authority before undertaking any works.

Carefully select plants

You need to choose plants that can handle short periods of inundation and long periods of dry weather, and form an appropriate cover. They also need to be adapted to the local growing conditions and have a deep, fibrous root system once established.

Generally, low-growing native grasses and sedges work best. Species that have been used successfully include Lomandra sp., papyrus (Cyperus papyrus), swamp paperbark(Melaleuca ericifolia), knobbyclub-rush (Ficinia nodosa), hop goodenia(Goodenia ovate), wetland grasses (such as Carex sp.) and rushes (such as Juncus sp.). It’s a good idea to check with your local nursery and ask the horticulturist to recommend which plant species are best suited to your locale.

Ongoing maintenance

Rain gardens are relatively low maintenance. After planting, the only maintenance required is ongoing weeding, watering while the plants are becoming established and during prolonged dry spells. Once established, the plants will need to be thinned as they mature. If organic mulch is used, it will need to be topped up periodically.

If you would like to learn more about rain gardens, there are several very useful websites with fact sheets and step-by-step instructions. One of the best can be found at the Water by Design website (www.waterbydesign.com.au), where you will find a very informative fact sheet with detailed instructions. Melbourne Water (www.melbournewater.com.au) also has a handy set of guidelines you can download, and you can find a good explanation of how rain gardens work and why they’re important on the Sustainable Gardening Australia website.