Rain gardens in their simplest form are shallow depressions with absorbent, yet free draining soil and planted with vegetation that can withstand temporary flooding. They are generally self-watering, low maintenance gardens designed to protect our waterways and lakes by capturing stormwater which runs off hard surfaces after it rains. They also mimic the natural water retention of undeveloped land and reduce the volume of rainwater running off into drains from impervious surfaces (surfaces that fluids, like water, cannot pass through). They also treat low level pollution and nutrients in stormwater by using physical processes in the soil and biological properties of plants, roots and microbes.
How do rain gardens work?
A rain garden lets water collect and settle on the garden surface then soak through the plants and filter soil. Rubbish and sediment is trapped on the surface. Nutrients dissolved in the rainwater are used by the plants. The soil and plant roots work together to naturally filter the water and remove pollutants. It is important that the soil used is correct to let plants grow and hold moisture but also allow infiltration. There are many types of rain gardens:
A slight depression in the landscape which can be either grassed or planted with other vegetation.
*All images supplied on this page courtesy of Melbourne Water
Rain garden design
Start by either constructing your planter box or excavating your trench, depending on the type of rain garden you've decided to build. A waterproof liner is sometimes used around the outside of rain gardens when you need to:
- protect nearby buildings, foundations or infrastructure
- avoid creating problems with saline groundwater or reactive clays
- capture all stormwater for reuse.
If you're building an in-ground style rain garden (including an in-ground and infiltration rain garden or swale), dig the area with a gentle slope away from the house.
- Check the base of the rain garden is above the surrounding groundwater level.
- Design your rain garden to treat stormwater runoff from gentle rainfall.
- Runoff from heavy storms should bypass the rain garden into the drainage system.
A saturated zone or submerged zone below the rain garden is recommended to provide water storage to help plants survive during dry periods and help remove nitrogen.
Note: a storage zone for water below the soil can support plants and improve treatment performance. This is especially useful in Canberra where there can be extended dry periods.
Rain garden plants
Be creative with your rain garden design using a variety of plants, rather than one species. This will prevent die-off and weeds. Mulch your rain garden with gravel to keep the moisture in. Avoid using bark or straw mulch as it will float and wash back into the stormwater system.
Your local nursery can guide you on what plants are suitable for your area. When choosing plants for your rain garden make sure they're able to tolerate heavy rainfall with temporary flooding as well as long dry periods. Native plants are usually more drought resistant and easier to maintain than introduced species.
The ACT Government’s Actsmart website includes a Water Right Garden Web Tool. It provides specific advice on the right plants for your garden to conserve water, avoid overwatering and fertilising and benefit our waterways.
Tips for a healthy rain garden
Rain gardens are easy to maintain, especially when planted with native Australian plants. They don't need to be watered, mowed or fertilised. Follow these simple tips to make sure your rain garden functions well.
- Cover your rain garden with gravel mulch to retain moisture.
- Weed your rain garden until the plants have matured.
- Evenly distribute water flow into your rain garden to limit erosion from heavy rainfall. Strategically placed rocks may help with this.
- Inspect your rain garden – replace plants and repair erosion in your rain garden when necessary.
- Don't drive over or squash your rain garden as this will reduce its ability to work effectively.
If it doesn't rain, water your rain garden until your plants have established in compliance with local water restrictions.
Need help building your rain garden?
The easiest and cheapest way to build a rain garden is to do it yourself. The materials you need can be purchased from most hardware or garden supply stores.
You could also contact an professional landscape gardener to help you build your rain garden and provide advice. A licensed plumber can assist you with any modifications and diversions to your stormwater pipes.