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A rain garden is a low area of ground that is designed to use rainwater runoff effectively. It can be planted with water-loving plants that can survive and thrive in boggy areas, and also cope with drier spells. They can be created in a front or back garden where there is runoff from hard surfaces such as roofs, patios, driveways, and so on.
As our climate changes and we are experiencing more frequent heavy downpours and wetter winters, combined with prolonged summer droughts, drains can become overwhelmed as rainwater pours off hard surfaces. A rain garden helps to manage the excess water, and mitigates against the risk of flooding.
Rain gardens are not the same as bog gardens, because they are designed to dry up during dry weather. Bog gardens are designed to be permanently damp areas of soil, and are planted with species that are able to constantly withstand waterlogged ground.
What are the advantages of a rain garden?
A rain garden soaks up excess water after a heavy downpour, putting less pressure on drains and reducing the risk of flooding and erosion. They also make a haven for wildlife, and will attract butterflies, bees, and birds to your garden, especially when planted up with native perennial plant species.
Once the rain garden is established, it requires little maintenance, and adds interest and variety to your garden.
Where should a rain garden be sited?
Rain gardens should be sited at least 16 ft away from your house, to avoid the risk of damage to the foundations. The ground should be on a slight incline of 10% or less, according to the Royal Horticultural Society (RHS) and in a well drained area that is in full sun or partial shade.
If your garden has any hollows or natural dips or shallow areas, then this may be the logical site for the rain garden. It should be at the end of a gentle slope down from the hard surfaces where water runs off, or there is a direct route from a downpipe, ditch, or other water channel.
You also need to test the soil type to check how permeable it is. The RHS advises that heavy clay soils that take longer than an hour to soak up ½ inch of water (1.25cm) are not suitable for rain gardens.
How do you build a rain garden?
First, you need to work out how large the rain garden should be. The RHS explains that it should be approximately 20% of the roof area that will provide the rainwater runoff for the garden (which will be collected in a downpipe). To calculate the depth, you need to test how quickly water drains into the soil.
Faster draining soils of 5 cm of water per hour would need a depth of 15cm, with a deeper point of 5 to 10 cm to allow for extra wet weather. Slower draining soils will need extra depth, based on the rate of water absorption per hour. If the soil drains very quickly as some sandy soils may do, you could add some compost or topsoil to help it hold more water.
Once you have worked out the size and depth, dig out any lawned areas or vegetation from the site. Create gently sloping sides that lead to the deepest point of the rain garden, but ensure that the centre is flat to allow for even drainage.
Put the excavated soil to one side, and when you have created the required shape and depth, use it to build a lip around the sides. This is known as a berm, and it will hold the water in place. Pack the soil firmly to ensure it is robust, and build it to a height of about 30 cm.
To divert runoff directly into the rain garden, you may need to install a diverter to the roof downpipe. You can then dig a channel or create one from bricks to direct the flow of water into the rain garden (this can make an interesting water feature in itself). Add some stones or gravel at the entry point to help prevent soil erosion.
Plant up the rain garden
The final stage is to add some plants to create visual interest and attract wildlife. They should be chosen carefully to be able to cope with both waterlogged soil and drier spells. The Wildfowl & Wetlands Trust recommends plants such as Yellow flag iris, Meadowsweet, Granny’s Bonnet, Coneflower, and Primrose.
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