As I discussed last week, rain gardens are used to slow down rain water and then filter this water before it goes off into the stormwater system and then often into local waterways. For the water to enter these areas, the actual garden bed area needs to be in a depression to allow the water to flow into it from the surrounding streets or garden. This then allows the water to soak into the ground and have pollutants filtered out.
Beyond slowing down water and filtering out pollutants before the water moves onto its final destination of our waterways, how are rain gardens any different to the average ‘normal’ back garden?
To be able to do the job that they are designed to do, the area of a rain garden is about twenty percent of the total area from which the water will drain into it, but the important thing is that any size is always helpful (and a ‘normal’ garden that’s well cared for is great, as it reduces hard surfaces, so allows more water to soak into the soil).
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The position and shape of the garden are also important. They should be longer than they are wide, with the longer sides positioned across the slope, rather than down the slope (apparently the slope can’t be steeper than 12%, others say 10% maximum) to allow them to catch as much water as possible. Keep at least three metres away from any buildings too. Ensure the selected area isn’t near tree roots and also not where water naturally pools-the rain gardens wont work here.
A rain garden also has shallow channels that funnel the excess water into them. The shape of the actual garden bed is a bit like a bowl so that the water can be collected there. Some soil removed to make this shape can be used to make the sides higher again, the rest is mixed with organic matter like leaf mould or compost and put back into the actual garden bed that will be your garden bed.
In a rain garden, the plants need to be quite tough in specific ways. These gardens are supposed to spend the vast majority of time as areas where there is no supplementary watering between the rain, so plants have to be drought tolerant, while also being able to tolerate the temporary waterlogging that will happen during rain. These gardens also have to have good drainage as the water shouldn’t stay for longer than 48 hours-you don’t want to encourage mosquitoes.