If you missed the rain garden UK training from ARK or need a reminder here are some tips to help you get started with creating a rain garden.
What is a rain garden?
In its simplest form, a rain garden is a shallow depression, with absorbent, yet free draining soil. Rain gardens are planted with vegetation that can withstand occasional temporary flooding as well as being drought tolerant. Rain gardens are designed to reduce the volume of rainwater running off into drains from roofs and other impervious areas.
Despite the name, rain gardens only contain shallow water for brief periods (24-48 hrs) after heavy rain. Rain gardens need to be free-draining so clay soils may not be suitable.
Where to put a rain garden
The first task is to identify a suitable site for your rain garden, follow these tips to help you decide;
Decide where you want to take rainwater from - identify downpipes or other inflows.
The recommended minimum distance from building foundations is three metres.
The best spot is usually the lowest point convenient to inflows, using gravity to move water from a down pipe on your house to the rain garden.
A free-draining area is needed so avoid areas that already have problems with water-logging.
Don’t build higher upslope than the house.
Avoid proximity to underground services (including septic tanks).
Avoid tree roots
Think about where excess rainwater could go if the rain garden in a big storm? A large lawn, second rain garden or flower beds could be options.
Avoid steep slopes with a gradient over 12%.
What size should my rain garden be?
As a simple rule of thumb your rain garden should be one fifth of the size of the roof area it will be taking water from. You can make more exact calculations by following the guidance here.
Testing to check if your soil is free draining
You can dig a test pit to check whether your soil is free draining. Here's how to do it;
Dig a test pit, roughly 25cm square and 30-40cm deep.
Fill the pit completely with water and let water fully drain.
Fill again, then measure the rate that water soaks in per hour. Placing a long ruler in the test pit is a good way to do this.
Ideally, water should infiltrate at a rate equal to or greater than 50mm (5cm) per hour.
Getting water to your rain garden
Once you have decided on a rain garden spot, you’ll need to think about ways to get the rain to it. If you are looking for alternatives to simply extending the downspout pipe, here are some ideas.
Swale
A swale is a shallow vegetated channel flowing towards your rain garden. Be sure the first section runs through a gutter, pipe, flexible hose or is lined, to protect your building foundations from damp. After that water can move naturally towards your rain garden and some will soak in on the way.
Rill
This is a simple solution making use of materials like roof ridge tiles, C-shape purlins or decking boards to make a water channel. It doesn’t matter if it’s a little bit leaky along the way.
Dry creek bed
This can be as formal or informal as you like. Line the first few metres from the house with butyl pond liner to keep water away from the house foundations. From that point you can let rain soak into the ground as it makes its way to your rain garden.
Find out more about the Great Bedwyn rain garden project here.
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