What is a Rain Garden?
A rain garden is commonly mistaken as a water garden, or a wetland. Contrary to popular opinion, rain gardens are typically dry for the majority of the time. It will typically hold water only during and after a period of rainfall, but with an added bonus; they will typically drain within 24-48 hours.
Where Should I Plant a Rain Garden?
Planting a rain garden should ideally be done in an area where the soil can drain easily. The drainage rate should be between 1.25 and 5cm per hour, and this can be tested by:
- Digging a small hole (about 25cm deep) and fill it with water
- Once empty, refill and observe the drain time
You should aim to plant a rain garden a minimum of 10 feet away from your house, so as to avoid flooding and damp in foundations. Also, it’s important to check that no underground wires or pipes will obstruct any planting.
Rain gardens should be located in area exposed to full sun or partial shade, ideally on a very gentle incline.
How Big Should a Rain Garden be?
The surface area of the rain garden can be almost any size, but time and cost will be important factors in determining the ideal size. Any reasonably sized rain garden will provide runoff control from excess rainwater. Typically, rain gardens are between 100-300 square feet (9.2-27.8 square metres).
The size of the rain garden will depend on:
- How deep it will be, which will depend on how quickly it will drain. As a guide it should be between 10-20cm deep on a level surface
- What type of soils the garden will be planted in
- How much driveway surface water will drain to the garden
The next step in determining your rain garden size is to find the area that will drain to the rain garden. For the purposes of this article, let’s call this the runoff area. As the size of the drainage area increases, the rain garden size should increase as well.
You can calculate the area of a rain garden based on the runoff area; the garden should only constitute 20% of this area. Measure the surface area of the driveway, and then multiply the area of the driveway by 0.2 to get the area that will accommodate typical UK summer rainfall in your garden.
What Plants to Use?
This will also depend on your soil and also average rainfall in your area. If your soil takes a while to drain, your garden could potentially get waterlogged, particularly if you live in an area that gets frequent heavy rain. This means the rain garden plants that could handle damp areas would be more suitable due to their high moisture absorption. Plants like these could include:
- Cardinal flowers
- Yellow iris
- Arum lilies
- Pendulous sedge
For when the weather is drier, a good rain garden contains plants that can tolerate these conditions as well as wet weather. Some plants that can withstand both wet and dry conditions include:
- Siberian iris
- Korean feather reed grass
- Clustered bellflower
- Tufted hair grass
Benefits of Rain Gardens
- They’re very low maintenance
- They soak up to 30% more water than a conventional garden
- Plenty of birds and insects are attracted to the types of plants that complement a rain garden
- They offer plenty of opportunities to plant perennials
- There’s no need to sink a soakaway
- They reduce erosion by slowing heavy rainfall