Rain Gardens: What, Why and How
A rain garden is a garden which takes advantage of rainfall and storm water runoff in its design and plant selection. Usually, it is a small garden which is designed to withstand the extremes of moisture and concentrations of nutrients, particularly Nitrogen and Phosphorus usually from fertilizers, which are found in storm water runoff. Rain gardens are ideally located close to the source of the runoff and serve to slow the storm water as it travels downhill, giving the storm water more time to infiltrate and less opportunity to gain momentum and erosive power before entering storm sewers, lakes or streams.
On the surface, a rain garden looks like an attractive garden. It may support habitat for birds and butterflies, it may be a formal landscape amenity or it may be incorporated into a larger garden as a border or as an entry feature. What makes it a rain garden is in how it gets its water and what happens to that water once it arrives in the garden.
Rain gardens should be located close to the site of the runoff Rain gardens can attract wildlife
There are two basic types of rain gardens: under-drained and self-contained. Both types of rain gardens are used to improve storm water quality, reduce runoff volumes and generally facilitate infiltration of cleaned water. Which type of garden is selected to be built is a balance of volumes of water to be treated, existing soil conditions, available space, and budget for the project.
In both types of gardens, the ground is excavated and the planting media is imported to the site. The imported planting media should be clean and free of weed seeds. A liner may or may not be used, depending on the local conditions.
Rain gardens are designed to be drained within four hours after a 1-inch rainfall. Under-drained rain gardens typically are designed to drain within two hours of the storm. This is achieved through the use of highly porous planting media and under drains which carry the cleaned rainwater away from the garden. As a result, the plants need to be able to withstand both the extremes of flooding and drought. Plants on the upper edges of the garden are often xeric (adapted to an extremely dry habit) with plants lower in the garden being more adapted to floodplain conditions.
Rain Garden Plants
Red Maple, Acer rubrum
Shrubs and Vines
Red Chokeberry, Aronia arbutifolia
|Perennials and Herbaceous Plants|
Northern Maidenhair Fern, Adiantum pedatum
This plant list includes plants that can tolerate temporary pooling of rainwater as well as dry periods. Most of the plants are native species and those that are known for attracting butterflies or hummingbirds are noted. Some of the native plants included in this list may not be readily available at all garden centers. Call your local nurseries and garden centers to find out availability.
A variation of plants can make an attractive rain garden
Plant list and photos courtesy University of Rhode Island Extension
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