A rain garden is a landscape feature that is specially designed to utilize rainwater and storm water runoff to support the plants contained within. A rain garden can take many shapes or forms: it can be incorporated into a larger landscape feature garden, it can form a border or entrance to an existing garden, or it can be a stand-alone focal point of a garden. What defines a rain garden is how it is irrigated, and what happens to the water after it reaches the rain garden.
Controlling Storm Water Runoff
In urban areas that consist largely of paved and tarred surfaces, rain washes off rooftops, driveways and other impervious paved surfaces and flows into storm water drains, eventually making its way to streams, rivers and lakes. As the water flows over these surfaces, it removes loose topsoil and debris, as well as surface contaminants such as pesticides, fertilizer, oil, and animal waste, which are all carried to our surface waterbodies. Excess sediment loads as a result of erosion can cause our rivers and lakes to become silted up, while excess nutrients from fertilizers and animal wastes can cause eutrophication and harmful algal blooms – both of which can have a severe impact on natural ecosystems and the wildlife they support. Toxins can pollute surface waters, posing a risk to wildlife, and they can also contaminate drinking water sources, posing a health risk to humans too. As the Pacific Northwest becomes more and more developed and urban areas expand, the problem is intensifying.
Rain gardens slow this runoff, allowing it to filter into the ground in your yard rather than gushing downhill. Surface pollutants are either removed as the water filters through the soil, or in the case of nutrients such as nitrogen and phosphorus from fertilizers and animals waste, are absorbed by plants and utilized for growth.
Healthy Natural Ecosystems
On the surface, a rain garden provides a healthy wildlife habitat. Below the soil’s surface there are a number processes taking place that mimic the hydrology of a natural forest ecosystem. A rain garden is essentially a small bioretention system that cleans and reduces the volume of storm water that enters the system. Plants remove nitrogen and phosphorus, reducing the levels of these nutrients, together with sediment loads from storm water. The soil structure needs to be carefully engineered, and plants wisely selected to ensure that the system functions effectively.
As plants need to be able to withstand extreme variations in soil moisture and nutrient availability, plants selected for a rain garden therefore need to be able to tolerate extreme variability in moisture, as well as high concentrations of nutrients such as nitrogen and phosphorus that are often present in storm water runoff.
Deep-rooted plants with fibrous root systems tend to thrive in a rain garden and also provide optimal cleaning and filtering action. Choose native plants that are well adapted to local conditions where possible, however, non-invasive ornamental plants that are well suited to the conditions may also be used.
Siting a Rain Garden
Rain gardens should ideally be sited as close to the source of the surface runoff as possible, so that they are effective at slowing storm water runoff as it flows downhill. This gives the storm water a chance to infiltrate the ground rather than washing downhill, thus preventing erosion. When there are a number of rain gardens capturing storm water in an area there will be a cumulative positive effect on the quality and volume of the surface water runoff.
A Rain Garden Offers Multiple Environmental Benefits
Rain gardens play an important role in preventing flooding by reducing the volume of water that is transported to rivers during heavy rain storms. Instead, rain water seeps into the ground to replenish groundwater sources, which helps to keep your garden naturally irrigated, alleviating the need to water often, and thus saving water. By carefully choosing the plants that go into your rain garden you can create a wildlife refuge that not only attracts frogs, but also butterflies, bees and birds, while managing storm water runoff at the same time.
The following resources provide more information on how to go about setting up a rain garden on your property, including advice on costs, siting, plant selection, maintenance, as well as step-by-step instruction on layout and construction.