When it rains, runoff takes the path of least resistance. Some water goes in the ground, and some does not. In urban areas, most water becomes surface runoff. This is a major component of the water cycle and the primary agent in water erosion.
When runoff flows along developed surfaces, it can pick up soil contaminants that become discharge or nonpoint source pollution. The first flush of runoff often carries with it concentrations of pollutants that have accumulated during the period of dry weather between storms, which could be one day or several months.
In addition to causing water erosion and pollution, surface runoff in urban areas is a primary cause of urban flooding, which can result in property damage and street flooding.
The best direction for water to flow is into the ground, and a rain garden is an ideal way to steer runoff away from the municipal water system. It’s also a way to give back and put something into Mother Nature’s bucket.
A properly built rain garden benefits the environment and beautifies your yard. A successful rain garden requires four elements: proper position, correct depth, perfect size and deep-rooted greenery.
Position
The location of the rain garden should be near a source of water runoff such as a downspout, roof drain, walkway or driveway. It should not be in an area with a lot of debris – such as leaves and branches – because this will impede the flow of water and could clog the area, precisely what we don’t want to happen. Do not place the garden near a septic tank or sewer service or where water already pools in the yard. Its purpose is to infiltrate the ground.
Select a level area, if available, that is at least 10 feet from the dwelling to prevent seepage into a basement or beneath the residence. The water source, such as a downspout, should flow directly toward the chosen spot.
Depth
Before placing a shovel into the ground, call 811, the universal “know what’s below” number, to make sure there are no gas lines or cables in the area. A rain garden should be three to eight inches in depth, depending on the slope of your yard – and keep in mind the area should be as flat as possible. If there is a small slope, dig about three to five inches into the ground; medium slope, six to seven inches; and large slope, eight inches.
Dirt composition also matters and the southern regions of the country often have slow-draining clay soil. If this is the case in your yard, remove the top 10 to 12 inches of soil and replace it with 40 percent sand, 20 percent topsoil, 20 percent compost and 20 percent clay to support plant growth and improve infiltration.
Size
The size of your rain garden depends on downspouts, soil quality, slope and the dimensions of your dwelling. The Rain Garden Alliance has a simple online calculator to help determine the ideal size. You are now ready to start digging but remember one more tip: The water that flows into the garden will try to run off into the surrounding area so build a small berm along the edges and at the bottom of the garden as you shovel.
Greenery
The final step for a rain garden is the selection of plant and grasses. This is important because of the nitrogen and phosphorous found in stormwater runoff. Choose native plants and mix heights, shapes and textures.
Select plants with a well-established root system. Apply double-shredded mulch over the plant bed approximately two inches thick but don’t bury the crown of the new plants. Suggestions include perennials and herbaceous plant such as ferns and switchgrass; shrubs and vines such as chokeberry and holly; and trees such as red maple and birch.
Beneath the surface is an ecosystem that mimics the hydrologic action of a healthy forest. Stormwater is cleaned and reduced in volume. Above the surface, the site can be very attractive for birds and butterflies. It’s a win-win for the environment and for your yard.
Summary
The first rainfall in an urban area after an extended dry period can create runoff that is polluted with litter, soil, animal droppings and contaminates such as fertilizer and pest control products. Given the simplicity, effectiveness and aesthetic benefits of rain gardens, they are a perfect solution for protecting one of our most essential natural resources.