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Rain gardens are a great way to treat stormwater runoff from roofs, parking lots and driveways.
Rather than piping all of that water to a stormwater pond, these depressions in the landscape trap the water and allow it to infiltrate just as it did in its natural state.
Also known as bio-retention areas, rain gardens got started in Prince George’s County, Md. The idea was to create attractive spaces in the landscape that would also help remove nutrients and pollutants from the water falling on impervious surfaces like streets, roofs and sidewalks. Rather than building the planting areas up, lower areas are created to temporarily hold the water. Trees and shrubs are used that tolerate the temporary pooling of water.
One question that always comes up when we talk rain gardens is, “Won’t that attract mosquitoes?”
If the water pooled for several days at a time, mosquitoes would have time to breed. But, rain gardens are designed to dry out in no more than 48 hours. In most rain gardens the water moves out much more quickly—usually four to six hours. So, a properly designed bio-retention cell will not become a breeding ground for mosquitoes.
Rain gardens are a great way to remove some of the bad stuff in stormwater like excess phosphorus, heavy metals and bacteria.
Soil, mulch and plants create a little biological filter that ties up and breaks down these pollutants. And, since the area dries up completely between rainfall events, bacteria are exposed to sunlight and dryness. Bacteria just can’t survive without lots of water.
Designing a rain garden for your home landscape is straightforward. It’s just a matter of figuring out how much surface area of roof or paving you are draining and then calculating how large the garden must be to store that quantity of water.
Small rain gardens we’ve installed in the area usually end up being about 10 percent of the impervious area we’re working with. For instance, if we have 1,200 square feet of roof area, the rain garden will have to be about 120 square feet to hold the water.
It’s obviously important to make sure the trapped water drains fairly quickly because of the aforementioned mosquitoes and keeping plants alive.
Our sandy soils work well because they have great internal drainage. Sticky clay soils may not work or may require that an under-drain be added to move the water through.
Another consideration in the soils of the area is the depth to the water table. You’ll need at least a foot or so between the bottom of the rain garden and the top of the water table. If you dig out an area and it permanently fills with water, it’s not going to function properly as a rain garden.
Al Hight is the county extension director and horticulturist with the Brunswick County Cooperative Extension Service. Call 253-2610 or e-mail email@example.com.