How To Build a Rain Garden

North Carolina Science Now Reporter's Blog

So how do you build a rain garden?

Rain GardenAfter reporting on the raincatchers project, I decided to look into building a rain garden at my house. It’s a perfect location. The lot slopes into the back yard and there is a creek at the back of the lot. Well, it’s usually just a dry ditch. But when it rains, the ditch quickly fills with fast moving water. I did a little research and found the ditch flows into Bond Lake, which is in the middle of Bond Park in Cary. While the lake isn’t a source of drinking water, people do fish and swim there, so keeping pollution out of the lake is a good thing.

There are two reasons to build a rain garden. The first is to capture the runoff and filter the water before it enters the waterway (the creek behind my house). The second reason is to slow the water down. After a rain the creek fills up really fast and there’s a lot of water moving through it. That causes erosion and washes a lot of dirt into the stream. 

Laura Webb Smith, the public education coordinator with the City of Durham Public Works Department, suggests my rain garden should be about 6-10’ long and located at the bottom of the steepest part of the hill on the side of my house. That’s where the runoff flows at the fastest rate.

Ideally, a rain garden is about 8-10” deep with a depression in the middle. You should use the soil that is dug up to create a berm along one side of the garden. In my case, the berm should be on the downhill sloping side of the garden. She suggests breaking up the soil at the bottom, then adding about 3-4” of topsoil/compost and then adding about 2-3” inches of hardwood mulch on top of that. That creates a porous surface that will absorb the runoff. It also provides several inches of ponding depth for the water to sit in. The mulch is important for pollution removal, maintaining soil moisture and preventing erosion.

Here’s the basic plan for what I’m hoping to build. It’s a design that fits our climate:

Rain Garden

A rain garden should not hold water for more than a day or two. After all, they are designed to absorb runoff. There’s no need to worry about mosquitoes because they require several days to hatch and the water would be absorbed by then.

Webb suggests planting native flowers, bushes and shrubs that are native to the area in and around the rain garden. Native plants fit the soil and climate. They also develop more extensive root systems than ornamental plants that will anchor the soil around them and survive long dry periods.

- Frank Graff

Frank Graff is a producer/reporter with UNC-TV, focusing on North Carolina Science Now, a weekly science series that airs Wednesdays, beginning in August 2013, as part of North Carolina Now on UNC-TV. In addition to producing these special segments, Frank will provide additional information related to his stories through this North Carolina Science Now Reporter's Blog!

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