From Rain to Garden Rain Gardens Filter Polluted Runoff—And Bring Added Beauty

Rain Garden, Takashi .M, FlickrCC

Precipitation has etched an intricate web of watersheds into the Earth’s surface. Natural landscapes, such as forests, have very little runoff thanks to a combination of soil, leaf litter and vegetation. That’s not the case in cities and developments, where rainfall slides off rooftops and pavement into gutters and storm drains, sometimes in volumes that pipes can’t handle, and takes pollutants along for the ride. According to the U.S. Environmental Protection Agency (EPA), stormwater runoff is the number-one cause of water pollution and the greatest threat to our country’s estuaries. Rain gardens offer a solution—and help beautify your yard—as they recapture part of the Earth’s natural hydrological cycle.

Rain Garden Basics

There are two basic types of rain gardens—self-contained and under-drained. Self-contained gardens are appropriate for soils that drain quickly and can percolate water efficiently. Ann English, a RainScapes Planning Specialist for the Montgomery County Department of Environmental Protection, says: “The overall profile of the garden should be 30 inches deep and have a flat bottom and a flat surface. There should be a three-inch layer of mulch. Rain gardens typically have a dish shape.” If the soil doesn’t percolate water or drain quickly enough, it will require an under-drain. English says that people often make the mistake of placing their rain garden in a spot that’s too low, which can lead to over-saturated soil and even to a mosquito problem if water becomes stagnant. Instead, she recommends planting right above the lowest spot, which will saturate the soil just enough and give it room to drain.

Maryland’s RainScapes program—started in 2004—has evolved into an incentive program for the local community, providing technical and financial assistance in the form of rebates. The program not only promotes rain gardens for reducing runoff, but also conservation landscaping, permeable paving stones (in place of concrete driveways and walkways), green roofs, dry wells and rain barrels.

English says that rain gardens are “very efficient at trapping certain types of pollutants. EPA studies have shown up to an 88% reduction in total suspended solids—silt—that is retained in the soil rather than washed into storm drains.” Phosphorus and nitrogen are also trapped in various quantities depending on the types of plants the garden features.

Picking Your Plants

When selecting plants for your rain garden, it’s important to consider soil type, geographic area and the level of pollutants they’ll have to withstand. “The water that runs off first is the dirtiest,” English says. “For example, if you have copper flashing on your roof, the water will be very acidic. You have to use plants that can handle whatever the water brings with it.” The RainScapes program advises participants to use soil that is 50% sand, making it very absorbent. Put drought-tolerant plants around the garden’s perimeter, since that will be the driest area. The center of the garden is the wettest zone, which is best for plants with fibrous root systems, not taproots. The Brooklyn Botanic Garden has a useful list of native rain garden plants by region, such as blue lobelia and cinnamon fern in the Northeast; cardinal flower and giant coneflower in the deep South; and ironweed, monkey flower and big-toothed sunflower in the Midwest. As a landscape designer, English’s advice is to keep the plant palette simple. Rain gardens also don’t have to be stand-alone islands, she says, they can be incorporated into the rest of the natural landscape. The size of a rain garden can be determined by accounting for the size of the roof as well as how much rainwater you intend to collect.

Digging In

The Low Impact Development Center, a nonprofit dedicated to water and natural resource protection, has a number of rain garden design templates on their website. The cost of a rain garden varies based on size and the required amount of excavation, labor and materials.

Depending on whether you do the labor yourself or hire a professional landscaper, rain gardens can cost as little as $400 for 450 square feet, or as much as $7,000 for 120 square feet. Rain gardens are flexible and can be tailored to meet a variety of landscape or budget limitations, improving the quality of stormwater runoff and your curb appeal.

CONTACTS: American Water Works Association; Brooklyn Botanic Garden Rain Garden Plant Guide; Low Impact Development Center; RainScapes.