A real watershed: A backyard rain garden install

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Updated Feb 2, 2018

Picture your neighborhood before it was full of houses and people – and impervious surfaces such as rooftops, parking lots and driveways. What happened when a hard rain fell? Trees and plants had a job to do: They minimized and slowed runoff to streams and rivers, allowing excess moisture to percolate through the earth and become groundwater.

But in heavily developed areas today, that natural process is substantially compromised. Runoff is a growing environmental challenge: It overburdens sewer and stormwater-treatment systems, wreaks havoc on built and cultivated environments, sweeps sediment and pollutants – such as phosphorus from fertilizers – into streams that supply drinking water, and ultimately exacerbates erosion and drought. In this climate, and under government pressure, savvy contractors are becoming aware of ways that nature’s stormwater management tools – plants and trees – can function as cost-effective “green” fixes for manmade problems.

Like a natural sponge
Rain gardens, or bioretention basins, are one attractive ecological solution landscapers can use on sites of any size, from intimate back yards to substantial public green spaces and commercial properties. A rain garden is simply a shallow depression – rather than your typical raised planting bed – filled with moisture-loving plants. Like a natural sponge, its plant material and soil matter soak up, filter and evapotranspire excess water, mitigating peak flow. Though not yet widely familiar to consumers, rain gardens are poised to be the next hot trend as interest in sustainable landscaping and low-impact development continues to grow. In addition to functionality, they provide a colorful and low-maintenance habitat for birds and butterflies.

Rain gardens are most effective when planted with native plant species, a fast-growing segment of the nursery business. For Mike Berkley and Terri Barnes, cofounders of GroWild, a nursery and landscape design firm in Fairview, Tennessee, this means rain gardens are one more way to encourage awareness and use of native plants instead of invasive exotic species like Bradford Pears and burning bush. Under the auspices of GroWild, which carries one of the largest selections of North American native plant species and cultivars in the Southeast, Berkley has installed rain gardens for numerous government and corporate clients. Lately, he says, he’s getting increased interest from the residential market.

Spreading the word
Rain gardens are already prized by some local governments and communities that seek ways to deal progressively – and cost-effectively – with drainage problems and water pollution. The term “rain garden” first entered landscapers’ lexicons in the early 1990s when construction began on Somerset subdivision in Prince George’s County, Maryland. The 199-home development is considered the first example of bioretention in residential use. Working in partnership with Prince George’s County Department of Environmental Resources (officials there actually coined the term “rain garden”), the developer designed each home lot to include a 300- to 400-square-foot planted drainage swale at a low point on the site. This cut costs to $100,000 from $400,000-plus for conventional runoff management. Homeowners are given information on how to maintain and update their rain gardens. The county estimates that runoff has been curbed 75 percent to 80 percent. And the effort proved to be economical as well. Officials estimated each rain garden cost a total of $500 – $150 for excavation costs and $350 for plant installation.

In Burnsville, Minnesota, last year, city officials wanted to explore new ways to improve water quality and reduce runoff in areas with existing conventional storm sewers. Residents demonstration neighborhood, responded enthusiastically to the gardens designed by Barr Engineering in Minneapolis. Working together, they helped plant their front and back yards with 17 separate rain gardens. The street curbs were retrofitted with slight dips to allow road runoff to flow into the gardens. A paired watershed study has shown that the Burnsville rain gardens reduce runoff volume into nearby Crystal Lake by 90 percent – decreasing erosion, improving water clarity and improving the health of fish populations. Barr Engineering places the project price tag at $8 per square foot for construction and $4.50 per square foot for homeowner education, design and construction supervision.

Kansas City, Missouri, is another city that has taken a proactive approach to bioretention by rain gardens. In 2005, the city launched its “10,000 Rain Gardens” public initiative. Directed by the Kansas City Water Services Department, the project’s goal is to encourage the installation of 10,000 rain gardens on public and private property in a five-year period. A Web site offers educational tips and resources, a list of area landscape contractors who have completed or who offer rain garden training, and a count of gardens registered by city and county.

Last spring, Bonnie and Carl Jannasch, a couple who’d just bought a home in a Brentwood, Tennessee, subdivision, hired GroWild to help them outfit their back yard with native Tennessee plants. Right away Berkley observed that their lot, located on a slight downslope in a cul de sac, presented some significant hydrology issues, so he suggested a rain garden as part of a native landscape redesign. The couple had never heard of rain gardens, but was immediately receptive. “We could understand their value,” says Bonnie Jannasch. “There was no doubt in our minds that we wanted to capture the water, put it back into the yard, and not let it run off into the sewer.”

Ground work: Go deep, get porous
Berkley observed that rainfall could leave the back yard bombarded by runoff both from the roof of the house and from a neighboring lot, from which water seeped in beneath a wooden fence. A nearby drainage ditch supplied a third potential source of runoff. Add to this a fair amount of tree canopy from a large sycamore, and the homeowners were looking at consistently moist ground conditions at best, flooding and bogging at worst. “What little turf grass there was back there was struggling because it was too shady and it stayed too wet,” Berkley notes. On the other hand, he recognized the yard’s potential as a prime dwelling for wildflowers and other shade-loving natives.

Before breaking ground, he consulted with Dodd Galbreath, then an environmental specialist with the Tennessee Department of Agriculture, with whom he’d worked on several large rain gardens at Ellington Agricultural Center in Nashville. They concluded that two small rain gardens would be more functional than a single large one at this site, due to sunny and shaded areas that required different plant communities.

Berkley connected the gardens by a shallow trough, or swale, about six feet long and one foot wide, so that the second, shadier basin acts as a catch valve for overflow from the first. In the event of very heavy rain, any remaining runoff that the second rain garden can’t absorb will flow over ground into a drainage ditch located just behind the backyard fence, and into the municipal stormwater system. “By the time it gets there, it will be filtrated, and flowing at a decreased volume and velocity,” Berkley says.

Berkley notes that it’s important to use surveying instruments to determine the grade of your site when planning the placement of a rain garden. In terms of shaping, sites should be longer than they are wide. “You can be creative,” he says. “They don’t have to have to be symmetrical. They can have a little oxbow or amoeba shape. The bottom line is that water enters into one or more sides.” In this case, concern for the roots of three dogwood trees helped dictate the size and shape of both gardens, which are about eight feet long and three feet wide.

The GroWild team’s first step was to gently grade the yard with a skid loader to encourage rainwater to flow naturally toward the gardens from the roof and neighboring lot. They also connected four downspouts on the back of the house to corrugated pipes placed underground, which feed into the garden area.

They then used a skid steer to dig down 21/2 feet – deeper than the gardens’ planned final depth. (Berkley notes that shovels would suffice for many small rain garden projects. But in this case, clay-rich soil and a stubborn limestone boulder made the light equipment essential.) Digging deeper than the level of the basin itself is crucial because it allows more benefit from the next step. “Really check the soil,” Berkley advises. “If it’s heavy clay, then you need to amend that soil to allow it to absorb.” The goal is the greatest possible permeability, he points out.

But keep an eye on costs. “If you spend all your labor dollars on digging before you put in plant material, you’re going to be outside the budget really quickly.” In this case, soil amendment and bed preparation added $300 to the final cost of the landscaping.

Generally, the site will dictate the amount of amendment required. “If it’s a very porous soil, you may not have to do anything but add compost,” Berkley says. “In [this] situation, the organic material in the soil was not bad, but it was heavy, typical Brentwood clay. So we dug that out and put in peat moss, sand and topsoil, and blended that together by tilling.” This new soil filled the excavated basin to a depth of 18 inches, leaving it ready for the addition of plant material – which, at $200 per garden, cost no more than what he would have used for a standard bed, Berkley says.

Rather than let all the removed clay go to waste, Berkley repurposed half of it into a gentle berm near the fence. The rest was added to the yard to further help channel water toward the rain gardens. Berkley also incorporated a stack of 6-inch by 9-inch pavers, left by the previous homeowners, into the bottom of the shallow connecting trough. There, they safeguard against erosion and add aesthetic interest in the appearance of a short path.

Not your typical back yard
Berkley selected eight herbaceous perennials for each garden. In the sunny rain garden he used colorful natives like Swamp Sunflower (helianthus), Blue Mist (ageratum) and Turtlehead – a wetland species with pink and white flowers. For the shady one that bisects the dogwood grove, he chose two varieties of Lobelia (Cardinal Flower and Big Blue), ferns and sedges. All plants were spaced about 18 inches apart, except for the Blue Mist which was planted tighter for a massing effect. Around the rain garden area Berkley installed several plants that thrive in damp conditions such as Winter Red Winterberry – a native whose abundant, bright red berries offers great winter interest. In the completed design the rain gardens are seamlessly integrated as gentle dips in the larger landscape, which is mulched with pine needles; these are less apt to float in wet conditions than bark mulch.

For this suburban site, Berkley selected only small plants, but he says that in larger rain gardens, native trees such as Bald Cypress and Southern Magnolias make excellent choices.

The Jannasches report that they’ve had fewer problems with drainage after rainfall, and they’re looking forward to watching the plants fill out and bloom in seasons to come. “We’re excited about it,” Bonnie says. “A small portion of the back yard is in grass, and the rest of it is in all these native plants. It’s not your typical back yard. You go back there and feel like you’ve cut a little segment of the forest out. The rain gardens add a different perspective.”

Their neighbors have been very curious about the rain gardens, and the Jannaschs happily show them off. “People will ask us, ‘What is that?'” Bonnie says. “As soon as we talk to them about it, they’re very interested.” Using landscaping to collect runoff – and ultimately recharge our aquifers – is “just a new idea to everybody we talk to,” she says. “It makes so much sense.”
–Susannah J. Felts

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