Go Green > Gardening with Rainwater

David Cedarholm, Durham’s town engineer, is passionate about rain gardens. On a stormy day—“the best time to see how a rain garden works,” he says— Cedarholm enthusiastically entices a visitor into the elements to see a sixteen-footby- ten-foot, stone-filled patch of ornamental grasses, irises and day lilies adjacent to the Durham Public Works (DPW) building.

Despite a howling wind and torrential rain, water isn’t visibly pooling in the garden—which is how it’s supposed to work, Cedarholm explains. “The water percolates down through the garden, and the overflow goes into a catch basin (in the parking lot),” he says. “People are amazed at how it works.”

Cedarholm started his garden by digging an eight-inch-deep “bowl”. He created a berm around it to delineate the area and keep water inside the garden when it rained.

Cedarholm is among a growing number of ecologically minded New Hampshire residents interested in using the water running off their roofs in their yards. Water conservation is the primary goal behind the rain gardening, where water runoff from roofs, yards and paved surfaces is absorbed in the garden. The water is filtered slowly through the garden’s plantings and into the soil, which cleans the water of pollutants before it reaches local streams, rivers and lakes via storm drains.

According to the Rain Garden Network—an online forum about rain gardening—these gardens can absorb between 30 and 40 percent more runoff than a conventional lawn, slowing the rush of a rainstorm and leavening its potential polluting effect on a community.

In addition to their practical applications, rain gardens add beauty and value to commercial and residential properties. “A rain garden makes your site more sustainable,” says Terrence Parker, principal of Terra Firma Landscape Architecture in Portsmouth and a designer of rain gardens for both commercial and residential use. “It’s more cost-effective in the long term, and the gardens are a visual amenity.”

Working With Nature—and Tougher Building Standards

Facing tougher state and federal regulations on potential pollution (commercial construction projects in New Hampshire must treat stormwater runoff from pavements and parking lots), developers are seeing rain gardens as an appealing water-management solution.

When Phillips Exeter Academy built three faculty houses last year in Exeter, the homes were designed with a rain garden in mind, says engineer Jeff Clifford of Altus Engineering in Portsmouth.

The runoff from the single-family homes is collected in a boomerang shaped rain garden adjacent to the school’s athletic fields. The garden— containing mostly wild, ornamental grasses—is designed to collect and “pond” water up to six inches before it’s filtered through the landscape and an underground drain carries the water away.

“It’s designed for a one-inch water depth,” Clifford says. The garden is between two feet and three feet deep, and there’s eighteen inches of soil— layers of loam, compost and sand—that filter the water. “It’s a perfect location because the soil is good and the layout of the project naturally leads the water away,” he says.

However, rain gardens aren’t limited to larger building projects. “Homeowners can build rain gardens on a small scale with ornamental appeal,” Parker says.

For example, Steve Lewis, an Atkinson land-use consultant and builder, has built two residential communities that incorporate rain-gardening techniques. The Village at Braemoor Woods in Salem features maintenance-free rain gardens, which add to the development’s appeal. “I use indigenous plants, like winterberry, in the landscaping,” Lewis says. “The gardens are meant to be a bio-cleaning fi lter, but they’re also very attractive.”

Building a Rain Garden

Fortunately for beginning gardeners, a rain garden doesn’t have to be sophisticated, but there are a few points to keep in mind.

First, locate your rain garden near the water runoff source. Cedarholm— who built the Durham DPW rain garden on Earth Day 2007 with his family—located the garden near the downspout of the building’s gutter. The most important time to treat stormwater runoff is during the first ten minutes of a rainstorm. “That’s when you get silt from the roof and junk off the road in the water,” Cedarholm says.

Second, be sure to consider the soil type. The area around the DPW’s rain garden has sandy soil that’s permeable and effective at filtering the rainwater. Cedarholm also put in layers of loam and compost to create additional filtering properties.

Third, use plantings native to New Hampshire climate and soils. Parker suggests native grasses, ferns, rushes, echinacea and black-eyed Susans as well as common plants seen in meadows, such as milkweed, aster, columbine, cornflower and irises. “You’re designing your garden for a dry situation, so plants have to be drought-tolerant,” Parker says. “Plants aren’t sustainable if they’re designed for constantly wet situations. They must be tolerant at both extremes.” Also, the local plants and flowers can become butterfly and bird habitats.

The Durham Garden Club gave Cedarholm the DPW garden’s plantings for free. Instead of bark mulch, which absorbs water, Cedarholm and his children placed dinosaur egg-shaped rocks found in a local quarry around the plants. “The plants we selected can tolerate flooding as well as droughts,” Cedarholm says. “The day lilies and irises flourished.”

The Many Missions of Rain Gardens

Cedarholm’s rain garden has proved successful on several levels. “We use [the garden] as a public outreach to discuss the town’s stormwater and water systems,” Cedarholm says. “I want everyone to know that this is doable and good for the environment.” He’s already planning a rain garden for downtown Durham as well as another behind the public works building.

For Parker, rain gardening is a simple way to help the environment and express individual creativity. “Everyone can do it at their own level,” he says. For gardeners and conservationists alike, that means a beautiful garden that puts water in its place.