Rain gardens deal with environmental issues

A rain garden is a deceptively simple-looking element. A swale backed by a berm, a rain garden may appear as an island of plants or look like a pond in the process of going back to dry land.

But beneath the vegetation, there is a careful composition of soil and a series of drains that are constructed to filter and hold the runoff that comes from impervious surfaces.

Rain gardens deal with several environmental issues. They catch and slow stormwater drainage, drastically reducing erosion and the clouding of streams. They capture and filter pollutants so that they are not fed into waterways — and eventually reservoirs. And they add to the beauty of the landscape and provide wildlife habitats while often reducing landscape maintenance.

The folks at the Unitarian Universalist Fellowship of Winston-Salem on Robinhood Road have embraced the concept in a big way. Their bio-retention watershed is 87,934 square feet. Almost 4,000 of those feet are impervious surfaces. That includes a roof, parking lot and pavement that direct water beyond the building to an area sloping rapidly toward Mill Creek. Mill Creek leads to Muddy Creek, and Muddy Creek to the Yadkin River.

Originally, the site used a large drainage pipe to capture the water and direct it to an open embankment. The water was soon cutting a nasty gouge into the landscape.

That is when Joan Wright got involved. Wright is a Davidson County master gardener and UU member. She served as chair of the grounds team and was sort of the general contractor for the project.

Wright attended a workshop by Linda Birdsong, coordinator of the Community Conservation Assistance Program with the Forsyth County Soil and Water District, and Wendi Hartup, an Extension agent and environmental specialist. She discovered that a rain garden was the solution for the UU drainage problem.

If CCAP approved the UU project, Wright knew that 75 percent of the cost would be refunded. An anonymous donor put up the $45,000, and the CCAP program refunded $33,750.

The goal of CCAP is to reduce the flow of runoff from roads, parking lots and other nonpoint sources of pollution into the streams, creeks and rivers of the state.

CCAP is a voluntary program designed to improve water quality by using techniques on land that is not directly involved in agriculture.

Soil and water-conservation districts provide technical, educational and financial assistance to landowners involved in projects such as impervious-surface conversion, stream-bank stabilization, stream restoration, backyard wetlands, rain gardens and others.

Birdsong said this project ranked high for approval because of the size of the watershed, the percentage of impervious surface area, the site's proximity to Mill Creek and because of the educational component at the church.

Because of the size, the site was designed by engineer Daphne Cartner with the Division of Soil Water Conservation. Any project more than 2,500 square feet has to be designed by an engineer.

"This garden was designed to hold the water and release it within 48 hours with energy dissipated and the water clean," Hartup said.

To do this, the gathered water is drained from a culvert into a "plunge pool." The plunge pool is an area of rock that breaks and funnels the water, slowing its velocity.

The water then moves into the rain garden. Four feet of native soil has been removed to form a large basin, which is surrounded by a 4-foot berm. An underdrain bed is composed of washed stone, a gravel varying from ¼ to ¾ inch in diameter. Perforated drain pipe lies below this. A carefully calculated soil mix is placed on top of the gravel at a depth of 3 feet. This loamy sand media has to have a certain pH and a phosphorus index of less than 30.

The object is to enable the soil to absorb phosphorus for 50 years. Nitrogen and phosphorus can be accounted for in a rain garden. The estimate for a project this size is that the site will remove 5.13 pounds of nitrogen and 0.57 pounds of phosphorus per year. Excess nitrogen causes algae blooms, which blocks out sunlight and kills the biology of the stream. Excess phosphorus causes eutrophication, a reduction of dissolved oxygen in the water. In both cases, the life in the water is threatened.

Birdsong and Hartup came out to inspect the finished project during a torrential downpour. They were pleased to see that the water leaving the rain garden and entering the flow to Mill Creek was running clean and clear. Much of the work of cleaning the water falls to the plants in the garden.

Hartup came up with the planting design.

"You need to have plants that can be inundated with water for 24 hours and are also drought-tolerant," she said. "The congregation didn't want anything to look weedy."

The plants are a mixture of natives, such as Joe Pye weed, sweet pepperbush and New York Asters. There are also unexpected plants, such as butterfly weed and dwarf yaupon holly.

"The plants were all contributed by people in the congregation who gave money or donated plants," Wright said. "We had big, big planting days," she said. "We knew we had to plant two trees, 10 shrubs and a couple of hundred other plants, and then we had to mulch it all. We had a brigade bringing down the mulch in wheelbarrows." The water garden, which took three weeks to build, is about 4,000 square feet.

That mulch proved to be one of the more difficult elements. A woodchip mulch floats when the water comes flowing in. It has to be put back in place several times until the garden settles.

"A shredded, hardwood mulch retains moisture and shelters the plants," Hartup said.

After a period, the mulch starts to settle down and the plants take on a major role in the stabilization and cleansing of the runoff. Plants are placed according to their ability to withstand periods of immersion.

Baptisia, planted on the sloping edge of the garden, can take two days of immersion while sweet pepperbush can handle three days.

"You are not supposed to fertilize, so you have to be a little more patient with these plants," Hartup said. "It can take up to three years before plants are established and showing their true potential."