Rain gardens

April showers bring May flowers — but only if the soil is friable and well drained and the rain is gentle enough that the soil can soak it up and make it available to growing roots.

When the rain comes fast and hard and the soils are compacted or covered with impermeable surfaces, the soils (and, ultimately, the plants) miss out on this much-needed nourishment.

To add to the problem, the water that doesn’t make it into the soil or the roots of plants runs off into ditch lines, culverts and storm drains that deliver it to ponds, creeks and rivers downhill or downstream. Typically, this runoff carries sediment, animal and plant wastes, pesticides, chemicals from automobiles and landscapes — and anything else that gets in its way. And this, my friends, is what our plant and animal friends — and we, ourselves — end up drinking.

Perhaps you’re wondering what all this has to do with gardening. But if you live under a roof, live in community and are concerned about your water supply, these are critical issues. As development continues and we become increasingly urbanized, our environment includes more and more impervious surfaces — which, in turn, create more runoff. Municipalities are busy creating complex infrastructures to handle the onslaught of this underutilized and largely abandoned resource. And we as individuals can also make a significant contribution to reducing storm-water runoff.

Installing a rain garden is one easy and direct way to handle runoff from a roof, parking lot or other impermeable surface before it can cause erosion or pollute a pond or stream. When water is diverted, it’s often a case of “out of sight, out of mind.” But why not put this precious resource to good use instead? Rain gardens, designed to hold water for a short period of time after a storm, feature plants adapted to such variably wet conditions. River birch, Virginia sweet spire, cardinal flower and blue flag iris are among the many possibilities for a rain-garden plant palette.

There’s a quiet, humble beauty to a rain garden. Besides creating additional gardening opportunities, these multipurpose systems increase the amount of water that soaks into the ground, helping replenish local water supplies. They help protect areas from flooding and drainage problems. They shelter wetlands and waterways from pollutants. Best of all, they’re beautiful and provide valuable habitat for birds and many beneficial insects

At this point, the skeptic in you is probably wondering about the two M’s: mosquitoes and maintenance. But rain gardens are designed so they dry out between rainfalls, rather than continuously holding standing water. And since mosquitoes need seven to 12 days to lay and hatch their eggs and the plantings offer enough diversity to create habitats for predators, mosquitoes shouldn’t be a problem. As with any other garden space, some weeding and watering will be needed for the first several years; but once the plants are established, little maintenance is required.

Siting and installing a rain garden does require some up-front planning. First, consider how it can be incorporated into the existing garden or landscape. You’ll also want to think about such typical design factors as views, current use of the area, garden shape, and plant textures and combinations. To be effective, a rain garden should be about one-twentieth the size of the area it treats. So if your roof is 1,000 square feet, the rain garden would need to be about 50 square feet (or roughly 5 feet by 10 feet).

A quick survey of the site will help avoid problems later and give you the best chance of success. Among the factors to consider are existing vegetation and slope. Site your rain garden downhill and away from the foundation of your house, in an area where flooding will not be an issue. You don’t want water seeping into the foundation and basement. Do not place a rain garden over a septic system or under a large existing tree (to avoid disturbing the roots). Flatter areas work best: They minimize the amount of grading needed, and they easily accommodate a small soil berm or raised area to hold the water. Smaller storms will create a temporary pond, but in larger storms, the water will flow right over the berm.

But the single most important factor for a rain garden is soil infiltration (how well the soil drains). Although some areas in the mountains have sandy soils with high infiltration rates, many sites will probably have heavier clay soils requiring the addition of a sandy soil mix. You may also need to install an underdrain. The North Carolina Arboretum’s Plant Professional Landscape Garden includes two rain gardens built using this method. Sited downslope from the Production Greenhouse, they’re designed to catch water running off the parking lot, road and neighboring lawns.

This project and others are the fruits of a recent partnership between the arboretum and N.C. State University, with grant support from the U.S. Environmental Protection Agency and the N.C. Department of Environmental Resources. The arboretum’s construction projects showcase many innovative water-management and conservation techniques, such as rain gardens, storm-water wetlands, infiltration strips, permeable pavement and stream restoration. The soon-to-be-completed Operations Center incorporates a variety of these features, as well as a vegetative “green” roof to help mitigate the effects of storm-water runoff. To learn more about these projects, visit the Arboretum’s Web site (www.ncarboretum.org). To learn more about N.C. State’s Water Quality Group and French Broad Training Center, or to subscribe to the Training Center’s listserv to be notified about upcoming educational workshops and tours, visit www.ncsu.edu/waterquality/frenchbroad/.

This topic is hot. Many state and federal agencies, universities and environmental groups around the country are doing good work on protecting water quality, and it’s only a matter of time before such measures become part of our urban way of life. So the next time we get a good, hard rain, step outside and watch where the water goes. Even if you’re not interested in building your own rain garden, you could at least advocate for them as part of the new development that’s undoubtedly headed our way. The soil, plants, animals and streams — not to mention future generations of people — will be glad you did.

[Alison Arnold is director of horticulture at The North Carolina Arboretum. She can be reached at: aarnold@ncarboretum.org.]