Gardens for a Rainy Day
Rain gardens are the hottest new landscaping trend. More aptly called storm gardens, these flower patches clean up gritty storm water.
By Tom Dickson
A few years ago, I was fishing the Mississippi River for walleyes below the Ford Dam, wading the shallows near the Minnehaha Creek confluence. A thunderstorm had rolled through the Twin Cities earlier that day, and as I systematically cast and retrieved my lure, drops of rain dripped off the tall cottonwoods onto my shoulders.
Thanks to state and federal laws regulating discharge of industrial and municipal waste, the Mississippi where I was wading usually runs clean and clear. Except when it rains. That's when oil, gasoline, fertilizer, dog waste, and other pollutants from roofs, lawns, sidewalks, driveways, streets, and parking lots get carried into storm sewers, streams, and eventually the river.
As I waded downstream toward the mouth of Minnehaha Creek, effluent from the upstream urban watershed gushed past me. I continued to cast until I looked down and noticed that the oily water was leaving a residue on my legs. I reeled in and headed home to shower.
Mary Nolte has never fished the Mississippi. But she's glad to know her rain garden-more accurately called a storm garden-is helping to keep pollutants from spoiling the great river downstream from her home in Minneapolis. The 8-by-5-foot garden, set in a slight depression partly bordered by large river stones, blooms extravagantly in summer. Around Joe-pye weed grow prairie phlox, black-eyed Susans, New England asters, grasses, and sedges. Buried PVC pipe channels storm water from her house's downspout into the garden, where it pools for several hours or more before seeping into the ground.
Previously, storms would send volumes of rainwater gushing from downspouts onto Nolte's lawn. Because lawns can typically soak up less than half an inch of precipitation from gentle rains, the storm water would pour in sheets off the lawn and into the street and then to nearby Minnehaha Creek or Lake Harriet. Now, the airborne pollutants that collect on roof shingles wash into the bowllike garden where plants can absorb them.
"I know my little garden won't save the environment," Nolte says, "but it's helping. And if more people around here had them, we could really make a difference."
Popular in Maryland, Seattle, and other parts of the East and West coasts, these attractive water-filtering systems are now showing up in yards around the Midwest, and the trend appears to be growing.
"Each year we see more developers and city engineers discussing rain gardens and more homeowners putting them in their yards," says Jay Riggs, an urban conservationist with the Dakota County Soil and Water Conservation District. "We're at the beginning of what's about to become a huge burst of activity all over the region."
Storm-Water Woes
Downpours cause the environmental damage that rain gardens help reduce. In a natural landscape, such as a meadow or prairie, storm-water runoff pools up in shallow depressions and wetlands before seeping into the aquifer. But in an urban setting covered with impervious surfaces-asphalt, concrete, roofing materials, and even lawns-storm water moves sideways, not down, picking up all kinds of contaminants, from gasoline and motor oil, to leaves and grass clippings, to lawn fertilizer and phosphorus-laden dust blown in from western Minnesota farm fields. When carried into lakes, the chemical pollutants can kill insects and fish. The excess nutrients create blooms of algae, which, when they die and decompose, use up dissolved oxygen needed by fish and other aquatic life.
Paved surfaces also increase water flow and speed of storm water, causing greater flood peaks and intensity. When conveyed into urban streams, the rushing torrents tear away at stream banks, causing massive erosion. "Pretty much every stream in the metro region has sections that have been blown out from storm-water runoff," says Riggs.
And because this fast-moving urban storm water doesn't get a chance to soak into the ground, groundwater levels decline, which leads to dry streams during drought.
There's more. Storm-water runoff picks up animal waste. According to the Maryland-based Center for Watershed Protection, fecal coliform levels in urban storm water are usually 15 to 20 times the recommended safety level for swimming.
The Mississippi River, where most of the Twin Cities' storm water ends up, is the source of drinking water for millions of people.
It doesn't take much asphalt and pavement to cause environmental harm. Studies cited by the Center for Watershed Protection show that when impervious surfaces cover as little as 10 percent of a watershed, streams show a sharp decline in mayflies, caddis flies, and other aquatic life. When they cover 25 percent of a watershed, an urban stream is "greatly impaired" and sees marked decreases in fish species diversity and even an elimination of fish altogether.
The amount of impervious acreage across Minnesota continues to expand. From 1986 to 2000, impervious surface coverage in the seven-county Twin Cities region increased 60 percent, according to Marvin Bauer, head of the University of Minnesota Remote Sensing Laboratory.
Simple Trick
The environmental harm of more pavement, asphalt, and roofing can be greatly reduced, say storm-water experts, if runoff can be intercepted before it reaches the storm sewer.
"The key is to break the connection between the hard surface and the storm sewer," says Fred Rozumalski, an ecologist and landscape architect with Barr Engineering in Minneapolis.
This can be done any number of ways, from the small homegrown solution of rain gardens, to the large-scale re-engineering of streets and parking lots.
The easiest big fix is to design parking lots to usher storm water into vegetated depressions. Once the water slows, sand and silt settle out. Then plants absorb nitrogen and phosphorus. Bacteria in the soil convert gasoline and oil into simple organic compounds before the filtered water seeps down into the groundwater below.
Among the public parking lots using this method are the Minnesota Landscape Arboretum in Chanhassen and the new visitors center at Dakota County's Lebanon Hills Regional Park (which also sports a new "green roof" covered in grass and soil that absorb rain and snowmelt).
In Brainerd, the Northland Arboretum plans to install two rain gardens this summer as part of a million-dollar expansion and remodeling project.
Some businesses are also redesigning to lessen runoff pollution. Land O' Lakes in Inver Grove Heights recently built an infiltration system and grassy swales to catch, hold, and filter storm water. So did Deluxe Business Systems in Shoreview. Schmelz Countryside Volkswagen in Maplewood repaved its lot with a permeable surface. And the colorful rain garden at H.B. Fuller Co. in St. Paul cleans runoff while providing bird and butterfly habitat.
Homegrown Bioretention
Generally described as any slight depression containing deep-rooted perennial plants that captures and holds runoff (a process called bioretention), rain gardens can be as large or as small as a homeowner wishes.
With more than 300 gardens planted since 1996, Maplewood is Minnesota's rain garden mecca. Over the past eight years as it repaves old streets, the city has offered residents the option of having new curb-and-gutter systems installed or going curbless and having the water drain off the road into boulevard rain gardens. Many neighborhoods voted for the rain garden approach, though not every home on a curbless block had to put in a garden.
"Most of our residents do it for the aesthetics," says Virginia Gaynor, horticulturist and open space coordinator for the city of Maplewood. "But the environmental benefits are also important to people."
According to assistant city engineer Chris Cavitt, the cost of putting in residential rain gardens is less than installing new curbs and gutters. The city pays to excavate the depression and buys the plant plugs. Homeowners do the planting.
"It was one of the best things that has happened to us since we moved here," says Michael Hafner, who, with his wife, Stacie, planted two rain gardens in 2000. "They're beautiful, and they attract all kinds of birds. You walk down the street, and it just looks like a friendlier neighborhood with all the gardens out front."
Most rain gardens are being planted in the Twin Cities, though a few are popping up here and there in greater Minnesota. Dan LaFrance, a landscape designer with Landsburg Landscape Nursery in Brainerd, says he installed three rain gardens last year and hopes to do more as people learn how they work.
"There's a lot of potential up here with so many people moving in and around lakes," he says. "Rain gardens are a beautiful solution to water-quality problems."
Stumbling Blocks
Rozumalski, who has designed dozens of rain gardens, says the colorful plantings don't need to be wild looking to help the environment.
"The messiness issue is a big one with a lot of homeowners," he says. "Not everyone likes the fuzzy look. If people want to use cultivars and maintain tidy and clean edges, then I think that's fine. These are urban settings, after all, and the main goal of the gardens is to get the water clean."
Another concern of homeowners: Will rain gardens produce mosquitoes? "They don't," says Rozumalski.
"Mosquitoes need several days of standing water to reproduce, and rain gardens are designed to dry up before that." In fact, he says, rain gardens often attract dragonflies, which eat mosquito larvae.
Attracting wildlife was one of many reasons Burnsville homeowner Dianne Rowse designed her own rain garden in the fall of 2002. As part of her street reconstruction, the city excavated a shallow basin in her yard and replaced existing clay soils with well-drained ones. Then Rowse used the Department of Natural Resources' Restore Your Shore CD to select plants suitable for her 17-by-20-foot garden.
"I just popped in the CD, pulled up a plant list, entered various parameters, such as not wanting anything over 3 feet tall, and it told me the plants I needed to get," she says.
Now in its second growing season, the rain garden will soon be abloom with prairie blazing star, cardinal-flowers, wood-lilies, and wild geraniums. Indiangrass, ironweed, and path rush add texture and diversity. High-bush cranberry bushes provide windbreaks and fruit for wintering birds.
"I really wanted a lot of color in summer, and I like that the garden attracts songbirds and butterflies," says Rowse. "I also like the look of the grasses and seed pods in the winter. The birds are attracted to the seeds, which keeps them here year-round." An unexpected bonus: "The rain garden holds the snowmelt too."
Do They Work?
Rain gardens can be beautiful. But do the planted basins actually help keep water clean?
According to the Center for Watershed Protection, several studies on the East Coast have shown that rain gardens capture pollutants and keep downstream lakes and streams healthier. And local research indicates so too. A 1998 study by the Ramsey-Washington Metro Watershed District found that, in one year, the H.B. Fuller rain garden captured 55,000 cubic feet of untreated storm water and 500 pounds of leaves, grass clippings, and other matter that would have entered a nearby wetland.
A current study funded by the Metropolitan Council is comparing lawn phosphorus runoff from several Burnsville streets with 17 rain gardens to streets without rain gardens. All of the streets drain into Crystal Lake, which has become increasingly algae-laden during the past several decades.
Meanwhile, observation strongly suggests that rain gardens do effectively retain and filter contaminated rainwater.
"During a big rain, you drive down a street [in some parts of Maplewood] and see water pooled in the [rain] gardens," says Cavitt. "Then you come back later in the day, it's all gone, seeped down into the ground. That's water that would have washed down the streets and into the storm sewer."
As communities across Minnesota ooze outward, more concrete and asphalt cover the land. Consequently, storm water has fewer places to go. Rain gardens won't solve that problem entirely, but they're an easy and attractive way for homeowners to help lessen the water-quality problems their sidewalks, driveways, and roofs create.
Tom Dickson, longtime contributor to the Volunteer, is editor of Montana Outdoors.