USGS: Rain Gardens Work Regardless of Soil Conditions

Five-Year Study Shows Native Prairie Grass Outperforms Turf

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Posted: Monday, February 21, 2011 11:31 am

A five-year study by the U.S. Geological Survey's Wisconsin Water Science Center has confirmed that even simple, residential-style rain gardens can capture nearly 100 percent of stormwater runoff from the roof of an adjacent building, even in thick clay soils. The key in temperate climates is to use native prairie vegetation, the long roots of which can penetrate the silt/clay layer of the subsoil, according to the study.

USGS Hydrologist William Selbig presented the results of the study at the 4th Annual "Put a LID on It" Low-Impact Development Workshop in Dubuque, Iowa, on Thursday. The study was conducted in Madison, Wis., and the USGS issued its Scientific Investigations Report in 2010.

Two rain gardens, one planted with turf grass and the other with native prairie species, were constructed side-by-side in 2003 at two locations with different dominant soil types, either sand or clay. Each rain garden was sized to a ratio of approximately 5:1 contributing area to receiving area and to a depth of six inches, according to the report.

Selbig said the study confirmed that small rain gardens, even when the ratio is increased to 10:1, are extremely effective in preventing rainwater from running into nearby rivers and streams, especially when native prairie plants are used.

"One of the key findings was that the prairie grass roots penetrated the clay," Selbig said. "Some thought the roots would just spread out laterally when they hit the clay, but that didn't happen."

Infiltration rates steadily improved throughout the study, as the long roots of the prairie plants pushed deeper into the subsoil, he said.

"This study was unique because it looked at typical residential applications," Selbig said. He said the small gardens were constructed by the city of Madison following standard specifications written specifically for homeowners. While two of the four cells in the study were planted with typical turf grass, the other two were planted with 18 varieties of wildflowers and prairie grass.

"Some people tend to lean on the cynical side and say, ‘It's just a bunch of weeds, and why would I want that?' But it doesn't have to look like that. There are all sorts of varieties of native plants," Selbig said.

Not surprisingly, the rain gardens in sandy soil performed superbly, even with the turf grass on the surface. But, it was the gardens in the clay soil that made the prairie grass stand out. Selbig said a large rain event in early May of 2004, when five inches of rain fell in a day and a half, gave the rain gardens their biggest challenge. During that event, the stormwater overflowed the banks of the turf grass swale in the clay soil, while the prairie grass absorbed it all at an infiltration rate as high as a half inch per hour.

In the fifth year of the study, the USGS routed all the rainwater from the roofs to the prairie grass cell of the rain garden in clay soil, in order to stress the native vegetation even more. The results, Selbig said, were that even with a contributing/receiving area ratio of 10:1, the garden performed very well.

When the experiment was over, scientists dug up the rain gardens to study the soil underneath. That's when they discovered the cell containing the turf grass, with roots penetrating only a few inches down, showed signs of water ponding above a thick layer of clay only two feet below the surface, and the native prairie grass roots were penetrating the clay.

The bottom line, Selbig said, is that rain gardens work regardless of soil conditions, and native prairie vegetation is the best ground cover to use.

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