Study explores effectiveness of rain gardens

Although many rain gardeners swear by prairie plants and other native species, vegetation plays a lesser role than other factors in how well rain gardens trap storm water runoff and coax it into the ground.

So say the results of an ongoing experiment conducted by University of Wisconsin–Madison horticulture professor John Stier and graduate student Jake Schneider. Since the study began in October 2005, the pair has seen little difference in the performance of rain gardens planted with prairie species versus those sporting a lawn of common turfgrasses.

Instead, construction parameters appear to hold sway, including garden size and the presence of a water-retaining berm.

"What we’ve found so far is that berms are the main factor controlling runoff," says Schneider, whose award, the Terry and Kathleen Kurth Wisconsin Distinguished Graduate Fellowship, supports the work. "I think the bottom line is that if you put in a rain garden as a catch basin, it’s going to be incredibly effective, no matter what type of vegetation you have."

Homeowners hoping to do their part for urban water quality have made rain gardens one of the fastest-growing features of the home landscape. Positioned under downspouts, these shallow depressions intercept storm water that ordinarily flows off roofs and onto city streets, allowing it instead to seep into the ground.

The enthusiasm for rain gardens is easily seen on the Internet, where a quick search unleashes a flood of how-to guides. In place of the turgrasses people usually have in their yards, these manuals often recommend planting prairie species, whose deep root systems reportedly help water permeate the soil.

But this claim has gone untested for the most part, says Schneider; in fact, rigorous, scientific studies of rain gardens amount to barely a trickle.

"Native prairie plants are supposed to be the best. But why they’re the best, we don’t really know. Are they the best? We don’t know," he says. "We wanted to see whether turfgrass would be the same, better, or worse at controlling runoff and the nutrients contained in the runoff."

At UW–Madison’s turfgrass research facility, the O.J. Noer Center, Schneider built 16 rain gardens to Wisconsin Department of Natural Resources specifications, each of which receives drainage from a 200-square-foot section of rooftop. Four of the eight gardens that he sodded with turfgrass are also surrounded by a 6-inch-high berm; the other four are not. The same goes for the eight gardens planted with a mix of prairie species.

Since Schneider began monitoring the experiment a year and a half ago, the bermed gardens have consistently produced less runoff and allowed greater volumes of water to penetrate the soil. At the same time, plots planted with prairie species and those covered in turf have performed similarly, despite the difference in the plants’ root systems.

Although they can’t say for sure why this is, Schneider and Stier suspect it relates to plant density.

"The roots of turfgrasses generally aren’t as deep as the roots of prairie plants, but turf does have a higher plant density," says Schneider. "Especially in the early stages of rain garden development, like we’re studying here, plant density is going to affect infiltration, which is one of the keys both to decreasing runoff and increasing percolation."

Denser plant growth, he explains, prevents the "soil crusting" that occurs when rain drops striking bare ground disperse small soil particles. These particles then clog pores in the soil, reducing the amount of water that penetrates the surface.

Because water quality is also a concern, the scientists caution that they won’t know the full story until analyses of nitrogen and phosphorus in the runoff and soil "leachate" are complete. When present at excess levels, these nutrients act as major water pollutants.

"We want to look at nutrient loading because if rain gardens don’t produce a lot of runoff, but they still produce a lot of nutrients, you’re still going to have that nutrient enrichment of surface and groundwater," says Schneider.

In the meantime, says Stier, "I think Jake’s work is providing some good data for people who want to make sound, science-based decisions on the installation of rain gardens. Because previously there has been very little in the scientific literature about their effectiveness."