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Water pollution prevention is the most effective way to protect our most valuable resource, Kraus says. "Once pollution is in water, its expensive and hard to clean up, but soil and plants do it perfectly," Kraus explains. Her solution—Grow A Rain Garden. Rain gardens are plant beds grown in recessed areas where water naturally flows. "The premise is that you get the polluted storm water into the garden and the soil holds it so that the plants can take up the nutrients," she says. Kraus and her team have been researching how to optimize rain gardens by investigating how soil substrates—soils mixed with minerals—are able to collect and hold polluted water.
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Mineral content is important because minerals dictate a soil's texture and consistency. "Soil consists mostly of minerals—about 45 percent," explains Deborah Martin, author of Rodale's Basic Organic Gardening. "Tiny particle of rock, categorized by their size as sand, silt, or clay, make up the mineral portion of the soil. The percentages of each of these particle types can have a significant effect on soil drainage and fertility."
The researchers looked at three common substrates—sand, slate, and a soil boosted with organic matter. They found that all three soil types supported plant growth well while slate removed the most nitrogen and phosphorus, sand had a good removal of phosphorus but not nitrogen, and soil wasn't quite as good as either of the other substrates tested.
Fortunately, despite its lower performance, Kraus still recommends organically boosted soil as a fine substrate for homeowners wanting to grow rain gardens. "It's more of a concern in a commercial situation, but in a home situation it's acceptable to just add compost to soil," she says. "It's still better than not having a rain garden at all."
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Kraus recommends using low-nutrient waste—like lawn clippings or fallen leaves—instead of cow or poultry manure to boost the soil in your rain garden—to create a compost pile just make sure that you don't use any of the 7 Things You Should Never Compost. If you have a lot of clay in your soil, she also suggests adding pea-gravel to allow water to infiltrate the soil quickly. "During a hard rain, this garden will get hundreds of gallons at a time and you want to let it get as much as possible," Kraus says.
Plants are the other half of the equation. "The engineers who designed rain gardens hadn't thought about it from the plant's point of view," Kraus says. Plants in rain gardens have two challenges—first, they only get watered when it rains, and second, the nitrogen and phosphorus levels in the polluted water are actually lower than most plants typically need—so they need to be able to survive on low nutrients.
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Kraus' top three plant recommendations are Dura heat River Birch—Betula nigra duraheat—Virginia sweet spire—Itea virginica—and narrowleaf sunflower—Helianthus angustifolius.
"Every landscape should have a rain garden," says Kraus, "because we're all making micro-pollution events—our rooftops and driveways are making pollution. If we all do a little bit, we'll make a big impact, but if we wait for someone else to deal with, it won't be as big as an impact because pollution happens at so many points. It's a neighborhood concept."