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How Does Your Garden…Drain?

Vegetated swales are integrated into parking lots to infiltrate and treat a portion of stormwater volume.

When it comes to rain, it is well known that the Pacific North“wet” is at the top of the list for soggy weather in the continental United States and southwest Canada. With increasing urbanization and development, the rain that nourishes lush greenery now runs over and off concrete and asphalt sidewalks and streets, scouring the various oils, pesticides, and other contaminants into stormwater drains. This runoff then travels unfiltered into streams and rivers, not only polluting waters, but causing erosion and mudslides, depleting groundwater sources, flooding low-lying areas, and damaging property. According to the Puget Sound Partnership, stormwater is the number one polluter of Puget Sound and has harmed salmon, trout, and shellfish. It’s a no-win situation.

Biofiltration—used in low impact development, best management practices—is the win-win solution that, if built correctly, helps remove pollutants from stormwater runoff, nourishes plants, and helps filter clean water into local waterways and groundwater. A process in which the landscape, plants, and soil are used to filter out pollution, biofiltration construction is useful in minimizing runoff and drainage problems and can be a beautiful addition to landscaping. Using appropriate vegetation, and depending on how the water is collected and distributed, biofiltration can also reduce the cost of landscape maintenance by using rainwater rather than piped water to irrigate.

One type of biofiltration is a bioretention area, popularly known as “rain gardens.” Used in smaller sites of about 5 acres or less, a rain garden is a depression in the ground that filters or “percolates” the runoff vertically (instead of horizontally, such as in bioswale channels) through layers of carefully prepared mulch and soil, and then often into a storm drain. The advantage of a rain garden is that it can be installed in almost any soil or landscape that has a relatively shallow slope. The native soil has to be evaluated carefully so that the right soil, gravel, mulch, plants, and other filtration devices can be properly layered for it to work.

Regardless of the type of biofiltration, care must be taken to select plants for the rain garden’s microclimate. Though the Pacific Northwest is known for being very rainy, not all areas have the exact same conditions of soil, moisture, wind, and other environmental conditions. In addition, the depth and size of a rain garden or bioswale and how much water drains into it, how much it retains, and seasonal variations are additional considerations. Both native and climate-appropriate plants can be used; although for easy maintenance, native plants are preferred. It’s also important that the plants are chosen appropriately for the three zones of a rain garden: the upper edges, slopes, and bottoms, because of the different moisture conditions of these three zones.

If all this seems like a lot of work, in the long run, it’s not. Once the biofiltration area is established, it requires much less maintenance than a lawn because it doesn’t need to be mowed, fertilized, or watered. It also saves money. According to the U.S. Environmental Protection Agency, use of native or climate-appropriate plants can decrease outdoor water use by 50 percent, which can mean a considerable decrease in water bills.