Rain gardens can reduce polluting water runoff

The rainy season is upon us, and often when it rains, it pours. Heavy rains can turn your landscape into a soggy, muddy mess and lead to more serious environmental problems such as flooding, erosion and water pollution. In fact, storm water runoff is the greatest source of pollution of Hawaii’s streams and coastal waters.

Storm water runoff is created when rainwater flows over land and impermeable surfaces, such as rooftops, roads, driveways, sidewalks and parking lots, rather than seeping naturally into the soil. As the runoff washes across the landscape, it picks up contaminants, including fertilizers and pesticides, exposed soil particles, motor oil, solvents and other pollutants, and carries them into storm drains, which feed directly into streams and the ocean, where they pose a risk to people and aquatic life.

Storm water runoff is a problem of our own design. In shaping the places we live, we have drastically altered the way water flows through the watershed.

By replacing natural vegetation with impermeable surfaces, we have impaired the land’s ability to absorb and filter rainwater.

Native forests, shrublands and grasslands are highly efficient at capturing, absorbing and filtering rainwater. Natural vegetation intercepts rainwater, slowing its fall to the ground, while healthy, undisturbed soil acts like a sponge, soaking in and filtering the rain before gradually releasing it to groundwater.

In developed areas, rain flows very rapidly over the landscape. Impermeable surfaces and even turf, with its shallow roots and underlying compacted soil, produce considerably more runoff than undeveloped areas with deep-rooted plants.

Designed to mimic natural systems, rain gardens are a simple, low-cost way to reduce the amount of runoff your property generates and restore natural water flows through your landscape.

Rain gardens are simply excavated landscape features, planted with native perennial plants, that are engineered to capture runoff and release it gradually into the soil. Rain gardens should be downslope from a runoff source, such as a downspout or a driveway.

In addition to reducing runoff and filtering impurities, rain gardens can reduce the risk of floods, protect property from water damage, provide habitat for beneficial insects and birds and enhance the beauty of your yard.

Next time it rains, watch the way water flows through the landscape and look for signs of soil erosion, standing water or low spots and depressions that remain moist after it rains.

You may be a good candidate for a rain garden. For guidance on how to design and install a rain garden, refer to the Hui o Ko’olaupoko Hawai’i Residential Rain Garden Manual, available at your local library or online at www.huihawaii.org/rain-gardens.html.

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Kim Perry is a junior extension agent and Kauai Master Gardener Coordinator with the University of Hawaii Cooperative Extension Service in Lihue. Email her at kperry2@hawaii.edu.