Building a rain garden in the home landscape contributes to the health of streams and wetlands by collecting, absorbing and filtering stormwater runoff from roofs, driveways and lawns.
What they are Rain gardens are basically depressions in the landscape that are dug out and partially refilled with a composted soil mix that allows water to soak in rapidly. Typically, the finished garden sits 6 to 12 inches below the surrounding soil level to allow for temporary ponding. Moisture-loving hardy plants, often native varieties, are planted in the depression for an attractive appearance. Rain gardens are the workhorses of low-impact development (LID), a new approach to managing stormwater runoff.
Why they’re needed Unchecked storm runoff carries pollutants – such as oil from driveways and fertilizers and pesticides from lawns – into storm drains, which eventually feed into wetlands, streams and Puget Sound. Besides filtering these pollutants, rain gardens help recharge groundwater sources, provide habitat for beneficial insects and birds and reduce flooding on neighboring property.
Design basics When sizing a rain garden, homeowners should consider soil type (fast or slow draining), slope and the square footage of any impervious surfaces (like driveways and roofs) that will drain into the garden. Runoff can be directed into the garden through a pipe, across landscaped areas or via a swale. Choose a garden shape that suits your style and landscape constraints. During heavy storms, a rock-lined overflow channel will direct excess water to the storm drain or across the landscape.
Right time Fall and spring – when the ground is moist but not saturated – are the best times to build a rain garden. Digging while the ground is wet can compact the soil and reduce its ability to soak up water; digging during the winter can create erosion problems during big rainstorms.
Learn more Detailed directions on sizing, locating, building and planting rain gardens is available in the “Rain Garden Handbook for Western Washington Homeowners,” written by Curtis Hinman, a watershed ecologist and member of the WSU Extension faculty. The handbook is available online at www.pierce.wsu.edu.
Jean Parietti is a Federal Way freelance writer. E-mail her at southsoundgarden@aol.com.
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