In the garden: Why have a rain garden in Washington?

Dana Kelley Bressette

guest columnist

The Puget Sound was one of the featured waterways recently covered in the PBS Frontline special “Poisoned Waters.” Unfortunately, I missed the program, which aired the evening before “Earth Day.”

The Frontline Web site emphasized that, although Puget Sound may look pristine and beautiful on the surface, there are many chemicals severely impacting marine wildlife and the food web underneath.

In the past, the biggest polluters were big industrial companies. Today, most of the pollution comes from small commercial and residential developments; much of it from seemingly innocuous sources, such as beauty products, medicines and household cleaning products that are flushed into our sewers and storm drains.

A rain garden is one of the strategies now being used in the field of “low-impact development,” along with permeable paving, rainwater collection systems and living roofs.

Rain gardens are specially designed to filter pollutants and reduce the amount of water that runs off into storm drains and roadside ditches. Having a place for rainwater to go also may reduce damage caused by flooding after severe storms.

The first step to building a rain garden is to determine the best locations in your yard where water naturally flows. Test the soil to find out how well it drains.

Check out any county or city requirements before you begin — especially for new construction. Many local governments are starting to require the installation of rain gardens for new construction projects.

Also, call before you dig to make sure there are no underground utilities at the site.

The second step is to plan the size and shape of your rain garden. By calculating the square footage of impermeable surfaces, such as driveways and roofs, you can determine the percentage of rainwater that will be held or absorbed, depending on the size of your rain garden and how well your soil drains.

To build your rain garden, you will need to excavate between 18 and 30 inches of soil. If the existing soil doesn’t have too much clay, you may incorporate compost and reuse it; otherwise, you may need to purchase an appropriate soil mix.

Replace enough soil so you leave about 6 inches of depth for ponding. Create an entry for the water, and make sure any pipes are appropriately sloped. Provide a rock-lined overflow.

Although fall and winter are generally considered to be our best planting seasons, it’s recommended to plant a rain garden in the summer, so rainwater doesn’t turn your project into a mucky mess.

Select plants for three wetness zones. Native plants are a good choice, and they’re likely to be drought-tolerant in the summer. After you plant them, mulching helps keep the soil moist, adds organic matter, discourages weeds and prevents erosion.

Your rain garden should be irrigated the first two to three summers in order to encourage healthy root systems. Re-apply mulch to any bare areas. Make sure the inlet and outlet remain clear of debris.

Do not apply fertilizers or pesticides; the soil mix should provide adequate nutrients.

Many organizations have been sponsoring seminars in the greater Puget Sound area. For information on how to register for the Gig Harbor session on June 15, visit www.stewardshippartners.org/raingarden_workshops.html.

Also, there are several downloadable publications at www.pierce.wsu.edu/Water_Quality/LID.

In the Garden columnist Dana Kelley Bressette can be reached by e-mail at skyenergy@centurytel.net.