SEATTLE —- Three months ago, Karrie Kohlhaas didn’t know what a rain garden was. Now she has one in her front yard and there are nine others on her block.
That’s how fast rain gardens are spreading around the Seattle metropolitan area.
The gardens are part of a campaign by Washington State University and the non-profit Stewardship Partners. Their goal is to install 12,000 rain gardens in Puget Sound communities by 2016. So far, more than 700 gardens have been installed (see a map of them) and more are being added every week.
“Twelve thousand gardens will absorb approximately 160 million gallons of stormwater each year,” said Stacey Gianas, who is with Stewardship Partners.
That much water would fill 250 Olympic swimming pools. And its stormwater, which washes over roofs and streets, picking up all kinds of pollution. Usually that contaminant-filled water runs into storm drains that empty into waterways and rivers.
One goal of the campaign is to strategically build clusters of gardens in neighborhoods near streams and marine shorelines that are known to suffer from substantial stormwater runoff.
For instance, Longfellow Creek, which cuts through the North Delridge neighborhood, is so polluted by stormwater runoff that 88 percent of Coho salmon that return to this urban stream die before they can spawn, according to a 2003 King County Stormwater Services report.
Karrie Kohlaas, who lives in North Delridge, discovered rain gardens in an email.
“I didn’t know what a rain garden was,” Kohlhaas said. “But I thought it sounded interesting so I just stopped what I was doing and picked up the phone and called Stacey.”
Kohlhaas and Gianas met the next day and took a walk down Kohlhaas’ street, 25th Avenue Southwest, and started talking to neighbors about rain gardens.
How well do rain gardens work?
The new Washington Stormwater Center is doing the research. Read more about it.
Gianas explained that rain gardens look similar to most gardens, except they’re neither raised nor flat. They’re built in a shallow depression, like the shape of a bowl. Rain gardens take in stormwater runoff from impervious surfaces like roofs, driveways, walkways and parking lots. Once the water reaches the rain garden, a mixture of soils absorb the water and strain it like a sieve, filtering out sediment, heavy metals, oil, grease and other pollutants.
“Rain gardens mimic a forest because they’re built to have layers of vegetation and thick spongy soils,” Gianas said. “There’s not room to put a forest back into the urban area, but rain gardens are like mini forests and the more rain gardens we build, the more of an impact they can make.”
Rain gardens are not meant to hold water like a pond or a wetland, Gianas explained. A properly functioning rain garden will hold water for no more than a couple days after a storm.
During her visit to North Delridge, Gianas conducted a soil drainage test, also called a “perc test,” which is the first, and possibly most important, step in building a rain garden.
Gianas found that the neighborhood had well-drained soils, so she applied for a Community Salmon Fund grant through the National Fish and Wildlife Foundation on behalf of the neighbors to pay for the gardens. The hope is that this cluster of rain gardens will serve as a model for other residents in the area, Gianas said.handbook on building rain gardens from Washington State University