Right as rain: Rain gardens keep pollutants out of stormwater systems

Rain gardens keep pollutants out of stormwater systems

craig.sailor@thenewstribune.comMarch 12, 2014 

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Into each yard some rain must fall. And there it needs to stay.

At least that’s the goal of local environmental stewards who want to keep runoff out of stormwater systems and local waterways and put it into rain gardens where it can slowly seep back into the earth.

The urban environment is covered with surfaces impermeable to water: asphalt streets, concrete sidewalks and buildings large and small. The rain that hits those surfaces has to go somewhere. And along with it goes brake dust, fertilizers, pesticides, oil from leaky cars and soot from air pollution.

And that’s where rain gardens come into play. Think of them as a coffee filter for your yard. Rain flows off of your roof and into an area filled with plants that filter water and recharge the earth.

In an effort to reduce the impact on Tacoma’s stormwater system and reduce pollutants from Puget Sound and other bodies of water, the city is using a multi-tiered approach to encourage residents to build rain gardens.

A workshop on Saturday at the city of Tacoma’s EnviroHouse will cover design, siting and construction. In addition, the city is offering rebates to residents who build rain gardens in the Leach Creek and Flett Creek watersheds. (See sidebar.)

A rain garden consists of a slight depression, lined with gravel and filled with well-draining soil. Planted with mostly natives that can cope with both flood and drought, it takes in rainfall from pipes channeled from downspouts.

“The microbes in the soil help to eat the pollutants and some of the plants uptake the pollutants,” said Mike Carey, an environmental specialist with the city of Tacoma.

Rain gardens are not only functional but pretty as well, said Jessica Knickerbocker, an engineer with the City of Tacoma’s surface water division.

“It’s a nice amenity to add to your property. It definitely highlights one of our greatest attributes in the Pacific Northwest: The rain,” Knickerbocker said.

Knickerbocker helped make a rain garden at EnviroHouse, a demonstration eco-friendly property near the Tacoma Recovery and Transfer Center. She’s also helped build rain gardens at Cheney Stadium and along the recently refurbished Sprague gateway. She and Carey will be leading Saturday’s workshop.

Rain gardens consist of zones. The bottom-level plants are the most aquatic. But they also must tolerate periods of drying. “If you select the right plants, you should not have to water after the third year, except in extreme periods of drought,” Carey said.

Carey uses native red twig and yellow twig dogwood along with sedges and rushes in the bottom layer. The sides of the bowl should be planted with quicker establishing plants such as strawberries and grasses that help hold the slope. They need to be able to tolerate some excess water.

The highest portion of the rain garden would consist of typical landscaping plants. “You can use any of your standard ornamentals or natives. It’s intended to blend into your landscape,” Carey said.

Despite their occasional aquatic nature, rain gardens are not ponds. “If it’s designed right it will drain down within 24 hours,” Carey said. That’s not long enough for algae to grow or mosquitoes to hatch. “That’s why it’s important to design them,” Carey said.

A rain garden’s size is based on several factors: 1) How quickly the soil drains, 2) how much square footage is contributing water, 3) annual rainfall and 4) its depth. An 1,150-square-foot roof might use a 230-square-foot rain garden with an 18-inch depth.

One of the larger rain garden projects in the city will soon be on the University of Washington Tacoma campus. The city is collaborating with the school to build six 525-square-foot rain gardens along the Prairie Line Trail in the heart of the campus. It will treat stormwater from roads and developed areas upstream of the campus and will be completed by the end of the year. An even bigger rain garden will be built in Point Defiance Park in 2015 and treat a 720-acre area.

Homeowners need not worry about treating water that does not originate on their property or even all of the rain that does fall on their site. Rain gardens can be as small as 5-by-5 feet.

“You don’t need a circular pond-shaped area to build a rain garden. It’s possible to even use your side yard. You just send less of the water there,” Knickerbocker said.


Rain garden workshop

When: 10 a.m. Saturday (workshops will be repeated if they fill up).

Where: EnviroHouse, 3510 S. Mullen St., Tacoma at the Tacoma Recovery and Transfer Center (formerly the landfill).

Tickets: Free, open to residents of Pierce County; advance registration required.

Information: 253-573-2426, cityoftacoma.org/EnviroHouse, cityoftacoma.org/raingarden.

Resources: Rain Garden Handbook for Western Washington Homeowners can be found at raingarden.wsu.edu.

Craig Sailor: 253-597-8541

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