Photo by Heather Arndt Anderson

Rain gardens are a popular way to clean city stormwater before it hits sewers and waterways, but now cities like Portland are offering free greenery and landscaping as part of the deal

Dakota Kim  – January 8, 2020 | Updated January 10, 2020

Rainwater runoff pollutes urban sewers, but some cities in the West are turning to a win-win solution that will both clean the water and build beautiful rain gardens for homeowners.

A little background: When it rains or storms, the water flowing downward collects bacteria, metals, pesticides and chemicals, oils and other pollutants as it makes its way to the sewers of a city. Once this water enters storm drains, it will eventually enter our waterways and have a significant impact on urban waterway appearance and health. Urban runoff is the most significant human cause of bad water quality in ocean shoreline areas, per a National Water Quality Inventory 1996 Report to Congress. And during snowmelt and big rainstorms (a.k.a. “high precipitation events”), the excess input to sewers can lead to combined sewer overflow (CSO) out of wastewater treatment facilities, essentially, flushing raw sewage straight into rivers and streams.

A rain garden, also known as a bioswale, is the smart win-win solution Los Angeles and Portland are adopting to filter their water. Vegetation is planted in a small depression on a slope, and the roots of plants and trees literally filter out pollutants in the water before it can reach storm drains. Though a rain garden is usually dry, when it rains, it does its job efficiently. 90% of chemicals and 80% of sediments are removed, and 30% more water is soaked into ground aquifers than conventional lawns. The rain garden will even drain within 12 to 48 hours, preventing mosquitoes from becoming a pest. They’re attractive, too; pictured above is the rain garden installed in the front yard of Sunset garden editor Heather Arndt Anderson (photo taken in winter).

The word is out on bioswales, and their popularity is spreading. The University of California, Santa Barbara has installed rain gardens in their biodiversity and ecological restoration center. Patagonia has installed bioswales at its headquarters in Ventura, CA. The San Luis Obispo UC Cooperative Extension has transformed its parking lot into a bioswale demonstration. And Westwood in Los Angeles has created bioswales in order to improve water quality in the Santa Monica Bay.

But the biggest creator of bioswales in the West, and the city that’s giving free native greenery to its citizens and even landscaping it into their rain gardens, is a rainy one: the City of Portland. The past year has seen a major sewer system overhaul in the city, and the next phase of the project will be the installation of rain gardens free of charge for citizens—with folks even getting to choose their plants. In all, the project will add 83 green street planters to filter 7.1 million gallons of runoff before it enters the sewer system, in addition to replacing four miles of century-old sewer pipe.

The Portland City Council approved a Green Streets Resolution, Report, and Policy in April 2007, and has been implementing it ever since. The city is currently in the middle of its years-long sewer replacement project, adding rain gardens and bioswales as it goes along. The Portland project team is currently working in southeast Portland; it will be holding two open houses to discuss the new bioswales, or “green street planters,” as Portland is calling them. If the city isn’t planning on building the green planters in your neighborhood, you can apply for a % for Green project grant.

If you want to DIY a rain garden, follow this instructional for building your own bioswale. Talk to your local town or city council to find out how to petition them to pay for a bioswale in your yard, and check out our guide to the best rain garden plants.