Get Ready for ‘Damp Season’ with a Rain Garden

This year’s dry summer may have all but erased your memories of Seattle’s rainy winter weather. However, now is the perfect time to start prepping your garden for fall and winter downpours with a rain garden!

Residential Rain Garden in Seattle. Photo: Ashley Blazina, 2015

Residential Rain Garden in Seattle. Photo: Ashley Blazina, 2015

If you’ve ever walked through a wooded park on a rainy day, you likely noticed that the rainfall is much lighter under the protection of the trees. Their branches and leaves catch and slow down the rainfall, which is an important step for the environment. The tree’s roots and the surrounding soil act like a filter for the rain as it slowly trickles into the ground.

As Seattle has grown, many trees have been replaced with paved surfaces like driveways, sidewalks and streets. When rain falls on the pavement, it rushes into storm drains instead of slowly soaking into the ground. Along the way, it picks up oils, sediments, lawn chemicals and other pollutants. This polluted runoff ends up in our neighborhood creeks, Lake Washington, Lake Union, and Elliott Bay. When our nearby lakes are polluted, they’re less habitable to salmon and unsafe for swimming.

Residential Rain Garden in Seattle. Photo: Ashley Blazina, 2015

Residential Rain Garden in Seattle. Photo: Ashley Blazina, 2015

There are ways that we can help bring nature back to the city, though – and one of them is planting a rain garden. Rain gardens are shallow, planted depressions that mimic a forest’s way of slowing down rainfall. They’re filled with soil and compost and planted with native plants. As rainfall percolates through the rain garden soil and plant roots, pollutants are filtered out – which helps keep our groundwater and lakes clean.

Sound too good to be true? That’s not all they do. Adding a rain garden can also attract birds and butterflies. When rain gardens are planted with native species like snowberry and Oregon grape, birds are drawn in. Flowering plants, like red-flowering current, are magnets for butterflies and hummingbirds.

If environmental benefit, butterflies and birds aren’t enough, your very own rain garden will also beautify the landscape. Not much time for yardwork? Rain gardens are still effective when they’re small, and the compact landscaping can greatly improve the curb appeal of your house.

Oh, and one more thing – rain gardens can also reduce the potential for basement or other household flooding, which may help increase your property’s value. With a more beautiful garden and a higher property value, it’s a true win-win situation!

Rain Garden Basics

Rain gardens can take time to develop, but if done correctly, they can provide great benefits for your yard, local wildlife and the environment. Below are three main focus areas to think about:

PLANNING

Planning a rain garden is the most important step. Make sure you give yourself enough time to plan a dynamic garden before you do any building. Plan for these aspects in particular:

    • Distance from your home: Rain gardens must be placed at least 10’ away from any building foundation. If placed too close, rain gardens may actually make your basement or crawl more likely to flood, rather than less.
    • Size of the garden: Determining how much water will be flowing from your roof, driveway and other hard surfaces into the garden will help you design a rain garden that can accommodate the right amount of water.
    • Drainage levels: Check your soil’s drainage capabilities by performing a soil test. Soils that feel sticky and can be molded have high clay content, and will need to be amended with a rain garden soil mix.
    • Landscape slope: Determine the slope of your rain garden location before you finalize any designs. Rain gardens located on slopes greater than 10 percent require a geotechnical engineer’s evaluation.
    • Local Requirements: Washington State Department of Ecology has minimum size requirements for new development projects, so check with your local municipalities before you finalize any designs.

INSTALLING

Even after you’ve solidified your design, you’ll likely have to do some troubleshooting:

    • Get the right equipment: Once you’re ready to build, make sure you have all of the tools you’ll need, including shovels, a water source to test the garden and plenty of string to lay out your rain garden footprint. Have a leveler on hand, as it can be easy to over- or underestimate your yard’s slope.
    • Build entry and exit points: Rain gardens need a specified location for water to enter and exit. These can be built with clean cobblestones that slope in and out of the rain garden. Your particular garden may also need a pipe to direct inflow – experiment with how water flows into the space before you dismiss the need for a pipe.
    • Call before: Hitting a septic tank or a local pipeline can be a costly mistake, so be sure to call your local utility companies before you do any digging.

PLANTING

Rain gardens are filled with native plants that are split into three zones, each with their own specified purpose and necessary growing conditions. Be sure you plant each zone with the most appropriate flora:

Typical three-zone rain garden. Photo: Washington State Department of Ecology, 2013

Typical three-zone rain garden. Photo: Washington State Department of Ecology, 2013

  • Zone 1 is for plants that thrive in wet conditions, such as Pacific willow, common rush and red-osier dogwood. These plants can soak up the highest amount of water, and provide the best support during ultra-wet storms.
  • Zone 2 is for plants than can withstand both dry and wet conditions, such as western serviceberry, Nootka rose and wild ginger. These plants will catch only some of the water as it flows into the lowest part of the garden.
  • Zone 3 is for drought-tolerant species of the Pacific Northwest. These include Garry oak, lavender and coastal strawberry. These plants can survive on very little water, and are able to maintain strong roots and stabilize conditions along the upper edges of your garden.

 

Seattle is home to more than 970 rain gardens, and the larger Puget Sound region includes more than 1,500. Several community groups are currently behind the push to get 12,000 rain gardens in Puget Sound by 2016. To see which of your neighbors have installed their own rain gardens, use this this map!

In line with residential efforts, the City is currently developing a green stormwater infrastructure network to keep pollution out of our lakes and streams and help create more inviting public spaces. Here are a few examples:

Swales in Fall 2014. Photo: KPG Interdisciplinary Design, 2015

Swales in Fall 2014. Photo: KPG Interdisciplinary Design, 2015

The Swale on Yale

As any local skateboarder will tell you, Capitol Hill has some of the steepest slopes in Seattle. Stormwater, unfortunately, feels the rush of these

descents as well! Up to 190 million gallons of runoff flow into Lake Union from the hill every year, and bring contaminants like silt, heavy metals and automobile oils along with it. To help reduce the annual runoff from surging into Lake Union, four biofiltration swales are being constructed along two blocks of Yale Ave N and Pontius Ave N. Each swale will be 270 feet long and between 10.5 – 16.5 feet wide.

Location of the swales and complementary infrastructure. Photo: Seattle Public Utilities, 2012.

Location of the swales and complementary infrastructure. Photo: Seattle Public Utilities, 2012.

How it works: Water from Capitol Hill is diverted into a large underground tank that sits under Yale Ave between Stewart and John streets. Water then flows into a swirl concentrator, where rain is separated from any large solids or trash that may have been swept up from the sidewalk. Trash is collected in a connected sump. After traveling through the swirl concentration, the water flows into the landscaped swales, which slow the stormwater, causing the sediments and pollutants to settle out of the rainwater before it finally reaches Lake Union.

The first phase of this project was completed in 2013, and swales will be complete by 2017 and 2018.

Ballard Natural Drainage System

Currently, the greater Ballard region of Seattle is home to approximately 1/3 of all combined sewer overflow problem areas in the city.

A finished rain garden in the Ballard Natural Drainage System. Photo: Seattle Public Utilities, 2015

A finished rain garden in the Ballard Natural Drainage System. Photo: Seattle Public Utilities, 2015

The Ballard Natural Drainage System, a project that has been in the works since 2012, will help keep 1 million gallons of raw sewage and runoff out of Portage and Salmon bays every year. Planning for the project was a challenge – most of Ballard’s soils absorb water very slowly, which can make rain gardens into semi-permanent pools. These pools can become breeding grounds for mosquitos and other pests.

A street map highlighting proposed natural drainage system projects in the Loyal Heights neighborhood. Photo: Seattle Public Utilities, 2015

A street map highlighting proposed natural drainage system projects in the Loyal Heights neighborhood. Photo: Seattle Public Utilities, 2015

Because of this, each rain garden site for this system was heavily tested for its drainage abilities before it was finalized. Seattle Public Utilities and Seattle Department of Transportation have drafted plans for rain garden planting strips along 17 blocks. When completed, approximately 95 percent of the previous untreated overflow that would dump into Salmon Bay will be captured in the rain gardens.

 

For more in-depth information on the development and creation of rain gardens, check out the Rain Garden Handbook for Western Washington and the RainWise website.