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Rain gardens

By Candace Stoughton

Have you considered building a rain garden in your yard? Many of us in Portland have seen the swales being built all around the city to manage stormwater runoff and protect our rivers. Did you know you can do the same thing in your yard to capture the runoff from your roof, driveway, etc? When we build swales in our yards, we call them rain gardens.

What is a rain garden?

A rain garden is a “sunken garden bed” in your yard where you can direct runoff from your roof, driveway and other impervious surfaces on your property. The rain can then soak into the ground naturally rather than running off into storm drains.

Why build one?

When a landscape is covered in natural vegetation, most rainfall soaks into the ground. As we start creating impervious surfaces: roofs, driveways, sidewalks, and streets, much of the rainfall can’t soak into the ground anymore. This can create problems, not just for people, but also for streams. Rain gardens are a beautiful way to manage stormwater runoff because they allow rain to soak into the ground naturally. This prevents pollution from entering our local streams and wetlands, recharges groundwater and keeps water in our streams during Oregon’s dry summer months.

Urban stormwater runoff causes unnaturally high flooding downstream. In-stream erosion degrades water quality, and compromises habitat for wildlife. Erosion in streams imperils the “built environment” as well by undercutting bridge supports, exposing sewer lines and washing out roads. Non-point source pollution (carrying metals, oils, bacteria, fertilizers, etc straight to streams) is associated with excess runoff as well.

Rain gardens are becoming very popular because they are planted with beautiful, hardy, low-maintenance and drought tolerant plants. These gardens also provide food and shelter for birds, butterflies and beneficial insects. This approach to gardening is an easy way for all of us to do our part to protect our streams and rivers.

Frequently asked questions

Do rain gardens breed mosquitoes?
No. Because rain gardens are shallow and are only built on soils with sufficient drainage, they are designed to dry out before mosquitoes can reproduce.
Will my rain garden have standing water for more than a day?
Rain gardens are designed to infiltrate water in about a day. If it rains several days in a row, it is possible that your rain garden may have standing water until the rain stops and the water has time to soak in.

Don’t rain gardens require sandy soil?
If your soil can percolate two inches of water per hour, you have adequate drainage.

Can I install a rain garden if I have a septic system?
Yes, but it is very important not to place a rain garden over a septic system.

Get started!

Find a spot in your yard where you can easily direct the runoff from your downspout or other impervious surface. Do a percolation test to ensure that the soils in that spot can soak up rain water.
How to do a percolation test
• Dig a hole at least twelve inches deep.
• Fill it with water and let it drain.
• Fill it with water a second time. If the water drains at least two inches in an hour the second time you fill it, your soil has adequate drainage for a rain garden.
• To avoid drainage problems, place your rain garden at least six feet from your house if you have a basement (two feet if you don’t) and five feet from your property line. Call your local jurisdiction to find out if you need a permit to disconnect your downspout or if there are special requirements.
• Dig a shallow depression to create a rain garden area about six inches deep. You can make it as long and wide as you like - the bigger it is, the more rain water it can soak up. Don’t forget to call before you dig so you don’t hit any buried utility lines. In Oregon call (800) 332-2344.
• Use the soil you dig up to create a berm on the down slope side and direct the overflow safely away from nearby buildings. Make the bottom of your rain garden level. If you like, you can amend the soil in your rain garden with compost.
• Plant your plants and then mulch. Water the plants until they are established. (For sample rain garden planting designs, go to our web site:

For more information, or to get updates on our upcoming rain garden classes in 2008, please email Candace Stoughton at or call

(503) 222-7645x109.

Candace Stoughton is the Low Impact Development Specialist with East Multnomah Soil and Water Conservation District.

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