Rain Gardens: celebrate rain & protect our streams

As more development occurs in Washington County, rain runs over roofs, driveways and sidewalks carrying pollutants such as fertilizer, oil, pesticides and pet waste into storm drains and eventually the nearest stream or river. Rain gardens keep runoff from leaving your yard and pollutants stay in the garden where they can be absorbed by native plants. Rain gardens reduce flooding by limiting the amount of water transported directly to the stream during rain storms. Rainwater infiltrates into the ground and replenishes ground water while helping diminish flooding in local streams. As an added bonus, a rain garden planted with natives attracts birds, butterflies and bees.Rain gardens can cut down this pollution by up to 30%.

Considerations before you begin

• Do not place the rain garden over a septic system
• Do not build your rain garden where water already ponds - runoff will not soak in
• Do locate your rain garden where plants get at least partial sun - you can use a wider variety of plants
• Do try to put your rain garden on level ground - dig less

Build your rain garden

1. Do a drainage test

1. Dig a hole at least twelve inches deep. 
2. Fill it with water and let it drain.  
3. 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.

2. Calculate the size of your rain garden

1. Measure your impervious runoff source ie.: 20X20 foot section of roof
2. Minimum storage required: 3 inches (.25 feet)
3. Storage volume needed: 20 X 20 X .25 = 100 cubic feet
4. Pond design: A 1 foot depth with 3:1 side slopes and a bottom area of 4 feet X 10 feet provides a storage volume of 100 cubic feet.

Note that if the soil drainage rate is .5 inches per hour, a pond 1-foot deep will take 24 hours to drain when filled. Similarly, a small storm of .5 inches would produce a depth of about 3 inches and drain in 6 hours.

3. Choose your plants

In the bottom of the rain garden, use water tolerant plants, preferably ones native to the Pacific Northwest. List of native plant nurseries.

Garden floor	Slope	Upper Edge
Western mannagrass	Tufted hair grass	Bleeding Heart
Spike rush	Salal	Lupine
Slough sedge	Deer fern	Douglas fir
Sawbeak sedge	Red flowering currant	Rhododendron
Slender rush	Douglas spirea	Snowberry
Hardstem bulrush		Mock orange
Small-fruited bulrush		Kinnikinnick

 

4. Prep and Plant

Define the edges of the garden using a hose, string, or marking paint. Dispose of removed vegetation by transplanting, composting, or adding to the yard debris recycling bin. Dig the entire garden about 12 inches deep, sloping the sides at a 45 degree or less angle. Make the main “basin” of your rain garden as level as possible to ensure water spreads evenly across the garden and infiltrates the soil. Use excess soil from the excavation to create a berm or dam around the downhill edge of the garden so water remains in the garden after a hard rain.

Make sure to have at least a rough plan for which plants will be planted where. Lay out the plants as planned one foot apart in a grid pattern, keeping them in containers if possible until they are actually planted to prevent drying out before they get in the ground. Dig each hole twice as wide as the plant plug and deep enough to keep the crown of the young plant level with the existing grade. Make sure the crown is level and then fill the hole and firmly tamp around the roots to avoid air pockets. Apply double-shredded mulch evenly over the bed approximately two inches thick, but avoid burying the crowns of the new transplants. Avoid bark dust since it will likely fl oat away during a heavy rain storm. The large surface area of wood chips captures and holds pollutants, keeping them out of our streams and lakes. Wood chips also reduce your garden’s water needs during the drier summer months.

5. Maintenance

After planting, your garden will need to be watered deeply once a week plants are established. Weed as necessary. Leave the vegetation for the winter as it provides cover and food for birds. Cut off all the dead vegetation in the spring to encourage new growth. As the rain garden becomes more established, it will require less maintenance while you enjoy the benefits!

More information

• Clean Water Hero Program (technical assistance for residents)
• Rain barrel information
• Welcome the Rain website
• City of Gresham website
• Metro website
• Fowler Middle School rain garden project
• University of Wisconsin rain garden brochure
• Wisconsin Department of Natural Resources brochure
• Convention Center rain garden project brochure
• Washington State University brochure

For more information call Clean Water Services at (503) 681-3600.