Rain Garden Designs

Rain gardens—celebrate rain and protect our streams.

More development in Washington County means runoff from roofs, driveways and parking lots carrying pollutants like fertilizer, oil, pesticides and pet waste into storm drains which lead to streams and the Tualatin River. Rain gardens help keep runoff from leaving your yard so pollutants can be absorbed and filtered by soil and native plants. Rain gardens can cut down pollution by up to 30 percent. They also reduce flooding by holding water during rain storms and allowing it to soak into the ground instead of flowing directly to streams. As an added bonus, a rain garden planted with natives attracts birds, butterflies and bees.

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 put your rain garden on level ground - dig less

Building Your Own 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 until 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!

Additional Information

For more information call Clean Water Services at 503.681.3600.

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