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.
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.
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|
|Slough sedge||Deer fern||Douglas fir|
|Sawbeak sedge||Red flowering currant||Rhododendron|
|Slender rush||Douglas spirea||Snowberry|
|Hardstem bulrush||Mock orange|
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.
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!
For more information call Clean Water Services at 503.681.3600.