Bioswales

The soil and plants in these areas filter out some impurities before the water drains into sewers, groundwater, rivers, and streams.

A Bioswale is a linear depression that directs the flow of water while letting it percolate into the soil. Bioswales are vegetated, mulched, or xeriscaped channels providing treatment and retention as they move stormwater from one place to another.

Vegetated swales slow, infiltrate, and filter stormwater flows. As linear features, they are particularly well suited to being placed along streets and parking lots.

CREATING A RAIN GARDEN

Choose a spot where water can be easily directed through the landscape or from downspouts. Make sure to leave at least 6 feet from your house, and allow for overflow away from foundations and other structures.

Use the calculator in OSU’s Oregon Rain Garden Guide to determine the size and depth of your rain garden. Amend the soil so the mix is roughly 50% native soil, 30% compost, and 20% pumice.

To help prevent erosion, mulch with pea gravel or river rocks at the point where water. Regardless of which you choose, it should be thick enough so that no soil shows through. The rest of the rain garden should have 1-3 inches of compost added once a year. Preferably, this would happen in June, when spring rains taper off. This will help suppress weeds and maintain moisture levels during dry periods.

All plants (even drought tolerant ones) will need supplemental water in the first 1-2 years until they are well established.

One of the goals in creating habitats like this is to help keep fertilizers and pesticides out of local waterways, and both should be avoided in your rain garden whenever possible. If necessary, use granular, low phosphorous, organic fertilizer and the least toxic pesticide available. Consider replacing plants which are repeatedly susceptible.

Rain Gardens: Managing water sustainably in the garden and designed landscape
Nigel Dunnett and Andy Clayden – general discussion of water and waterside plants in the garden, plus a big plant list
Plants of the Pacific Northwest Coast
Jim Pojar and Andy MacKinnon – northwest native plants, not specific to rain garden or swale use.
Tree Maintenance
Pirone, Hartman, Sall, and Pirone – includes a valuable list of shade trees that survived a 10-day, 15” deep flood.
City of Portland: How to build a Rain Garden Video
Clean Rivers Rewards offered by the Portland Bureau of Environmental Services.
Manage stormwater and be eligible for possible funding: East Multnomah Soil and Water Conservation Department
Groundwater Foundation, a member of the National Ground Water Association’s Foundation for Groundwater.

Rain garden plants must be able to tolerate periods of drought and flooding, especially in the Northwest. Few plants can tolerate all of the different conditions found in a rain garden, so the plants listed in our Rain Gardens and Bioswales PDF are divided into 4 different categories.

Dry: The top of the rain garden where floodwaters never reach. Drainage here is good because it’s at the top of a slope. Please see the Drought Tolerant PDFfor more plant suggestions for this area.

Mesic: This level, just below the dry zone, experiences occasional winter flooding and summer drought.

Moist: The zone approaching the bottom of the rain garden experiences frequent winter flooding. The number of plants that can grow here without summer water is limited.

Wet: The bottom of the swale will be saturated for a large portion of the year. Pond plants can be grown here if supplemental water is given in the summer. Without summer irrigation, fewer plants can grow here.