A Rain Garden is a planted depression where run-off from roofs, driveways and other surfaces is directed so that it can soak back into the soil naturally rather than run into storm drains.

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 that provide 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.

Rain Garden Location

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.

Site and Soil Preparation

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.


Two kinds of mulch are important in a rain garden. A mulch of pea gravel or river rocks at the point where water enters will help prevent erosion; this mulch should be thick enough that no soil shows through. The rest of the rain garden should have compost 1-3” deep added once a year as spring rains taper off in June. 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.

Fertilizer and Pesticides

These should both be avoided in your rain garden whenever possible; part of the goal is to help keep these things out of local waterways. If necessary, use granular, low phosphorous, organic fertilizer, and the least toxic pesticide available. Consider replacing plants which are repeatedly susceptible.

Categorizing Plants

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

Dry (D) – 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 handout for more plant suggestions for this area.

Mesic (Mc)– This level, just below the dry zone, experiences occasional, brief winter flooding and summer drought.

Moist (Mt) – 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 (W) – The bottom of the swale will be saturated for a large portion of the year; water plants can be grown here if supplemental water is given in the summer. Without summer irrigation, fewer plants can grow here.

For a list of plants, download our brochure Rain Gardens and Bioswales for a list of the many varieties - or come in and see what we offer for Rain Garden plantings!

rain garden

rain garden

The Oregon Rain Garden Guide

Landscaping for Clean Water and Healthy Streams

You'll find detailed instructions for building a rain garden with sample plans and illustrations in the OSU: Oregon Rain Garden Guide.

Download The Oregon Rain Garden Guide: Landscaping for Clean Water and Healthy Streams (photo images not high-res).

Also available for purchase as a book or high-res pdf: The Oregon Rain Garden Guide.

Resources: Books

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.

Resources: Websites

• From the Environmental Services of Portland, Oregon: How to manage Storm Water: Rain Gardens

• 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.