Tuesday, March 15, 2011

Coastal Rain Gardens

Spring bulbs in rain garden
A little over a year ago, I wrote about dealing with our heavy rainfall on the coast of Oregon and mentioned in that post the idea of "rain gardens."  Then last summer I had the opportunity to attend a workshop hosted by Oregon State University on this topic, and wanted to share this idea a little further with readers.

The basic idea of a rain garden addresses this problem: too much of our heavy rainfall comes all at once in a large "event," and rushes across impermeable surfaces like roofs, sidewalks, parking lots and driveways, then flows directly into the storm sewers (or nearby water bodies) without any filtration.  When this happens, lots of pollutants including fertilizers and motor oil are swept along with the water, requiring treatment if they make it to the sewer system, or just polluting the water bodies if it flows there directly.

If instead we create a mechanism to slow down the water flow, and cause it to slowly drain through layers of soil, plants and rock, it is much cleaner by the time it gets to its final destination and reduces our need for storm sewer treatment capacity.
Flagstone set into a permeable rock/sand base
provides additional area for infiltration

So the rain garden is simply a designed area which intentionally collects water that might otherwise flow from your downspouts to the storm drain, or across a large paved area, and directs it to an attractive planted feature which can contain the likely amount of water from a single "event" and let it filter through slowly.
"Stream" catches water from center
downspout & sidewalk

Benefits of the rain garden obviously include improved water quality and a reduction in the need/cost of storm water treatment.  Other less obvious benefits to the homeowner might include a very attractive landscape feature including grasses, shrubs and flowering perennials; wildlife habitat when using carefully chosen native species; turning a difficult area of the yard into a useful landscape feature; and preserving your structures from additional erosion or mud-splash by directing the flow elsewhere.

Commercial rain garden installation

A terrific guide is available from Oregon State University (free) to help homeowners, builders and landscapers  design rain gardens into their ecologically-friendly landscapes.  This guide is very thorough including design specifications and plant lists, so I'm providing here some photos of local examples.  The tiny residential rain garden is in my front yard, installed last fall.  The larger, commercial project is located at our new fire station and the garden is about 3 years old.

Note the curb cuts which allow parking
lot to drain into rain garden

Looking north: varied planting includes
Oregon grape, purple-osier willow, vine maple and rushes

Further reading--


  1. Good article! Maybe, this is what I need to do with the part of our lawn which is soaked right now and is very mossy.

  2. I french drained most of my down spouts and created gardens around them...then I found areas in the backyard where the water drains naturally that filled up and have now built one rain garden that has to be finished a bit and will construct another...when I see persistently wet areas, it is a great thing to do..wonderful post and resource

  3. Great post! I am glad that this concept is catching on. Wouldn't it be great if more commercial landscaping used this concept? Thanks for the resource link.

  4. Love this post! Being at the bottom of a valley, and coast side, we get a LOT of rain in winter. Our driveway is solid concrete, and contributes to a lot of excess runoff (we're hoping to replace it with a permeable surface soon). We have trouble in numerous areas of the garden too though, especially at the base of some of our slopes. A rain garden area would make a lot of sense in those areas to help curb runoff and erosion, and sounds prettier than just a faux dry stream bed. Thanks for the link to the rain garden guide. I'm looking forward to reading this in more detail.

  5. Tatyana and curbstone, rain gardens can be a great solution to areas such as those you mention. It's important to study the guide for information on how to calculate the amount of water you get all at once and how soon it will drain through your soil, so you get the design right.

    Donna, great to hear you are already experienced with this type of construction! I think it's an area of gardening with huge potential, including exploring little-used plants like sedges and rushes.

    Karin, I am starting to see them more often around here in commercial landscapes... one key point however is maintenance. If they are not cared for like any other garden feature, they can become a 'bad advertisement' for rain gardens. I would love to see more of them!

  6. So forward thinking to have commercial projects rethink drainage. Permeable surfaces have been with us for a long time, but what is new here is the design aspect utilizing and filtrating the excess water into an aesthetically pleasing and ecologically sound solution.

  7. Very good post. We don't get a lot of rain, but I want to be sure to use all we get, and not have it run off.