Planning a rain garden

By Denise Ruttan

Photo by Derek Godwin

Rain gardens feature hardy, drought-resistant plants in a landscaping design with a purpose.

Oregon's winters are a good time to observe how water flows on your property so you can later create a rain garden.

These landscapes with a purpose are dug-out areas where storm water from a hard surface like a roof or driveway can soak into the soil instead of flowing into a storm drain or sewer system, said Weston Miller, a horticulturist with the Oregon State University Extension Service.

One benefit to these generally low-maintenance areas is that they help keep pesticides, oil and toxic materials from polluting streams, rivers and lakes.

"It's using horticulture to solve the problem," Miller said.

So what can you do this winter to plan a rain garden? You can start by reading the 44-page Oregon Rain Garden Guide at http://bit.ly/XwkyAX. It was co-written by Derek Godwin, a former watershed specialist with the OSU Extension Service and now a regional administrator. The publication offers the following tips for planning:

Observe and map your site

* Before you start, contact your local planning department to clarify possible regulations for rain gardens.

* Create a map of your site that includes measurements of all structures. Use arrows to indicate where the water flows after the rain lands on these surfaces. Walk through your yard and note any obvious slopes or low spots. Note areas where water might drain into neighboring property. Look for a location where water flowing into the garden will be higher than where water will leave the garden. Look for areas nearby where overflow from a rain garden can be absorbed or collected into an approved storm water collection point, such as a storm drain.

Determine the location of the rain garden

* The easiest place to build a rain garden is near a gutter downspout. Don't build a rain garden on top of a septic drain field, in a location that stays wet throughout the rainy season, where the seasonal groundwater table is within three feet of the bottom of the rain garden, or under a tree canopy because the roots will be damaged by excavation. Also, do not place rain gardens on slopes steeper than a 10 percent grade. Make sure the outer edge of your rain garden is at least three feet from a sidewalk, six feet from a basement and two feet from a crawl space or slab and 10 feet from a retaining wall.

* Measure the slope by attaching a survey line or string to two stakes, with one placed at the top of the slope and one at the bottom. Divide the vertical distance by the horizontal distance and multiply the result by 100 percent.

Determine the size of the garden

* Multiply the length of the impervious surface the rain will be running off (like a driveway) by its width. Then multiply this total by 0.1. The result will be the area of the rain garden in square feet. A rain garden should be eight to 28 inches in depth. Your soil should drain at least a half inch of water per hour.

* Rain gardens should be a minimum of five feet wide to accommodate gentle side slopes for plants and to minimize erosion.

* Consider ways to divert water from your impervious surface to your garden. You may later need to dig a trench, run gutter extenders or build artificial streams that run only when it rains.

Once you've finished the planning stage, you'll need to assess the soil's ability to absorb water, which is best done when the soil is not frozen and when groundwater levels are at their highest, such as in the spring. Then when the ground is dry, it'll be time to dig the garden. In the fall, you can plant the appropriate vegetation. You can build rain gardens throughout Oregon, but advice for designing, building and maintaining them differs depending on the area. More region-specific tips as well as sample rain garden layouts, suggestions for plants, and instructions for completing your rain garden are in the Oregon Rain Garden Guide.

For more information about why rain gardens are important tools to fight storm water pollution, check out this feature in Oregon's Agricultural Progress magazine: http://bit.ly/SUSZwA.