Benton Soil and Water Conservation District's

2008 Native Tree and Shrub Sale ~ BONUS feature

Rooftop runoff has become a hot issue lately. Or, as I read on one website, it's a "hot, fast and dirty" issue. That's because, if you live within city limits, your rooftop rainwater goes into your city's pipe system, which then enters nearby waterways warmer and faster than normal overland water and full of particulates. This is bad news for fish and other stream-dwellers, some of which are dwindling in numbers.

What can you do as a homeowner? One thing you can do is to disconnect your downspouts from the city's drainpipes and connect them to a "rain garden." With a rain garden, your rooftop water ends up in our waterways the way nature intended: cool and clean.

A rain garden, at its most simplest, is a depression in the land about 2-3'deep populated with plants that tolerate winter soil saturation and summer drought. Our native plants are great at this and the BSWCD is offering a good sampling of these tough-and-beautiful plants.

The area of your rain garden should be 5-10% of the rooftop that your downspout delivers. In Corvallis with our clay soils and about 6 inches of rain per month during the rainy season, I would opt for the high end (10%). (Perhaps as more people create rain gardens in the area, we can get a more accurate number.) So, if one downspout carries the rain for 500 square feet of rooftop, the rain garden should be about 50 square feet, or 7' x 7'.

In this design, I am imagining that you are disconnecting two downspouts that amount to 900 sf. of roof. (Actually, this is what I am going to do at my own home). The rain garden will be two depressions connected by a "dry creek" which will be lower than the adjacent existing grade but higher than the depressions. During downpours, my two rain gardens will be one big rain garden. One depression is about 36 sf. and the other 50 sf. (heavy dashed line). Each depression will be about 2-1/2' deep at the low point (+), gradually climbing to the undisturbed existing grade (thin dashed line). I will be adding pea gravel and large, rounded rocks here and there for visual interest (not shown). This design is for an area that gets strong afternoon sunlight in the summer.

You may need to water your rain garden in the summer a bit, but not much. After all, these are all natives and they have handled summer drought for millennia.

• Do choose a flat site-it's a lot less work!
• Don't place your rain garden where water already ponds (runoff will not soak in).
• Do locate your rain garden well away from your house.
• Don't install a rain garden over a septic drainfield.

For more information, Google "Harvesting Rain in the Pacific Northwest," a Washington State University Extension Publication that is available online in .PDF form.

It was fun choosing from the BSWCD list because each plant has so much to give! Just to name a few: the rose will provide summer fragrance; the huckleberry, strawberry and sword fern will stay green all year while the tiny huckleberry leaves contrast nicely with the fern's bold texture. The monkey flower and camas will naturalize and create dreamy drifts of yellow and purple. The broad foliage of the fringe cup will contrast well with the other perennials. And, columbine has such delicately complex flowers; every garden should have them. I expect that this small garden will be alive with birds, bugs and other critters throughout the year.