Chances are good that you’ve seen at least one drainage swale, those depressions in the ground – often filled with vegetation and rocks – used to collect runoff from roofs, sidewalks, parking lots or walkways rather than sending it directly into storm drains or creeks.
“The soil and plants are used to filter and treat the water by removing pollutants or converting pollutants to less toxic forms,” Laura Antonson – a landscape designer and owner of Laurus Designs in Silverton – said. “It also allows the water to infiltrate into the soil and reduce flooding.”
Otherwise known as rain gardens, detention ponds or stormwater planters, drainage swales have become a common feature of new commercial buildings and site improvements, thanks to requirements by many municipalities. But they can benefit the home gardener too, not only capturing runoff, but providing additional habitats for birds, butterflies and bees as well.
“There are so many options with plants and rock, and some people use snags and logs to increase habitat value,” Antonson said. “We have a rain garden in our front yard that allows about half our roof runoff to infiltrate into the ground instead of being piped into the street. It’s also planted with edible plants like huckleberries, currants (the deer love them as much as the kids), rhubarb, and Saskatoon. It used to have plants in the bottom like lupine, camas, iris, and blue-eyed grass, but my kids love to walk through it so now we use small river rock to keep the soil in place.”
Information abounds for those who would like to create their own backyard rain garden including the printable, “Oregon Rain Garden Guide,” found at https://seagrant.oregonstate.edu/sgpubs/oregon-rain-garden-guide/.
“Another option is reaching out to a licensed engineer or landscape architect if you have concerns about installing a rain garden on your property,” Antonson said. Because, while it may seem easy “to dig a hole and create a rain garden, there are some extremely important factors to keep in mind.”
Distance
“You must make sure to maintain enough distance from your house or building when locating the rain garden because you do not want to undermine your foundation.”
Location
“You will also need to locate your utilities – the last thing you want is to hit a buried water or sewer line!”
Drainage
“A rain garden should be able to drain completely within 24 hours. For example, do not locate it in an area that is naturally wet from a seep or spring and does not drain.”
Overflow
“If we have a large rain event and the depression fills up, it needs somewhere to go other than towards a structure. For example, my rain garden edge is slightly higher on my house side, but lower by the sidewalk so if it fills up, it will overflow towards the sidewalk into the rest of my yard.”
Planting
“Most rain gardens and swales have two or three moisture zones with the bottom being the wettest and the top being dry. It’s important to use plants in the bottom of the rain garden that can survive being continually submerged throughout the winter… Native plants are a good start like red twig dogwood, red-flowering currant, snowberry, evergreen huckleberry, Oregon grape, kinnikinnick, and many sedges, rushes, and perennial flowers.”
Maintenance
“General weeding, pruning and mulching will help keep the rain garden looking nice. I recommend using mulch only on the sides and top otherwise it floats away. Using rock in the bottom… is a good alternative… do a little research on plants for maintenance before any aggressive pruning.”
As you can see, there is more to drainage swales than meets the eye. “I could write a volume of books on stormwater,” Antonson confirmed. But, like so many gardening trends, it’s often best if you just get out there and try it.
