Rain gardens do triple duty.
They provide an attractive habitat for you and the neighborhood wildlife to enjoy and use. They can prevent basement flooding and “bog yard” by directing runoff away from your lawn and house foundation during the rainy season. And perhaps most important, they improve the health of the Salish Sea and other waterways in Snohomish County by reducing the amount of stormwater flowing into storm drains.
Runoff from roofs, driveways and other impermeable surfaces carries toxins and pollutants; a rain garden captures that excess water, filters out the pollutants and uses the water to sustain an attractive, low-maintenance garden.
The rain garden example here uses mostly native plants, but you can select plants to create a pollinator garden (to attract bees, butterflies and hummingbirds), a four-season-interest garden, a color-themed garden, or almost any design you like. A well-designed rain garden will be very low-maintenance, once it’s established.
Contact the Washington State University Extension (raingarden.wsu.edu) for the “Rain Garden Handbook for Western Washington.” It has detailed information about how to plan, design, build, plant and maintain a rain garden. The handbook includes plant lists for the three zones suitable for our area; how to test your soil for drainage rate; how to determine the best location; a list of experienced contractors, if you need some help with the construction; and a step-by-step guide to creating a thriving, useful rain garden.
The best time to build and plant is in fall or spring, when soils are moist. You can plant in summer, but keep new plantings watered the first year to get them established. Avoid winter construction.
Inflow
Lined with rocks. This is where the water is directed into the garden. (An outflow trench should be at the other end). Water can come to the inflow area via a rock-lined swale or a pipe.
Ponding depth
To allow six to 12 inches of standing water is typical. The size and depth of the garden depends on your soil type and the amount of runoff the garden needs to handle.
Side slopes
Gradually sloping sides (2:1 maximum) allow plantings to take hold and a measured water flow to prevent erosion.
Excavation depth
24 inches to 42 inches total excavation depth
Soil/mulch mix
A very important part of the system. 12 inches to 24 inches of soil mix ion top of existing soils is recommended in the bottom of the ponding area. You can excavate and replace existing soil with compost, or amend your existing soil with compost and reuse it in the rain garden, as long as it doesn’t contain too much clay. Compost should be mature, crumbly and well-developed , not all bark or sawdust, and not contain too much nitrogen (like manure).
Existing substrate
Sandy soil drains quickly. Clay soils drain very slowly. You can test the area where you want to put the rain garden to determine how much soil to excavate and how much compost to mix in to the sold you add back to the bottom of the garden.
Planting zones
- Zone 1 is the wettest area, at bottom of the garden.
- Zone 2 is on the sloped sides.
- Zone 3 is on the top edge of the rain garden, where the soil will stay drier.
Plant tips
- Don’t grow vegetables or eat nuts or berries from the plants in the rain garden. Remember, it filters pollutants.
- Choose a variety of heights and textures.
- Consider the mature size of the plants you select, including roots, spread and height.
- Use plants that are well-adapted for our climate (wet winters and dry summers). That’s why native plants are a good choice.
- Mulch! It is very beneficial to rain garden plants. It keeps the soil evenly moist, helps prevent erosion, suppresses weed growth and adds nutrients to the soil.
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