| Help
reduce flooding with a rain garden
Photo by WSU
Extension - Clark Cty. |
The damage from flooding
in Washington this winter left many with a feeling of helplessness.
But there’s actually
something Backyard Wildlife Sanctuary managers and other property owners
can do to help reduce flooding – harvest that excessive precipitation
with a “rain garden”.
A rain garden is a shallow
depression in a yard that collects rainwater and absorbs runoff through
plants. It doesn’t hold water for more than a few hours, so it doesn’t
breed mosquitos or create other problems; in fact, a well planned and
maintained rain garden attracts birds and insects that can control pest
populations.
It may seem like a small thing,
but collectively rain gardens might help stem the proverbial tide -- as
more development occurs in the Pacific Northwest, rain pours off roofs,
driveways, sidewalks, and other impervious surfaces into our streams and
rivers.
In addition to excessive water, these flows transport pollutants such
as fertilizer, oil, pesticides, and pet waste. Rain gardens keep runoff
from leaving your yard and pollutants stay in the garden where they can
be absorbed by plants.
As an added bonus, a rain garden planted with the right types of plants
attracts birds, butterflies, bees, and other wildlife.
The following information about
rain gardens is excerpted from the Watershed Stewards program of Washington
State University Extension of Clark County and the Clark County Clean
Water Program.
Planning the Rain
Garden
Think carefully about how the rain garden will function in your yard.
Is the area sunny, shady, windy, or sheltered? Do you want to view the
garden from inside your home or from an area in the yard? Consider the
color and bloom time of plants incorporated into the garden. The rain
garden should meld seamlessly with existing or planned garden features
such as arbors, patios, picnic areas, and benches. Rain gardens work particularly
well near other wildlife friendly features such as ponds, bird baths,
and feeders.
Choosing a Location
Look for a relatively level site that is slightly downhill from your downspout(s)
and at least 10 feet from the basement or foundation of the structure.
Talk to your neighbor if the garden will be located on or near the property
line. Do not locate a rain garden over a septic tank or its drain field.
You should also mark the underground utilities even when digging the garden
by hand. Shape the rain garden to your personal taste, but make sure rainwater
will flow and pool where needed. A kidney or horseshoe shape may be best
in the middle of the yard, while a long, narrow garden usually works best
along a property line.
Testing the Absorption
Rate of Soil
Before investing the time and expense of building a rain garden, test
your soil to determine its suitability. Some soils simply drain too slowly
to adequately infiltrate rain water fast enough. Test your soil by digging
a hole six inches deep. Thoroughly soak the ground around the hole and
fill the hole completely with water. Water should drain completely from
the hole within 24 hours. If it does not drain, the area will not support
a rain garden. In this event, look for another site in your yard with
better drainage.
Determining Size
Almost any size rain garden will help remove pollutants from storm water,
improve groundwater recharge, and reduce your impact on the environment.
However, you can accurately size your rain garden by following some simple
guidelines. You will need to estimate how much area will drain into the
rain garden, the general type of soil you have, and the slope of the land
where the rain garden will be.
First figure out the roof area
that will drain to the downspout(s) feeding the rain garden. For example,
a roof measuring 45 feet by 50 feet drains 2250 square feet. However,
you determine that only half that will drain to the downspout(s) watering
the raingarden. This leaves 1125 square feet of area. Remember to count
only those downspouts draining directly into the rain garden when determining
the roof area.
You can calculate the slope
of the land by running a string from a stake pounded
into the ground at an uphill spot to a stake pounded into the ground downhill.
Measure the length in inches of the string (your width measurement) and
then measure the height from the string at the downhill stake to the ground.
Divide the height by the width to get the slope in decimal format. Multiply
this times 100 to obtain the percent. (For example: a height of 26 inches
divided by width of 240 inches equals 0.108, which multiplied by 100 produces
a slope of 10.8 percent.)
A slope of 8 – 12 percent
requires a rain garden depth of 8 inches; 5 – 8 percent requires
6-7 inches of depth; 3 – 5 percent requires 3-5 inches. Your soil
type helps determine square footage of the rain garden, according to the
depth. For sandy soils use a factor of .08 for 8-inch depth, 0.15 for
6-7-inch depth, 0.19 for 3-5-inch depth. For silty soils use a factor
of o.16 for 8-inch depth, 0.25 for 6-7-inch depth, 0.34 for 3-5-inch depth.
For clay soils use a factor of 0.20 for 8-inch depth, 0.32 for 6-7-inch
depth, 0.43 for 3-5-inch depth.
Using the example of 1125 square
feet of area and a clay soil, multiply the area drained times the size
factor of 0.20 for an 8-inch depth to arrive at a rain garden area of
225 square feet.
Site Preparation
Define the edges of the garden using a hose, string, or marking paint.
Dig the rain garden about 12 inches deep, sloping the sides at a 45 degree
or less angle to reduce sloughing. Make the main “basin” of
your rain garden as level as possible to ensure water spreads evenly and
infiltrates the soil. Use excess soil from the excavation to create a
berm or dam around the downhill edge of the garden so water remains in
the garden after a hard rain. Place an outlet in the berm, usually just
a small dip in the edge, to drain excess water from a particularly large
storm into your yard without causing damage to your garden.
Next, determine how you will
divert water from your downspouts to the rain garden. Choosing to simply
allow water to flow across the lawn toward the garden can create muddy
areas in the yard. A corrugated, non-perforated plastic drain pipe provides
the simplest method to convey water to the rain garden. (Perforated pipes
allow water to seep out and sediment may eventually block the pipe.) Attach
the corrugated pipe to your downspout using an downspout adapter and bury
it in a 12-inch deep trench at a two percent or greater slope to your
garden. After testing the system, cover the pipe in the trench. Add a
few rocks at the outlet of the pipe inside the rain garden to break up
the flow during a heavy rain and prevent erosion.
Water can also be conveyed
in a dry creek bed. Dig a 4 to 6-inch deep trench about 12 inches wide
from your downspout to the rain garden. Line the trench with weed cloth
and cover with two or three inches of river rock. Add a meander or two
to make your stream look more natural and complement your landscape. Test
your layout by running water through the downspout. Be certain the water
runs smoothly through the stream or pipe and spreads evenly across the
bottom of your rain garden without eroding the sides.
Now work 3 to 5 inches of compost
into the soil over the entire garden using a roto-tiller or shovel. This
aids stormwater infiltration and gives plants a healthy start.
Plant Selection
A wide variety of plants work well in rain gardens, but species that do
not require well drained soil work best. Vigorous perennials work great
in sunny sites. Native plants are adapted to the area and usually resistant
to disease. Coneflower, Oregon grape, snowberry and others that also provide
food for wildlife are good choices.
After planting, mulch the entire
rain garden with wood chips 2 to 3 inches deep.
Avoid bark dust since it will likely float away during a heavy rain storm.
The large surface area of wood chips captures and holds pollutants, keeping
them out of our streams and lakes. Wood chips also reduce your garden’s
water needs during the drier summer months.
Maintenance
Rain gardens may require extra water during the first couple of summers
to become established. Water thoroughly, deeply, once weekly to encourage
deep roots and vigorous growth. The garden also requires more weeding
during establishment. Weeds decrease considerably as the plants become
established. After each growing season, stems and seed heads can be left
for winter interest, wildlife cover, and bird food. They should be cut
back in the early spring to allow room for new growth. As the rain garden
becomes more established, the need for maintenance will decline. |
