Too much water can undermine your home
If you worry about a wet or damp
basement, a busy sump pump, or
muddy puddles in your yard after
a heavy rainfall, this story is for
you. We want to introduce you to
a new tool to improve drainage—
a rain garden.
A rain garden is basically a
plant pond, that is, a garden bed
that you plant with special deep-rooted
species. These plants help
the water rapidly seep into the
soil, away from your house and
out of your hair. You direct the
rainwater from the downspouts
to the garden via a swale (a stone
channel) or plastic piping. The
garden captures the water and,
when properly designed, drains it
into the soil within a day. You
don't have to worry about creating
a mosquito haven; the water
drains before mosquitoes even
have time to breed.
If there's an especially heavy
rainfall, excess water may overflow
the rain garden and run into
the storm sewer system. Even so,
the rain garden will have done its
job. It will have channeled water
away from your foundation and
reduced the load on the sewer
system. A rain garden also
reduces the amount of lawn
chemicals and pet wastes that may otherwise run off into local
lakes and rivers. In some communities,
the runoff problem is
so big that homes with rain gardens
qualify for a tax break! Call
your municipality to learn your
local policy.
In this article, we'll tell you
how to design, build and plant a
rain garden suitable for your
yard. We've condensed it to a few
handy guidelines. You won't need
any special tools or equipment. A
shovel and a level will do. But
expect to sweat through some
heavy digging!
Figure A: Rain garden details
Figure A: Rain Garden Details
Create the rain garden by building a berm in a low spot in the yard, then build swales to channel runoff from the gutters and higher parts of the yard. The water is then absorbed into the soil through the network of deep plant roots. Use a mix of plants adapted to your area and to the different water depths.
Location and slope
Check the slope of your yard with
a level and a long, straight board.
You'll need a minimum slope of
1 in. in 4-1/2 ft. (2 percent) for
water to flow into your rain garden.
If you don't have this slope,
you'll have to do major landscaping,
both to create the slope and
to improve drainage.
- Locate your rain garden where
rainwater will feed into it from
downspouts, driveways or low
points in your yard.
- Lay attractive river rock (1-1/2
in. diameter and, if desired,
larger decorative rocks) or run
an underground 4-in. PVC
pipe to channel water from a
downspout to your garden.
Use PVC for a better flow if the
garden is more than 30 ft. from
a downspout.
- Place your rain garden at least
10 ft. away from your home.
Otherwise, water may saturate
the soil close to the foundation
or even back up against it. If
you already have water pooling
close to your home, channel it with an underground PVC pipe to the
garden. This may mean tunneling
under a walkway or other obstruction.
- Keep in mind “the big rain,” that storm
a couple of times a year that will overflow
your garden. Create an overflow
zone, a slightly lower area on one side
with stones that will channel water
away once the garden fills. Locate it
away from your house and your neighbors'
homes as well.
- Do not locate the garden over a septic
tank or underground utility lines.
Remember to call 811 (national number) to have your utilities
marked before digging.
Garden depth
You only want to capture as much water
as will sink into the soil in 24 hours after
a storm—a garden dug in sandy, well-draining
soil can be deeper than a garden
dug in poorly draining clay. To determine
the ideal depth, first test the porosity of
your soil.
Dig a hole in your garden area about
the size and depth of a large coffee can
(8 in. x 8 in. x 8 in.) and fill it with water.
Time how long it takes for the water level to drop. If in one hour the water level has
dropped by 1/2 in., you can figure the soil
drains an inch in two hours. At this rate,
the garden soil will handle 12 in. of water
in a 24-hour period, making the ideal
depth of this garden 12 in.
Figure B: Depth Profile
Determine the size and depth of the rain garden based on how quickly the soil absorbs the water. An average rainfall should fill most of the garden, but drain away in 24 hours. The easiest way to calculate this is to dig a small test hole in the garden area, fill it with water, and see how fast it drains. Then do a rough calculation of how much runoff will be coming down the gutters (see 'Garden size' below).
Garden size
To determine the best size for your garden,
estimate the volume of water that
would flow off the roof and down the
spout that feeds it during a 1-in. rainfall
(the rainfall from an average storm). To
do this, calculate the rough area of the
roof that drains down the spout. For
example, in a 2,400-sq.-ft. rectangular
home with a downspout at each corner,
you'd have approximately 600 sq. ft. of
runoff going to each downspout. Multiply
by rainfall depth (1 in., or 1/12 ft.) to get
the volume of water—50 cu. ft. in this
case. If your soil porosity can handle a
6-in.-deep (that is, 1/2 ft.) garden bed,
dividing by 1/2 ft. gives you a 100-sq.-ft.
(10 x 10 ft.) garden size.
However, it's OK to vary the size. A
smaller garden can still yield big benefits.
Rain gardens that are 30 percent smaller
than ideal still handle nearly 75 percent of
the storm water shed from a house. Of
course, you can also make it larger. In any
case, make sure the size of the garden fits
your landscape.
Figure C: Plant selection
Figure C: Plant Selection
While growing zones and soils vary
dramatically throughout the country,
plant selections for this type of garden are
fairly standard.
Aster, daylily, iris, sedum, coneflower,
artemisia and sedge are examples of good
rain garden specimens. Talk to your local university
extension or other garden experts about
other options for your area.
Choose plants that have “average to moist” water
requirements listed on their tag. Position them in the
deepest parts of your rain garden. On the higher
edges of the bed, position plants that thrive in
“average to dry” water conditions. While it may
seem intuitive to purchase moisture-loving plants
for your rain garden, don't do it. Since your garden
is designed to drain in 24
hours, the moisture-loving
plants will soon be left high
and dry.
While almost any plant
with the right moisture requirements
will do fine in a rain garden, there are some
good reasons to select native plants. Native
grasses, wildflowers and
shrubs generally have
very deep root systems,
sometimes burrowing down
10 ft. or more. Most native
plants also cast off their roots annually,
growing new roots and providing more
soil aeration and pathways for water to
flow. And because they're indigenous,
you know these plants will thrive in your
zone and soil conditions.
TLC for the first year
- Baby your garden its first year. Mulch
with shredded hardwood mulch (not
pine bark or wood chips, which will
float away) and weed regularly.
- Dig a notch into the berm on the low
side to allow about half the water to
flow out for the first year. Young plants
can't handle a large volume of water.
- Add large decorative rocks at the garden's
entrance to prevent heavy rain
from washing out young plants.
- Water your new garden about an inch
per week during dry spells. If you select
native species, you'll find that these
plants will be highly tolerant of dry
conditions once they mature.
