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How to Construct Rain Gardens to Save Water

Rain Gardens
BY VALERIE SUDOL

If you haven’t heard of rain gardens yet, it’s time to catch up with
the latest way to give purpose to your plantings.

Briefly defined, a rain garden is a shallow basin filled with native
plants that is designed to trap and filter rainwater. It captures
runoff from roofs, driveways and pavement that otherwise would find
its way to storm sewers, streams and, ultimately, our water supply —
complete with a freight of pollutants.

A rain garden is not a pond, since a well-built one will drain
completely in a day or so as water percolates through the soil and
recharges underground aquifers. Think of it as a compact bit of
man-made wetlands acting to neutralize contaminants that compromise
water quality in our reservoirs, lakes, rivers and bays. (Runoff
typically carries fertilizers, pesticides, motor oil and other
chemicals flushed from lawns and roadways.)

Like natural wetlands, rain gardens can control flooding, too,
especially in a built-up, paved-over state like ours. By keeping
water out of overburdened drainage systems, rain gardens curb the
flash floods and erosion that often result from heavy downpours.

“In rain gardens, personal gardening meets public water policy,” says
Isaac Martin, vice president of the Native Plant Society of New
Jersey and owner of Ladybug Landscaping LLC in Freehold. “This is a
great way to solve any number of problems at once.”

Larry Coffman, associate director of the Maryland Department of
Environmental Resources, is widely credited with developing the rain
garden concept in Prince George’s County, Md. in 1990.

Coffman worked with developer Dick Brinker to design a 300- to
400-square-foot bioretention area on the property of each home in one
new subdivision. This network of planted drainage basins cost about
$100,000 — $300,000 less than the standard curbs, gutters and storm
drains. The term “rain garden” came into use because it sounded more
appealing than “bioretention area” or “planted drainage basin,” and
an environment-friendly idea was launched.

Besides contributing to the health of public watersheds, rain gardens
have other positive values. Planted with native species adapted to
extremes of climate and fluctuating moisture levels, rain gardens are
oases for wildlife. If you build one, the bees, butterflies,
hummingbirds and dragonflies will come — but not the mosquitoes. By
drying out quickly, rain gardens deny these pests the standing water
their eggs need to mature and hatch, a process that takes at least a
week.

So, what looks like a simple patch of wildflowers can be a
hard-working flood-preventing, water-purifying, wildlife-attracting,
mosquito-killing supergarden that works in just about any residential
or commercial setting. The idea is so appealing and so effective that
it is catching on in many states, including Wisconsin, Michigan,
Pennsylvania and Missouri. Kansas City, Mo. has launched a campaign
to see “10,000 Rain Gardens” built over the next decade.

The Native Plant Society, headquartered at Cook College in New
Brunswick, is among the organizations promoting the rain garden
concept in New Jersey. At the group’s Web site, www.npsnj.org, you
can download a 35-page pamphlet that walks you through planning and
construction, recommends sources for native plants and even offers
eight rain garden designs suitable for the soil conditions found in
the state.

The Rutgers Cooperative Extension Service is getting into the act,
too. The extension’s Middlesex County office has built a
demonstration rain garden at its EARTH Center in South Brunswick.
Visitors are welcome to check it out any day during daylight hours.

RAIN GARDENS MUST BE located to intercept and temporarily hold
drainage water from roofs, downspouts and paving. Beyond that, they
are open to interpretation.

“You can get creative from there,” says Martin, who has installed a
half-dozen rain gardens in Monmouth and Ocean County this year. “They
are getting more popular as people realize that this is gardening
with a purpose.”

Bill Young, a fellow society member who lives in Point Pleasant,
installed a rain garden to process water pumped from his backyard
pool, which relies on biofilters rather than chlorination for
cleanliness. At the end of the season, Young can discharge pool water
into an area planted with species that don’t mind getting their
“feet” wet periodically.

Some homeowners build rain gardens near downspouts to slow the flow
from gutters or near sump pump outfalls to process water lifted from
basements. Others put them in their side yards, where the garden
becomes a privacy screen obscuring the view of nearby houses. Another
option is to locate these gardens in front yards, between
water-shedding surfaces such as roofs and driveways and storm drains
in the street.

What all rain gardens have in common is a shallow, level-bottomed
depression excavated about 6 inches below the surrounding grade.
Excavated soil is used to create a berm so water doesn’t flow in the
uphill end of the basin and out the downhill side, creating gullies.

It helps to know the composition of your soil and its drainage
characteristics — and here’s where a soil test available for about
$10 from your county extension service office is a big help. The
basin should be backfilled with a mix that water will pass through
readily. A good formula is 50 percent sand, 30 percent excavated soil
and 20 percent decayed leaves (leaf mold), often available free from
municipal composting operations.

As for size, 150 to 300 square feet is generally recommended.
Ideally, the size of the garden should relate to the size of the roof
area or driveway being drained, but even smaller rain gardens will be
an asset.

The location you choose should be at least 20 feet from the
foundation of homes with basements to prevent water from getting into
cellars. Shallow, grass-covered ditches or perforated plastic pipe
can lead water to the rain garden. Any outfall from the planted basin
designed to carry overflow from big storms should direct water away
from the house, too.

After the rain garden is excavated, filled with suitable soil and
planted, the final step is to cover the soil surface with an organic
mulch. Shredded hardwood bark is recommended as a good trap for heavy
metals and other contaminants; it won’t float and wash away as wood
chips tend to do.

Costs will vary depending on the number and species of plants you
need and whether you do the digging yourself or hire it out.
Typically, you can build a 150-square-foot garden for $300 or so,
including soil amendments and mulch. Hiring a designer will assure an
eye-catching result, but will add to the price.

Choosing plants for the rain garden is “the fun part,” says Martin,
and a process likely to introduce you to beautiful, undemanding
species not commonly found in the aisles of most home centers. Native
turtleheads, Joe Pye weed and meadow rue bear lovely flowers.
Cardinal flower (Lobelia cardinalis) and jewelweed attract
hummingbirds. Native grasses and sedges offer year-round foliage
accents.

“People have to stop thinking in terms of the overly familiar
evergreens and perennials you see in every yard,” Martin says. “They
need to develop a new aesthetic.”

The beauty of a rain garden is a bonus, a reward above and beyond the
satisfaction that comes from being part of the waste water solution
instead of part of the problem. When your new garden is aflutter with
birds and butterflies, when a great blue heron stops by for lunch or
a frog appears for a drink and a dip, your yard will seem like a more
hospitable place. You may even find yourself hoping for a rainy day
to fill your garden — and your sense of satisfaction — to the brim.

Rain garden resources

Native Plant Society, Office of Continuing Professional Education,
Cook College, 102 Ryders Lane, New Brunswick, N.J. 08901. Visit
www.npsnj.org for a PDF file that describes rain garden construction.

WikiHow, a do-it-yourself Web site, has a “How to Create a Rain
Garden” manual that takes you step-by-step through the process at
www.wikihow.com/Create-a-Rain-Garden.

“Rain Gardens: A How-to Manual for Homeowners” is worth checking out
at http://clean-water.uwex.edu/pubs/raingarden/rgmanual.pdf.

“Rain Gardens: Managing Water Sustainably in the Garden and Designed
Landscape,” by Nigel Dunnett and Andy Clayden (Timber Press, 2007) is
a recently published book on the subject.

Things to do,

things to avoid

What to do

Before you start digging, find out where your underground utilities
are located — it’s the law. Call (800) 272-1000 for a free mark-out.
Use unimproved native plant species, not hybrids, which generally
carry a specific name in quotes. For example, look for Asclepias
incarnata, the common swamp milkweed rather than Asclepias incarnata
‘Ice Ballet’ or ‘Cinderella.’ Natives evolved with local wildlife;
hybrids are products of human breeding.

Put the most moisture-tolerant plants in the center of the garden
where water lingers longest and the more drought-tolerant types
around the margins, which dry out first.

Water plants for the first season until roots become established.
Natural rainfall should be sufficient in later years.

Incorporate native grasses and sedges for low-maintenance cover. Some
designers say these should represent two-thirds of the plants in your
rain garden.

Keep your rain garden looking neat by giving it well-defined edges. A
surrounding of mowed grass or a border of flat stones will impose a
sense of order.

What not to do

Avoid locating your rain garden under large trees whose roots may be
damaged by excavation.

Don’t use species that will tower over you when mature. A
14-foot-tall reed will look out of place in a suburban yard.

Pesticides and herbicides have no role in a rain garden and will harm
the wildlife you are trying to attract. Hand weed until the plants
you’ve selected knit together.

Avoid placing rain gardens too close to house or garage foundations.
Put them 20 to 30 feet away from structures with basements and at
least 10 feet away from structures built on slabs.

Don’t over-feed rain garden plants. Most native species don’t require
fertilizer.

Don’t put rain gardens directly over septic drainfields of near
wells. Roots can infiltrate and compromise the function of these
utilities.

7 thoughts on “How to Construct Rain Gardens to Save Water

  1. Hi, I was hoping to receive a response to this inquiry. I live in a suburban environment in which a section of our yard floods each spring. This flooding occurs primarily due to the natural depression within the lawn section, a naturally high water table, and run off from neighbors sloping drives. If there is an especially heavy rainfall in the spring, the waters leave the “field” and flood our drive. Upon reading various articles, it appears that this location is a natural site to put a raingarden to help prevent additional flooding throughout our yard. Would you agree or is it possibly too much water (5-12 inches annually)? Thank you.

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