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What
is a rain garden?
A rain garden is an attractive landscaping feature planted with
perennial native plants. It is a bowl-shaped garden, designed to
absorb stormwater run-off from impervious surfaces such as roofs
and parking lots.
Rain gardens can be small, formal, home-owner style gardens, large
complex bioretention gardens, or anywhere in between.
Why do we need rain gardens?
Rain is natural; stormwater isn't. Government studies have shown
that up to 70% of the pollution in our streams, rivers and lakes
is carried there by stormwater. Although most people never think
about stormwater, about half of the pollution that stormwater carries
comes from things we do in our yards and gardens!
Planting a rain garden may seem like a small thing, but if you calculate
the amount of rain that runs off your roof, you would be very surprised.That
rain is supposed to soak into the ground, but instead heads down
the street to the storm drain, carrying pollution with it.
Keeping rain where it falls, by putting it into a beautiful rain
garden, is a natural solution. You not only get a lovely garden
out of it, you have the added benefit of helping protect our rivers,
streams and lakes from stormwater pollution. You can be part of
a beautiful solution!
Who Should Create A Rain Garden?
Rain gardens are suitable for any land use situation, residential,
commercial and industrial. A rain garden should be placed so that
impervious surfaces will drain into the depression area. Its purpose
is to minimize the volume and improve the quality of water entering
conventional storm drains and nearby streams.
Grass buffer strip
A grass buffer strip slows water as it enters the rain garden and
its surface filters particulates from the runoff.
Ponding area
The depression area stores the water, provides for evaporation,
and allows the particulate material, not filtered by the grass buffer,
to settle to the bottom. The ponding area should have a depth of
6 inches, sufficient to provide adequate water storage, but should
not pond in excess of four days (to avoid mosquito and other insect
breeding).
Components of a rain garden
Mulch/Organic Layer
This material provides for the decomposition of organic material,
and also plays an important role in the removal of metals. Shredded
hardwood mulch is the preferred choice, since it allows for maximum
surface area for binding and resists flotation/washout.
Planting Soil
Organic matter in the form of leaf mulch (20%) blended into a
sandy soil (50%) with and about 30% top soil. The planting soil
mixture provides a source of water and nutrients for the plants
to sustain growth. Clay particles adsorb heavy metals, hydrocarbons
and other pollutants.
Plant
Selection
A planting plan design should include species that tolerate extremes.
There will be periods of water inundation and very dry periods.
Most riparian plant species will do well in rain gardens. The
choice of species should include plants that mimic forest habitat
and have an aesthetic landscape value such as flowers, berries,
interesting leaves or bark. Groundcovers, perennials shrubs and
trees should be incorporated into the planting design.
Site
Considerations
Each site should be considered unique. Microclimates (light, temperature
and wind), and the size of the drainage area will influence the
size of the rain garden and plant selection process. Software is
being developed for sizing the gardens. The shape of the garden
is not as important as the area available for bioretention. The
size of the bioretention area should be 5% to 7% of the drainage
areas multiplied by the crop "c" coefficient (the ground
cover type). For example, a 3/10 acre drainage area would use a
rain garden of about 600 square feet, or 15 x 40 feet.
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