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A
rain garden is a garden which takes advantage of rainfall and
stormwater runoff in its design and plant selection. Usually, it is a small garden which is designed to withstand
the extremes of moisture and concentrations of nutrients,
particularly Nitrogen and Phosphorus, that are found in stormwater runoff. rain gardens are sited ideally close to
the source of the runoff and serve to slow the stormwater as
it travels downhill, giving the stormwater more time to
infiltrate and less opportunity to gain momentum and erosive
power.
On the
surface, a rain garden looks like an attractive garden. It
may support habitat for birds and butterflies, it may be a
formal landscape amenity or it may be incorporated into a
larger garden as a border or as an entry feature. What makes
it a rain garden is in how it gets its water and what happens
to that water once it arrives in the garden.
Below
the surface of the garden, a number of processes are
occurring which mimic the hydrologic action of a healthy
forest. Soils are engineered and appropriate plants selected
for the rain garden. The garden is a small bioretention cell
in which stormwater is cleaned and reduced in volume once it
enters the rain garden. Nitrogen and phosphorus levels and
overall sediment loads in the stormwater are reduced by the
action of the plants and growing media on the water.
Multiple rain gardens over an area will have a positive
cumulative effect on both the volume and quality of
stormwater run off.
There
are two basic types of rain gardens – under-drained and
self-contained. Both types of rain gardens are used to
improve stormwater quality, reduce runoff volumes and
generally facilitate infiltration of cleaned water. Which
type of garden is selected to be built is a balance of
volumes of water to be treated, existing soil conditions,
available space, and budget for the project.
In
some cases where infiltration is not desired, the underdrain
system can move excess water into a conventional storm sewer
pipe system. Cases where infiltration would not be desirable
would be if the bottom of the garden has less than 4’ of
clearance to the seasonal mean high water table or if the
adjacent soils are contaminated and the cleaned water from
the garden would become recontaminated by coming in contact
with the adjacent native solids.
Rain gardens are designed to be drained within four hours
after a 1” rain event. Under-drained rain gardens typically
are designed to drain within 2 hours of the design storm
event. This is achieved through the use of highly porous
planting media and underdrains which carry the cleaned
rainwater away from the garden. As a result, the plants
selected for the bioretention cell need to be able to
withstand both the extremes of flooding and drought. Plants
on the upper edges of the garden are often xeric in their
cultural requirement descriptions with plants lower in the
garden being more adapted to floodplain conditions. Many
riparian edge species are particularly well suited to the
extreme environments of rain gardens.
Rain gardens with no underdrain typically hold moisture
longer, particularly in the lower areas of the garden.
Plants selected for this garden should be able to tolerate
inundation for a more extended period of time. However, as
in the case of the underdrained rain garden, the surface is
drained within four hours, although the soil may be
saturated. As in the bioretention cell, soils are amended
with a very porous planting media, minimally to a depth of
8” and ideally to a depth of 2’- 3’. The lower the amount of
soil amendment added when the garden is built, the more
necessary it is to have plants adapted to prolonged periods
of wetness. As with the underdrained rain garden, the plants
on the upper edges of the garden will need to be more xeric
in their cultural requirements than the plants in the lower
areas.
In
both types of gardens, the ground is excavated and the
planting media is imported to the site. The imported
planting media should be clean and weed seed free. A liner
may or may not be used, depending on the local conditions.
In
both the under-drained and self-contained rain gardens, the
success of the garden is greater when you start with healthy
and smaller, rather than larger, plants. Some plants listed
are successful in rain gardens only when they are installed
small and have a chance to adapt to the conditions as they
grow.
Plants
with deep fibrous roots tend to have a competitive advantage
in a rain garden and provide the most cleaning and filtration
benefits to the environment. Typical rain gardens are
populated with natives or native cultivars because those are
most well adapted to a locality, but other ornamental
horticultural plants that are non-invasive but able to grow
in the garden conditions can also be excellent choices.
Most
of the examples of rain gardens that are available to review
are populated with either herbaceous perennials, woody
shrubs or trees. This does not mean that annuals are not a
possible choice for such gardens; rather it means that the
gardens constructed have been designed for habitat and low
maintenance goals rather than purely seasonal aesthetics and
color effects. Some annuals are good candidates for a
higher maintenance version of a rain garden.
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