new hampshire; residential rain gardens - new hampshire
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Residential Rain Gardens
They are
Shallow depressions that allowwater to collect and infiltrate
Effective at removing
particulates and somenutrients from stormwaterrunoff
Planted with native or adaptedplants that tolerate wet anddry conditions
Additions to the landscapeinstallation and/ormaintenance services youoffer
They are not
Suitable for every site
Limiting factors:
Poorly drained areas Underlying ledge
Heavy clay soils
High water table
Mosquito pools
Located near basements or
septic systems
High tech or hard to install
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Residential Rain Garden Installation
October 2010 installation at Portsmouth, NH
Rain garden training for professional
landscapers
Co sponsors: UNH Cooperative Extension,
UNH Stormwater Center, Hodgson Brook
Restoration Project, Northeast States &
Caribbean Islands Regional Water Center, NH
Landscape Assoc. and New England Grows!
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Sizing the rain garden
Measure area of impervious surface to be drained
garage roof =416 square feet
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Size: An average size is about 1/3 the size of the drainage area and 4-8 deep.
With an adjustment for the slope here, the size of the rain garden needed wasabout 150 square feet (4 inch depth).
outflow
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Perc test
An infiltration rate of 1.5 per hour minimum is recommended.
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Outline with flags, string or paint
Call Dig Safe now! Then remove the layer of sod, saving it to use on the
berm later, or move it to another spot.
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Excavation
The soil removed from the middle can be used to berm the edges on a
sloped site.
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Hand excavation
Finish up with shovels and rakes to get the surface contoured as desired.
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Inflow
Sometimes landscape fabric is pinned underneath inflows and outflows
to protect soil against erosion.
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Leveling
Check the depth using strings a carpenters level and measuring tape;
adjust as needed.
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More leveling
The bottom should be very level so water infiltrates evenly throughout.
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Add soil amendments
Compost, lime, nutrients based on soil test and site. Be aware of
state/local restrictions within shoreland protection zones.
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Grading outflow
You may protect the outflow and inflow areas with rocks to protect
against erosion during heavy flow. Besides, all those rocks we dug up
have to go somewhere .
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Grading outflow
Recheck depth and grade; remember water flows downhill!
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Placing plants
Rain garden plants remove water through their roots and release it to the
atmosphere. Their roots also help keep the soil in place and maintain
organic matter.
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Placing plants
Mostly native plants are used; in this case, herbaceous perennials and
grasses are mixed together. In larger rain gardens, shrubs and/or a tree or
two may be added. Plant selection is planned for wet to dry zones within
the garden, depending on soil and site characteristics. Most rain gardens
dont stay very wet very long.
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Planting
Fall is a good time for root establishment, even though tops are going
dormant.
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Minimize compaction
Be gentle and avoid compacting the soil, which reduces drainage. Plant
from the edges as much as possible.
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Mulching
Three inches of shredded hardwood mulch is commonly used. Chips or
nuggets may float. Recycled, shredded yard waste can covered with an
inch of a more attractive hardwood mulch, as shown here.
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Finished rain garden
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Maintenance requirements
Water and weed as needed for first season.
Replenish mulch until plants provide adequate
ground cover.
Inspect for signs of problems, such as a plugged
inflet, uneven flow or gullies during and after a
rain event. Does the rain garden overflow throughthe outlet during a heavy rain?
Look for sediment accumulation in the rain
garden. This means it is working! As it
accumulates, you will need to remove it
occasionally with a flat shovel.
As with other gardens, plants will self-select over
time. Dont fight the site!
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For more information on rain gardens in
the northeast:
Vermont, Maine and Rutgers (New
Jersey) all have excellent rain garden
manuals on line
This presentation and links to other siteswill be posted on our web page
http://extension.unh.edu/Agric/AGNLT.htm
Presentation by Dr. Cathy Neal, Extension Professor and Landscape Horticulture Specialist, Univ. of NewHampshire, Durham NH. Additional photos courtesy of Margaret Hagen of UNH Cooperative Extension andCandace Dolan of the Hodgson Brook Restoration Project.