Residential Rain GardensThey are… They are not…• Shallow depressions that allow • Suitable for every site water to collect and infiltrate Limiting factors:• Effective at removing particulates and some • Poorly drained areas nutrients from stormwater • Underlying ledge runoff • Heavy clay soils• Planted with native or adapted • High water table plants that tolerate wet and dry conditions • Mosquito pools• Additions to the landscape • Located near basements or installation and/or septic systems maintenance services you offer • High tech or hard to install
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!
½ garage roof = 416 square feetSizing the rain gardenMeasure area of impervious surface to be drained
outflowSize: 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).
Perc testAn infiltration rate of 1.5” per hour minimum is recommended.
Outline with flags, string or paintCall Dig Safe now! Then remove the layer of sod, saving it to use on theberm later, or move it to another spot.
ExcavationThe soil removed from the middle can be used to berm the edges on asloped site.
Hand excavationFinish up with shovels and rakes to get the surface contoured as desired.
InflowSometimes landscape fabric is pinned underneath inflows and outflowsto protect soil against erosion.
LevelingCheck the depth using strings a carpenter’s level and measuring tape;adjust as needed.
More levelingThe bottom should be very level so water infiltrates evenly throughout.
Add soil amendmentsCompost, lime, nutrients based on soil test and site. Be aware ofstate/local restrictions within shoreland protection zones.
Grading outflowYou may protect the outflow and inflow areas with rocks to protectagainst erosion during heavy flow. Besides, all those rocks we dug uphave to go somewhere ….
Grading outflowRecheck depth and grade; remember water flows downhill!
Placing plantsRain garden plants remove water through their roots and release it to theatmosphere. Their roots also help keep the soil in place and maintainorganic matter.
Placing plantsMostly native plants are used; in this case, herbaceous perennials andgrasses are mixed together. In larger rain gardens, shrubs and/or a tree ortwo may be added. Plant selection is planned for wet to dry zones withinthe garden, depending on soil and site characteristics. Most rain gardensdon’t stay very wet very long.
PlantingFall is a good time for root establishment, even though tops are goingdormant.
Minimize compactionBe gentle and avoid compacting the soil, which reduces drainage. Plantfrom the edges as much as possible.
MulchingThree inches of shredded hardwood mulch is commonly used. Chips ornuggets may float. Recycled, shredded yard waste can covered with aninch of a more attractive hardwood mulch, as shown here.
Maintenance requirementsWater and weed as needed for first season.Replenish mulch until plants provide adequateground cover.Inspect for signs of problems, such as a pluggedinflet, uneven flow or gullies during and after arain event. Does the rain garden overflow throughthe outlet during a heavy rain?Look for sediment accumulation in the raingarden. This means it is working! As itaccumulates, you will need to remove itoccasionally with a flat shovel.As with other gardens, plants will self-select overtime. Don’t fight the site!
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 sites will be posted on our web page http://extension.unh.edu/Agric/AGNLT.htmPresentation 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.
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