Garden Design

Rain Gardens

Rain Gardens

Drip, drip, drop, little spring shower.  Spring is traditionally a time associated with rain. Perhaps this is because the land is waking up from its winter slumber.  Although the soil soaks up a lot of the rain, some runs off driveways, roofs, and parking lots picking up contaminants as it rushes along.RainGarden1

Many people do not realize that storm water runoff does not go to treatment plants, instead most runs into streams and rivers.  What does this matter?  Well, any pollutants that the water picked up along the way will contaminate drinking water for all towns using water from that body of water.

Rain gardens are designed to blend with the landscape and improve water quality.  A rain garden is a shallow depression in the ground, which captures runoff from impervious surfaces (roofs and driveways) and allows water to soak into the ground instead of directly entering streams and rivers.

The rain garden is landscaped with water tolerant plants that aid in water absorption. Not only does a rain garden reduce runoff, but it also reduces flooding and the plants provide a habitat for wildlife.  During a heavy rain, the garden will fill with a few inches of water.  However, the water will only remain for a few days and does not provide a mosquito breeding ground.

To build a rain garden, first observe where water flows from and to during a major rain event.  Ideally, the garden should be placed between the source of runoff (roofs and driveways) and the runoff destination (drains and streams).  Avoid placing the garden near your house, septic tank, well head, or underground utility lines.  You can use corrugated plastic pipe to direct more water to the site, but be sure to calculate the added water into the size of your garden.

Check out the soil before you get started.  Sandy and well-draining soils work best, but clay can be used too.  Perform a water percolation test for impermeable soils: pour water into a hole at the site of your rain garden – if the water remains after 2 days, install a wetland garden instead.

Your rain garden should be able to handle the water falling on impervious surfaces around your house.  To estimate the size of your garden, calculate the roof area by measuring the dimensions of your house (area = length x width) and multiply it by the estimated amount (%) going to downspouts that drain to the garden.  Add to this number the surface area of any paved surfaces (driveways or patios) that would drain to the garden.  Now divide by 20.  This will be a rough estimate of the area your garden will need to be to handle runoff from 1 inch of rainfall.

Before constructing your garden, make a plan and outline the area.  Dig the garden to 4 to 6 inches with a slight depression in the center.  Use the soil to create a berm on one side – this allows water to be retained during the storm.  If the garden is on a slope, the berm should be located on the downhill side.  Add compost to the soil.  If you have compacted soil, consider adding gravel to improve infiltration.

Plant selection is where rain gardens get fun.  Rain gardens can be designed to blend with the landscape or add interest to an area.  The center of your rain garden will be the wettest, so you want to select plants that are water tolerant (DO NOT use wetland plants because the area will fluctuate between moist and dry).  The edges of the garden can be more typical plants that blend with the other areas in your landscape.

Native plants are recommended for use in the rain garden since, for the most part, they require less maintenance.  However, there are some non-natives, such as liriope (Liriope muscarii), siberian iris (Iris sibirica), and rain lilies (Zephyranthes spp.), which are also suitable.

The final aspect important to rain gardens is mulch.  You will want to add 2-3 inches of hardwood mulch (light mulches float in water) and maintain that amount of mulch annually.

A rain garden is an easy way to help our environment by improving water quality and reducing runoff.  For more information check out the website: www.bae.ncsu.edu/topic/raingarden/index.htm.

Plants Suitable for Rain Gardens**

Small Trees
Red Buckeye (Aesculus pavia)
Ironwood (Carpinus caoliniana)
Redbud (Cercis canadensis)
Possumhaw (Ilex decidua)
American Holly (Ilex opaca)

Shrubs
Beautyberry (Callicarpa americana)
Summersweet (Clethra alnifolia)
Winterberry (Ilex verticillata)
Inkberry (Ilex glabra)
Verginia Sweetspire (Itea virginica)
Possumhaw (Viburnum nudum)

Perennials
Blue Star (Amsonia tabernaemontana)
Butterflyweed (Asclepias tuberosa)
False Indigo (Baptista spp.)
Boltonia (Boltonia asteroides)
Mouse Ear Coreopsis (Coreopsis auriculata)
Tickseed (Coreopsis lanceolata)
Joe Pye Weed (Eupatorium dubium)
Texas Star (Hibiscus coccineus)
Cardinal Flower (Lobelia cardinalis)
Garden Phlox (Phlox paniculata)
Black-eyed Susan (Rudbeckia fulgida)

Ornamental Grasses
River Oats (Chasmanthium latifolium)
Pink Muhly Grass (Muhlenbergia capillaris)
Panic Grass (Panicum virgatum)
Indiangrass (Sorghastrum nutans)

**Plants included in this list range from tolerant of flooding to tolerant of drought due to the varying zones of a rain garden.  More plants for the Piedmont can be found at: www.bae.ncsu.edu/topic/raingarden/Plants_for_RainGardens-Piedmont.doc

Byline:
Stephanie Romelczyk works for North Carolina Cooperative Extension as the Horticulture Agent in Lee County.  You may reach her at 919-775-5624 or by e-mail at [email protected]