Building a Backyard Rain Garden
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NORTH CAROLINA COOPERATIVE EXTENSION


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Building a rain garden in your own backyard is easy! By following these few simple guidelines, rain gardens

can easily be installed on your residential lot.

Note: This guidance is meant for individual land owners with small drainage areas only. Larger installations (schools, commercial areas) may require profesional consulting. Please contact us for additional information.

 

Garden Location
Soils and Drainage
Sizing the Garden Garden Construction Garden Photos


Garden Location

 


To select a location for the rain garden, begin by observing your yard during a rainfall event. Notice where water is flowing from, and where it is going. Rain gardens should ideally be located between the source of runoff (roofs and driveways) and the runoff destination (drains, streams, low spots, etc.). Be sure to consider the following:

    • The garden should not be within 10 feet of the house foundation
    • Gardens should be located at least 25 feet from a septic system drainfield
    • Gardens should not be placed within 25 feet of a well head
    • Make sure to avoid underground utility lines
    • The best location for the garden will be in partial to full sun
    • Rain gardens should be constructed where the water table is at least 2’ below the surface of the soil. If you hit the water table when constructing your rain garden, consider turning it into a wetland garden.

 

Downspout

Pipe

Once a location is selected, you may decide to send additional water to this site. Flexible plastic pipe can be used to direct water from downspouts and collecting areas to the rain garden.Corrugated plastic pipe can be used to direct water from a distant downspout to the garden drainage area. Be sure to factor this additional water flow into your garden sizing calculations.

 

 

Make sure to avoid utility lines!

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Soils and Drainage

 


Rain gardens work best when constructed in well-drained or sandy soils, but they can also be installed on sites with less permeable soils such as clays. By digging a hole 1 foot deep at the rain garden site, the soil can be examined.

There are three signs of an impermeable soil.

 If you see any of these signs, then your garden will need to be designed as a backyard wetland garden, or another location should be selected. Otherwise, your site is suitable for a rain garden.

 

 

Wetland soil Perc test
An example of a wetland soil
Perform a water (perc) test for impermeable soils

 

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Sizing Your Garden

 


Rain gardens can be large or small – the size depends primarily on the site drainage area. The volume of water to be collected will be roughly equivalent to the amount of rain falling on impervious areas draining to the garden location, such as driveways and rooftops. In North Carolina, we typically try to capture runoff from 1 inch of rainfall.

To estimate the drainage area, first figure out the roof area draining to the site. The volume of water draining to the garden from the roof will be equal to the square footage of the house multiplied by the percentage of roof feeding the downspouts to the garden. For example, if the back half of your house will drain to the rain garden site, the size of the house’s drainage area would be one half the square footage of the house. Add to this number the surface area of your paved driveway. The driveway area can be estimated through actual measurement, using a tape measure. The other option would be to pace the driveway, noting that each step is normally 2.5 to 3 feet in length. The combined roof drainage area and driveway drainage area make up the total impermeable drainage area for your garden.

 

By dividing the total impermeable drainage area by 20, you will get a rough estimate of the garden’s area requirements for a water depth of 6 inches. For a shallower depth of 3 inches, divide this total area by 10.

 

EXAMPLE
60by60
A house is 60 feet by 60 feet and the owner estimates that 25% of the roof area is draining to the downspout. Further, she estimates that the driveway area draining to the location for the rain garden is 500 square feet. For a desired 6 inch ponding depth, what size should the rain garden be?
Roof area
60 X 60
3600 sq. ft.
¼ of the flow to downspouts
(3600 sq. ft) X 25%
900 ft
Roof area plus driveway
900 + 500
1400 sq. ft
Divide square footage by 20
1400/20
70 sq. ft.
ANSWER
The water garden should be at least 70 square feet.
A 5X14 or 7X10 foot garden design would be sufficient

 

Shallower water depths are possible, but will result in a larger required surface area for the rain garden. The Sizing Chart relates impermeable drainage area to rain garden size. Once you have determined the total impermeable drainage area for your rain garden, use this table to determine possible rain garden dimensions.

 

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Garden Construction

 


 

Once the location and size of the rain garden have been determined, it is time to start digging the rain garden. Prior to digging, it may help to outline the area using string or spray paint.

The garden should be dug 4 to 6 inches deep with a slight depression in the center. The dug out soil will be used to create a berm along one side of the rain garden which will allow water to be retained during a storm. If the garden is located on a slight slope, the berm should be located on the downhill sloping side of the rain garden. To prevent erosion, the berm should be covered with mulch or grass.

 

 

Berm

For very well drained soils, adding compost to the top layer of the garden will allow plants to establish themselves better and also allow the garden to retain more water. If you have compacted soils, you may add gravel or mulch to improve infiltration, or preferably a backyard wetland may be installed.

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Once the rain garden has been dug, planting can begin. It is important to note that plants in a rain garden will have to tolerate fluctuating levels of soil wetness. To help plants survive extended wet periods, it may help to plant the plants “high” on the edge of the rain garden or on mounds within the rain garden to elevate the roots above the ponded water level. Associated plant lists are available for guidance in plant selection.

Finally, the area should be mulched with 2-3 inches of hardwood mulch. Lighter mulches will tend to float, so avoid pine bark and pine straw mulches. Mulch is important in pollution removal, maintaining soil moisture, and in preventing erosion.

 

 

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NC Cooperative Extension is based at North Carolina's two land grant institutions, NC State University and NC A&T State University, and in all 100 counties and on the Cherokee Reservation.