Creating a Rain Garden

Locating a Rain Garden

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.

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!

Soils & 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.

• The site ponds water or remains saturated for several days after a storm event.
• The soil shows signs of being a wetland soil within 1 foot of the surface. A wetland soil is often gray with ribbons or areas of brown color.
• Water poured in the dug hole is still there after two days, provided it hasn’t rained.

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.

Sizing Your Rain 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.  However, if you can capture more rainwater from a 2-inch or 4-inch storm, even better.  On the other hand, if you don't have enough space for a rain garden to catch a 1-inch rain, even a smaller rain garden will help.  Better yet, reduce the amount of impervious area or install rain barrels or cisterns to store the runoff.

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.

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.

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.

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.

What kind of plants should I use?

Use container-grown plants with a well-established root system. It’s fun to sow native wildflower seed, but experience shows that this doesn’t work too well in a rain garden. Flooding, weeds, and garden pests will be hard on your seeds, and the garden will be mostly weeds for the first few years. You can start plants from seeds indoors, grow them for a few months, and then move the plants outdoors. You might be able to get transplants from a gardening friend, or you could participate in a plant rescue where groups dig up plants from construction sites before the heavy equipment moves in.

Plants that are native to your area should need the least maintenance – they have adapted to the climate and rely on the insects that live in your area. Using little or no fertilizer and pesticides works toward our goal of improving water quality. There are many spectacularly beautiful plants that are native to North Carolina to choose from. Where you live in North Carolina and where you place your garden will determine what type of plants are best for you. Associated plant lists are available for different regions of North Carolina.

Planting several species in your rain garden can create a long flowering season, and give your garden depth and dimension. 

Make sure to dig a wide enough hole for the plant to thrive well. After planting, lightly tamp down the soil around the plant to eliminate air pockets.

Your rain garden will have a couple of different wetness zones in it. In the deepest part of the garden, you can put plants that withstand a couple of days of flooding at a time. In the shallower parts and on the edges, you can put more typical landscape plants.

Water immediately after planting, and then water twice weekly (unless rain does the job for you) until the plants are well established. After the first growing season, you shouldn’t need to water the plants unless there is a lengthy drought.

Add mulch two inches thick, but avoid burying your new plants with the mulch. You want mulch that won’t float away…hardwood mulch is best.

Information about the best plants to use in different regions:

• List of native plants to use in rain gardens
  
• Piedmont
• Plants to use in piedmont rain gardens
• Example of piedmont rain garden in sunny location
  
• Example of piedmont rain garden in shady location
• Mountains
• Plants to use in mountain rain gardens
• Example of mountain rain garden in sunny location
  
• Example of mountain rain garden in shady location
• Coastal Plain
  
• Plants to use in coastal rain gardens