rainwater

You’ve decided to build a rain garden. Now what? In Part I of our series, you learned what a rain garden is and why they’re beneficial to your residential landscape and the environment. Part II gives you the basics on planning and building your rain garden!

Choose the location
First, decide where to put your rain garden. Obvious places include low points in your yard where water accumulates when it rains, near downspouts (if extended outward from the house foundation), or where underground drainage pipes daylight. Also consider areas where rainwater flows onto your property from the street or a driveway.

Location requirements. Your rain garden must be:

• At least 10 to 15 feet away from the house foundation
• 25 feet away and downslope of a septic system drain field
• 10 feet from a well head
• Not directly over underground utility lines

It is helpful to put the rain garden in part or full sun for the widest selection of plants and the best drying effects from sun. 

Mountains present special challenges. When locating a rain garden on slopes, a series of cascading rain gardens can be a viable solution on a hillside. Avoid slopes greater than 12%, which will require different stormwater runoff management methods.

Evaluate the soil conditions and drain time
Proper construction and plant selection for a rain garden requires knowing your soil and how well the site drains. Is the soil hard clay, sandy, or loamy? Is it highly compacted and impermeable? To find out, conduct a perc test.

Dig several one-foot-deep holes in various areas of the proposed garden and fill them with water. Time how long it takes for the water to drain out of the holes. Drain time for a standard rain garden should be 12 to 72 hours. A fast-draining rain garden will empty in 12 hours; any area that takes more than 3 days to drain is a wetland.

Determine the size
The size of your rain garden depends on the impermeable surfaces (roof top, driveway, street) that send water to your garden. The amount of impervious surface determines your rain garden’s width. The depth of your rain garden depends on the results of your perk test—longer drainage times require deeper gardens.

In North Carolina, we recommend designing rain gardens to hold a temporary 10-inch deep pond of rainwater, based on a 1-inch rainfall. The size of the rain garden should be about 10% of the area of impervious surface draining into it.

For example:

• First calculate the impervious surface area that will drain to the garden. This will usually be a portion of your roof plus your driveway. If there are other areas draining to the garden, such as sidewalks and patios, you’ll need to add those surface areas as well.
• Assuming that your house is 60-feet by 60-feet, and that one-fourth of your roof area drains to the garden—this can be determined by the number and location of downspouts—then the impervious surface runoff from the house is 900 square feet (60 ft. X 60 ft. X 25% = 900 sq. ft.).
• Assuming that your driveway is 50-feet long by 10-feet wide, and that the entire driveway drains to the garden, then the impervious surface runoff from the driveway is 500 square feet (50 ft. X 10 ft. X 100% = 500 sq. ft.)
• Add together the total impervious surface areas that drain to the garden: 900 sq. ft. from the house roof plus 500 square feet from the driveway for a total of 1400 square feet.
• Using the rule of thumb cited above, the size of your garden should be about 10% of the total impervious surface areas draining into it. In this example, your rain garden should be about 140 square feet in size (10% of 1400 sq. ft. = 140 sq. ft.).

Our example is for sizing a standard-draining rain garden. But what if your soil is hard clay or compacted and does not drain well? Your options include enlarging the square footage area of the garden, digging a deeper basin, and layering several permeable materials—such as filter fabric, perforated pipe, and gravel—beneath the final amended soil layer.

Dig the basin and form the berm and weir
Now that you know the size of your garden, you’re ready to lay it out and dig. Use a rope, garden hose, or can of spray paint to outline the boundary. It can be any shape you choose—round, oval, kidney, teardrop—but it should fit the space and complement the surrounding landscape. Be sure to get your underground utilities marked before digging; within North Carolina just dial 811!

Digging the basin. Create the basin by digging out the soil to a 13-inch depth—10 inches for rainwater plus an additional 3 inches to accommodate mulch. Turn or till another 6 inches of soil in the bottom of the basin to loosen the dirt and improve water infiltration.

What about the rain garden in heavy clay or compacted soil? You may need to excavate the garden as much as 3- to 4-feet deep to accommodate layers of permeable materials. Layering helps prevent standing water in dense and compacted soil. Line the bottom with a permeable landscape fabric, then add layers of gravel, perforated pipe, sand, and soil. Be careful to leave 10 inches of ponding space and 3 inches for mulch in the basin.

Creating the berm. Pile all the excavated soil along the sides and lower edge of the garden, creating a berm and keeping the bottom surface area of the basin as level as possible. Make the berm about 3- to 6-inches tall. You can cover it with plants, sod, mulch, or rocks.

Making the weir. You’ll need to create a weir or exit ramp for water to overflow the garden in heavy rains. Dig the weir opening on the downhill side of the berm, about one-foot wide and slightly lower than the rest of the berm. You can cover the weir with rock or sod to prevent erosion.

Fill with soil mix
Mix a couple inches of topsoil or compost with the loosened soil in the bottom of the basin. Next, you’ll add plants and, finally, 3 inches of hardwood mulch to help maintain soil moisture, prevent erosion, and remove pollutants.

Stay tuned for Rain Gardens—Part III: It’s Time to Plant!

 Learn more

Rain Gardens
by Missouri Botanical Garden
Provides planning and constructing information and a good plant list.

Start to Finish Rain Garden Design: A Workbook for Homeowners
by Faribault County, Minnesota, Soil & Water Conservation District
Provides detailed guide and templates for planning and construction a rain garden.

Rain Garden Site and Soil Assessment
by U.S. Department of Agriculture, Natural Resources Conservation Service, Connecticut
Provides detailed worksheet for calculating garden size.

Tips for Rain Gardens in Clay Soils
by Rutgers, The State University of New Jersey

Amending Clay Soils
by Durham County, NC Cooperative Extension

Article written by Beth Leonard, Extension Master Gardener^SM Volunteer.

Next time it rains, head outside and take a look at the route stormwater runoff takes onto and through your property. Does the water come off the street, driveways, your roof, downspouts? Does it go directly into a storm drain, an open drainage ditch, the neighbor’s yard? Maybe it sheets across your lawn or cuts channels down your slopes. If it comes to rest in a low-lying area, leaving you with an unwanted “pond” and a soggy landscape, your yard could be a good candidate for a rain garden!

What are rain gardens?
Rain gardens are shallow depressions planted with deep-rooted native plants and grasses that are especially effective for residential use. Even very small areas can absorb as much as 30 to 40% more runoff than a standard lawn. While rain gardens won’t solve all stormwater runoff and erosion problems, they serve many beneficial purposes when properly constructed and positioned.

Why have a rain garden?
The primary reasons for rain gardens are diverting water away from your house and handling standing water in the landscape. When well-designed, a rain garden creates a lovely, interesting focal point in your garden!

Important environmental reasons to embrace rain gardens include:

• Minimizing runoff into storm drains and creeks
• Filtering pollutants and sediment carried in stormwater runoff through rain garden plants, mulch, and soil
• Gradually soaking rainwater into the ground
• Nourishing plant life and replenishing groundwater tables
• Providing habitat for wildlife, butterflies, and beneficial insects

Will my rain garden become a pond?
The short answer is “no,” although several types of stormwater management systems are called “retention ponds.” These include wet ponds, dry ponds, and rain gardens:

• Wet ponds are constructed basins on large drainage sites that have permanent pools of water, at least through the wet season. They are complex ecosystems that hold and treat rainwater before it flows downstream.
• Dry ponds—usually constructed of concrete or rock beds—immediately remove concentrations of rainwater from an area.
• Rain gardens slow down stormwater runoff and hold it for a short period of time—no more than 24 to 48 hours. Water collected in a rain garden usually seeps into the ground within an hour or two. Therefore, a rain garden should not contain any permanent pools of water.

A word about mosquitoes. They won’t find rain gardens to be good breeding areas. It takes mosquitos 10 to 14 days to go from egg to adult, and any water standing in the rain garden is long gone before then!

Things to think about
Multiple factors inform the way you design your rain garden—its depth, width, lining, layers of materials, and types of soil. Factors to consider when planning a rain garden include:

• Soil composition
• Slope
• Volume of water
• Space available
• Distance from the house
• Amount of sun and shade

You’ll also have to decide what plants to use. A combination of trees, shrubs, grasses, and flowering perennials will produce the most aesthetic appeal, but some plants may need to handle both wet and dry conditions. Others may need to tolerate different levels of water—both amount or volume—and force or velocity.

Learn more
The winter months are a good time to plan and get started constructing your rain garden. To learn how, watch for these blogs over the next several days: building a rain garden, planting a rain garden, and maintaining a rain garden.

Backyard Rain Garden
by NC Cooperative Extension, 2015

Rain Gardens: Green Solutions to Stormwater Pollution
by Clemson University, South Carolina, Feb. 2009
Provides A to Z information with good diagrams. Be sure to adapt plant and soil recommendations to N.C. Mountains.

Article written by Beth Leonard, Extension Master Gardener^SM Volunteer.