Should Weed Block Be In The Bottom Of A Rain Garden?

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!

Rain garden filled with native plants_MDurham — Swamp iris, cardinal flower, wild hydrangea, and fern fill this partly shaded 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.

Rain garden, 5 ft deep and 20 ft wide, sized for stormwater runoff from slope, driveway, and house roof_MDurham — Rain garden at base of slope sized to hold stormwater runoff from hillside, house roof, and steep driveway.

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.

Rain garden protects nearby creek from runoff_MDurham — Rain garden with rock-covered berm protects nearby creek from stormwater runoff.

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.