How to Build an Effective Rain Garden to Manage Stormwater Runoff
A rain garden is a low-maintenance, eco-friendly method of managing stormwater runoff and enhancing the beauty of your landscape. It serves as a natural filter of pollutants through the plants, mulch, and soil used to build a rain garden.
By capturing rainwater from your roof, hard surfaces, and lawn and redirecting it to a rain garden, you can...
Optimize the drainage in your yard
Reduce the burden on municipal stormwater infrastructure
And minimize pollution of nearby surface waters
The good news? Wilmington, NC (predominantly sandy soil) and Asheville, NC (well-drained cecil soil) are ideally suited for rain gardens.
While establishing a rain garden may be new or overwhelming to you, it can be simple if you know the right steps to take. Read on to learn the basics of building a rain garden.
1) Identify the Right Location
The first step is site selection. Your rain garden should be positioned downhill from a source of runoff, somewhere between the source and the runoff destination. You'll want to pick a location that gets direct sunlight for much of the day. Never place your rain garden uphill of your home, septic system, or wellhead.
2) Test Your Soil
If you don't know much about soil, now is the time to learn a few soil science basics.
You'll want to test an area's soil texture and drainage before building your rain garden in that location. The ideal soil for a rain garden is permeable (with significant sand content), but soils with loamy or clay content can also work with the right amendments.
To determine the drainage of your soil, you'll need to conduct a simple percolation test. First, dig a 12-inch hole in the area you wish to place your rain garden and fill it with water. Ideally, the hole should drain completely within 24 hours. But if an area takes more than four days to dry up, it's not a good area to install a rain garden.
The time it takes your soil to drain will determine the kind of rain garden you'll need. The three most common types are:
Quick-Draining Rain Gardens
Standard Rain Garden
Wetland Garden
Wondering which type of garden is best for YOUR landscape? Contact the Thorpe Landscapes team today for a free consultation.
3) Determining Your Garden Size
The size of your rain garden depends on the amount of runoff that location receives.
First, multiply the length and width of the impermeable surfaces that create the runoff (your roof, patio, sidewalk, driveway, etc.) to determine the total area of those surfaces.
Next, multiply that number by 0.1 (you'll want your rain garden to absorb 10% of the runoff from the impermeable surfaces).
This will give you the ideal area for your rain garden.
4) Prepare the Site
After calculating the size of your rain garden, mark the garden area with spray paint or flags. Then, remove existing vegetation and grass, preserving the top layer of sod as much as possible to help build a berm around the rain garden.
The key to proper rain garden drainage is to create a bowl-shaped depression where the water will collect and nourish your plants. The depth of this basin should be between 6 and 12 inches.
There is a simple method you can use to determine the ideal depth for your rain garden:
Dig an 8-in x 8-in x 8-in hole in your garden area and fill it with water.
Use a ruler or tape measure to record the starting water level.
Set a timer for one hour.
Once the hour is up, check the water level again. If the water level drops by at least half an inch, you can conclude that the soil can completely drain 12 inches of water in 24 hours. In other words, 12 inches is the perfect depth for your rain garden.
NOTE: Always be sure to call NC 811 before you dig anywhere in your yard to avoid hitting utility lines.
After digging a basin to your calculated depth, make sure the bottom of the basin is level. This will ensure rainwater is distributed evenly across the entire garden. Avoid compacting the soil. A rough surface will help with permeability and plant growth.
5) Install Drainage Features
The next step is the create an elevated berm around the perimeter of the basin with excavated dirt and/or sod. This will ensure water flows into the garden when it rains.
You'll also need to create a spillway or overflow zone. As the name suggests, this carved-out section of the berm will allow excess water to escape the garden in case of a larger storm event.
6) Apply a Permeable Soil Mixture
Depending on the specific content of your existing soil, you may need to add certain amendments to create a small layer of high-drainage soil.
For the best filtration, you need a topsoil mixture with this approximate composion:
50-60% sand
20-30% compost
20-30% native soil
The final mixture should have a distinct salt-and-pepper look. If your native soil drains quickly without any amendments, you may not need to incorporate much sand. Regardless, it's still important to add compost to the topsoil layer for optimal plant nutrition and growth.
Once you've gathered all the soil amendments, loosely combine the mixture with a rake or shovel.
Don't forget to account for a 2-inch layer of mulch. Remember, the lowest part of your rain garden surface should still be between 3 and 6 inches lower than the natural grade (a.k.a. elevation level) of your surrounding yard.
7) Plant Your Rain Garden
Choose hardy, native perennial plants to ensure your rain garden thrives in the local climate all year round. Always be sure to pick plants that work with nature rather than against it.
In both Wilmington and Asheville, NC, your plant selections should be wet-tolerant and able to withstand occasional periods of drought. Remember to consider wind tolerance, time and duration of sunlight, and specific soil type requirements for each plant.
For Quick-Draining Gardens, plant...
Switch Grass
Daylilies
Monarda
Juncus
For Standard Rain Garden, plant...
Inkberries
Cardinal Flowers
Goldenrods
Black-eyed Susans
Joe Pye Weeds
For Wetland Gardens, plant...
Lizard's Tails
Sedge
Woolgrass
Juncus
Softstem Bulrush
Waterlilies or Spatterdock (in the center where water will pool for longer)
As you arrange plants in your garden, be mindful of plant spacing and other specific recommendations for each different type of plant.
8) Add Mulch & Decorations
Install a 2-inch layer of wood mulch around the plants to suppress weeds, prevent erosion, and retain moisture in the garden. Feel free to add decorative elements like rocks, stepping stones, or other garden decorations for a more personal touch.
9) Connect Your Rain Gutter
Lastly, create a clear inlet for water to flow directly from the rain gutter attached to the side of your home to the garden. If your rain garden is located a ways away from the house, you'll need to extend the downspout with corrugated plastic tubing or PVC piping.
Dig a trench to house the tubing, carefully peeling and preserving the turf. Line the trench with crushed stone or gravel to prevent erosion, backfill with a layer of soil, and reapply the turf over the top to encourage quicker grass regrowth.
Extend the tubing into the rain garden basin approximately one foot. Place flat or crushed stone underneath the tubing where it enters the garden to ensure heavy rains don't drown or wash out young plants or erode the inlet.
Pro Tip: This final drainage step should be completed last to avoid flooding the garden area before the project is finished.
10) Maintain a Thriving Garden
In the first month or so after installation, you'll need to keep a close eye on your rain garden to ensure it drains properly after rain events.
To help young plants thrive and grow...
Weed the garden regularly
Replenish mulch as needed
Dig a notch in the berm to allow some water to escape as your plants establish strong roots
Water your garden around one inch every week (if it's not raining)
While the garden may require extra work in the beginning, the long-term results are extraordinary. In a couple of years, your beautiful, low-maintenance rain garden will be easily managing runoff all on its own!
Build Your Rain Garden With Thorpe Landscapes
Ready to install a rain garden in your yard? You're not in this alone! Let our drainage experts leverage our extensive experience to build you a stunning rain garden for the very best results. Contact us today to get started on your rain garden project.