Tired of that muddy puddle in the middle of the yard, or that washed-out mini-gulley that forms whenever a downpour loads up your home’s downspouts? If so, it might be time to get a handle on all that runoff and put it to good use by learning how to build a rain garden. These shallow saucer-shaped gardens, commonly described as “nature’s water filters,” are designed to capture excess runoff that can potentially wreak havoc on your soil and pollute waterways. All you need to create your own rain garden is a well-designed plan, a handful of native plants, and some good old-fashioned elbow grease.
How rain gardens work
Rain gardens are designed to catch storm runoff from rooftops, patios, sidewalks, roads and other impervious surfaces. During a storm, rain gardens will fill with a few inches of water that gradually filter into the ground. When properly designed and constructed, these structures can hold water for around 24 hours and will not attract mosquitoes — they’re much more likely to attract birds and beneficial insects.
Rain gardens can be helpful wherever water runoff is an issue. Ideally, you will want to situate the rain garden in between the source of the runoff and the runoff destination.
Before you break ground, make sure the garden is at least 10 feet from any buildings or structures, and at least 25 feet from any septic system drain field. Also be sure to avoid underground utility lines (call 8-1-1 at least 48 hours before digging), and if you have trees on your property, avoid disturbing established root systems.
Conduct a soil evaluation
Sandy and loamy soils work best for these backyard havens since they tend to drain well. Clay soils can become waterlogged and may not be suitable for a rain garden. Test the drainage of potential sites by digging a percolation test hole that is at least 8 inches wide and 8 inches deep. Fill the hole with water and let stand. Ideally, the water should drain at a rate of about an inch every hour. If you are working with hard clay soil that won’t drain, remove it and replace it with a mix of approximately 60 percent sand, 20 percent topsoil and 20 percent compost.
Calculate rain garden size
A rain garden can be almost any size, but most residential rain gardens range from 100 to 300 square feet. Many are the shape of a saucer or kidney bean, with the largest side facing the source of runoff. Use site conditions as a natural guide in shaping the garden. Rain gardens are generally 4 to 8 inches deep. Anything deeper than 8 inches may pond water, and rain gardens less than 4 inches deep may not provide enough water storage for proper infiltration. The slope of the land should help determine the depth of the garden.
Use the following general guidelines to determine rain garden depth. If the slope is less than 4 percent (4 feet vertical in 100 feet horizontal), build the garden 3 to 5 inches deep. If the slope is between 5 percent and 7 percent, build a garden 6 to 7 inches deep. If you are working with a slope in the 8- to 12-percent range, make the garden about 8 inches deep. Remember, rain gardens are easier to install and tend to work best in places where the ground is relatively level.
To determine the length and width of the rain garden, think about how it will catch runoff. Runoff should spread evenly across the rain garden so water doesn’t pool at one end or spill over before it has a chance to filter into the ground. Plan the garden so that the longer side faces upslope. This will ensure that the garden catches as much runoff as possible, and that the water spreads across the entire length of the garden. The width of the rain garden will depend on the slope of the land, but most residential versions are between 10 and 15 feet wide. Remember, if a rain garden is too wide, it may become necessary to add additional soil to the downhill half of the garden.
Here is a relatively simple formula to approximate measurements for your rain garden. First, find out which hard surfaces will be producing the storm runoff for your garden.
Let’s say you are planning to use roof runoff as the main source of water. In this example, you would measure only the parts of the roof that will be “feeding” the garden. Measure the width and length (in feet) of that part of the roof, then multiply the two numbers to get the square footage. This number is your “drain area.” If you are gardening with sandy or loamy soil, plan on making your rain garden about 20 percent to 30 percent of the drain area size. For instance, if the part of the roof that will provide runoff to the garden measures 1,000 square feet, you’ll want to make the rain garden 20 percent to 30 percent of that number, or 200 to 300 square feet in size. If you are working with clay soils, plan on making the rain garden larger to compensate for poor drainage.
Prepare the site
Start by defining the borders of the rain garden by laying a string or hose around the perimeter or marking it with fluorescent spray paint. You can use a rototiller or backhoe, or dig by hand, depending on the size and depth of your garden. As you dig to the desired depth, heap the soil around the downhill edges to create a berm. Once the soil excavation is complete, use a hand level to make sure the bottom of the garden floor is level. Mix and add necessary soil amendments. Again, if you are working with clay soil, create a simple garden soil mix of sand, topsoil and compost to amend or replace the difficult soil. Once the soil has been amended, let it settle overnight before planting.
Planting and care
Choose native plants appropriate for the sunlight exposure and soil conditions of your rain garden. Plants need to tolerate standing water for up to 48 hours, as well as some periods of drought. First-time rain gardeners are encouraged to contact their local soil and water conservation district for a list of native plants that work well under local conditions. Keep identification tags on the plants — or otherwise label them. Some plants may thrive better than others, and this can help you start to identify plants that work best in your garden.
Now it’s time to plant. As a general rule, plant shrubs 3 feet apart, perennials 1 foot apart, and annuals 6 to 8 inches apart. Apply 3 inches of mulch, and water plants immediately after installation.
Most rain gardens do not require much maintenance. Check plants periodically for signs of wilting, and weed as necessary. Also, check for berm failure and ponding. If standing water occurs longer than two days, this is a sign that the garden is not draining properly.
A rain garden offers an opportunity to use your imagination, so be creative as you design a beautiful oasis on your property.
A writer and gardener living in Phoenix, Patricia Escarcega is always looking for and experimenting with new ways to conserve water while keeping a garden in the desert.