A verdant lawn can make your neighbors green with envy, but traditional turf grass has its downsides. Beneath the surface, shallow roots and compacted soils can't absorb water, which leads to excessive stormwater runoff. Typical urban lots can shed thousands of gallons of water during a rain event, and that water will eventually make its way to rivers, lakes and streams, as well as pollutants it picked up along the way.
One way to stop runoff is by planting a rain garden. These landscaped depressions intercept and filter rainwater coming off hard surfaces like lawns, roofs and driveways, and give that water a chance to be absorbed by the ground.
If a rain garden sounds right for you and your yard, we’re here to help. We’ve put together a step-by-step guide on how to plan and build your own rain garden.
1. Select your location
Think about where water flows on your property when it rains. Positioning your garden downhill from downspouts is usually a good bet, and you can even get a contractor to re-route your downspouts if they’re in a tough spot.
After you've identified where water flows, you should then eliminate from consideration areas where it would be problematic to have standing water. This includes under trees, over utility lines or other places where water already pools in your yard. Pooling water indicates that a rain garden in that spot may become waterlogged and won’t have a chance to dry out. Similarly, placing a rain garden over tree roots or utility lines might cause disruptions if water is confined in that space.
2. Calculate the size of your garden
The size of your rain garden will depend both on the amount of water running to it from a downspout or other surface and the speed at which water infiltrates the soil.
First, measure how many square feet of surface area will feed your garden. For rain gardens positioned below a downspout, this will be the surface area of the roof that feeds that downspout. If your roof is mostly square, you may be able to measure from the ground, but Google Earth also has a measurement tool that can help you calculate the square footage of an irregularly shaped roof.
Next, you’ll need to test the soil infiltration rate. This is the speed at which water drains into the soil. You can do this by digging a 12-inch deep hole where you plan to put your rain garden and filling it with water. Measure the water level at the start and then again after one hour. You need a soil infiltration rate of at least 0.5 inches per hour in order to build a rain garden.
Calculating your desired rain garden size. When you know the square footage of your surface area and the soil infiltration rate, you can use the table on page 31 of this guide to figure out the size of your rain garden. As an example, a standard six-inch deep rain garden where the soil infiltration rate is one inch per hour will need to be 10% of the surface area that feeds it. This means that for a 500 square foot roof, you’ll want a rain garden that is 50 square feet.
3. Design your garden
Once you’ve chosen the location and calculated the size of your rain garden, it’s time to design! You’ll need to choose a shape for your garden — something that’s easy to maintain and mow around — and you’ll also need to choose your plants. ISU Horticulture Extension recommends native plants because they have deeper roots that are better at soaking up water and they attract diverse pollinators. However, non-natives like irises, lilies and hostas will also work.
In need of a little inspiration? The Polk County Rain Campaign site has plenty of pictures and examples to get you started.
4. Building the garden
Excavating and landscaping a rain garden can be a lot of work, so you may want to hire a contractor or landscaping company to help. However, if you want to do it yourself, you can knock out a rain garden in an afternoon with a small team of friends and a solid plan.
There are several steps involved in building a rain garden, including digging, improving the soil with compost and other amendments, and planting. For a full guide on how to do it yourself, check out iowastormwater.org.
And remember: Contact Iowa One Call at 800-292-8989 to have someone mark your utility lines before you start digging.
5. Maintaining your new rain garden
The wonderful thing about a rain garden is that it’s self-watering! However, while your plants are establishing their roots, you’ll need to give them a little extra help. For the first year or so, make sure your plants are watered thoroughly once or twice a week.
Once they’re established, you’ll only need to water during dry spells. And, as with any garden project, you’ll want to weed your garden by hand, without chemicals.
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Want to take your stormwater control to the next level? Consider ditching your turf grass completely for a native lawn!
