Diverting runoff into a home rain garden is a great way to support a drought-resilient landscape.
By Michael Hoyal, Intern at SAWS Conservation
During a rain event, much of the stormwater that falls on roads, parking lots and rooftops collects pollutants and runs off quickly into our creeks and rivers. Diverting this runoff into a home rain garden is a great way to not only clean the water, but also support a drought-resilient landscape that won’t require water from the Edwards Aquifer.
What exactly does a rain garden mean in a dry climate like ours? Think of it as a shallow basin or depression in your yard, planted with native plants that can handle occasional saturation but still be able to get by when the rain stops falling.
A seven-year study of rain gardens next to a parking lot in Santa Fe found that rain gardens could be essential to drought-proofing the landscape. Despite only receiving average annual rainfall of 14 inches, the soil in the rain garden retained enough moisture year-round to maximize plant growth. These rain gardens were able to support trees among other plants with no need for external irrigation — next to a parking lot. Also, rainwater harvesting can guide unused runoff to irrigate areas of need.
The principles of a rain garden are simple: water diverts from an impervious surface (such as through a downspout from your roof) to a shallow basin or bowl specifically dug into your soil and planted with native plants. These plants filter pollutants out of the water, which drains through the soil infiltrating deeper into the ground.
In the middle of the basin, you can plant plants that are more moisture reliant, like those that naturally reside on the shores of creeks. And along the sides of the basin plant flora that prefer drier climes.
To build a successful rain garden, there are a few things to keep in mind.
- Carefully consider the location. You want the rain garden to be at least 6 feet (ideally 10 feet) from your home’s foundation and where water naturally runs off your roof or other surfaces during a rain event. You can find this out by observing your property during a storm.
- Call 811 to make sure there are no utility lines in the area where you plan to install the garden.
- Test your soil with a perc test. To do this, dig a 12-inch deep by 12-inch-wide hole in the area of your rain garden. Saturate the hole with water by filling it and come back when that water is gone. Then, fill it again and measure how fast that water goes down. Ideally a rain garden should be able to absorb about 6 inches of water in 24 hours, so if you filled the hole full depth, that water should be gone in 48 hours. If your soil drains slower than that, you might need to dig out more soil and mix in sand with it to improve absorption.
If necessary to help divert water into the rain garden, construct a shallow trench to guide the water to the entrance of the rain garden. Some people use PVC pipes for this, but you can line the trench with something aesthetically pleasing like river rock to create a dry streambed.
In cases where diversion, not infiltration, is the goal, a bioswale can be constructed to divert water over longer distances. A bioswale is a vegetated depression used to help guide water runoff. You can see examples at Garcia Street Urban Farm, where bioswales divert runoff from a nearby neighborhood into an orchard of fruit trees.
Typically rain gardens can absorb rain from an impervious surface three times their size. If you can’t build one large enough, you may still find it useful, but it may not hold all the water during a large rain event.
Next it’s time to dig. Rain gardens should have a depth of at least 6 inches. If the area is steep, you can use the dug-out soil to construct a berm on the downhill side to contain the water. (You can also add rocks at the entrance of the garden where the water will flow in to slow the flow.)
Amend the soil with compost and fertilizer and you’re ready to plant! Plants such as frogfruit and inland sea oats are great for the center of your garden, and Gregg’s mistflower, scarlet sage and fall aster are all great choices on the sloping sides. You can also plant milkweed to support monarch migration.
Fall planting season is approaching — let SAWS help you offset the cost of planting material with our Watersaver Landscape Coupons if you’re converting a grass area to a rain garden!
Remember: The area transformed from turf to landscape bed must be at least 200 square feet. If your rain garden is not that large, consider planting the area around the rain garden to equal the minimum square footage required for coupon eligibility.
