How to Make a Rain Garden

As Diana Maranhao explains in Water-Smart Gardening, we are (slowly but surely) becoming wiser about preserving and protecting our water resources. The underlying goal of creating a water-smart landscape is creating healthy environments in which plants can grow, and a rain garden is a design champ for supporting drought-tolerant landscapes.

Rain gardens are a solution for plants that use rainwater as their sole source of water. A deep catch basin is dug first and the excavated soil is used to build an earthwork berm that surrounds or frames it. In areas with problem soils, berms are built using imported soil, creating a deep base of well-draining soil that can support deep-rooting plants.

The berm in the background forms the walls of the basin. A rain garden survives on water runoff from the berm and from rainwater captured in the basin. / Water-Smart Gardening

Materials for building berms can be soil of any type except sand, which would not hold its shape over a long period. If you are excavating or landscaping, use the soil you remove from projects to build the berm. If you are plagued with large amounts of clay or waterlogged, lean, or poorly structured soils, then importing clean fill soil may be necessary to build the berm. Building landscape berms requires some planning and measuring, plus a plan drawn to scale.

Use a garden hose to outline the berm and provide a good visual of its shape and size. Berms that are too small end up looking like little anthill mounds, out of proportion and not accomplishing their function.

Scale and slope are critical to minimize water runoff and erosion. The higher the berm, the wider and broader the base needs to be. The rule of thumb is that a berm 6 inches tall needs to be at least 24 inches wide at its base. In easy math: for every inch or foot of height, you need four times that measurement for the base. This calculation also ensures that the berm will not have overly steep sides, which makes them subject to caving in or eroding away with the first good rain. The height of a larger earthworks berm should not exceed 4 feet after compaction with a width of the base at 16 feet. Taller mounds of soil require retaining walls to hold their shape and to control erosion.

How to Make a Rain Garden

Water-Smart Gardening

1. Choose a site, size, and shape for the rain garden, following the design standards outlined above. Use rope or a hose to outline the rain garden excavation area. Avoid trees and be sure to stay at least 10 ft. away from permanent structures. Try to choose one of the recommended shapes: crescent, kidney, or tear drop.

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2. Dig around the perimeter of the rain garden and then excavate the central area to a depth of 4 to 8 in. Heap excavated soil around the garden edges to create a berm on the three sides that are not at the entry point. This allows the rain garden to hold water during a storm.

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3. Dig and fill sections of the rain garden that are lower, working to create a level foundation. Tamp the top of the berm so it will stand up to water flow. The berm eventually can be planted with grasses or covered with mulch.

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4. Level the center of the rain garden and check with a long board with a carpenter’s level on top. Fill in low areas with soil and dig out high areas. Move the board to different places to check the entire garden for level. Note: If the terrain demands, a slope of up to 12% is okay. Then, rake the soil smooth.

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5. Plant specimens that are native to your region and have a well-established root system. Contact a local university extension or nursery to learn which plants can survive in a saturated environment (inside the rain garden). Group together bunches of 3 to 7 plants of like variety for visual impact. Mix plants of different heights, shapes, and textures to give the garden dimension. Mix sedges, rushes, and native grasses with flowering varieties.

The plants and soil cleanse storm water that runs into the garden, leaving pure water to soak slowly back into the earth.

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6. Apply double-shredded mulch over the bed, avoiding crowns of new transplants. Mulching is not necessary after the second growing season. Complement the design with natural stone, a garden bench with a path leading to it, or an ornamental fence or garden wall. Water a newly established rain garden during drought times—as a general rule, plants need 1 in. of water per week. After plants are established, you should not have to water the garden. Maintenance requirements include minor weeding and cutting back dead or unruly plant material annually.

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You can have a colorful, striking garden that’s drought resistant, too!

Water-Smart Gardening gives you all the tools needed to create a water-smart landscape and garden. Drought is spreading throughout the country. Even areas that previously had plentiful supplies are feeling the strain, and the price of water is climbing. If you have to water your garden during non-drought years, many of the water-saving techniques from this book could still pay for themselves in no time.

Choose water-smart plants that survive and even thrive in low-water situations. Tap into the power of evolution and use plants native to your area. Time your irrigation and install water-collection devices such as cisterns and rain barrels. Creating a water-efficient garden can even be as simple as designing your landscape to harvest as much rainfall as possible, using berms, terraces, and raised beds.

Gorgeous photos throughout Water-Smart Gardening will inspire you with beautiful garden ideas and help you see your way to a garden that sips water instead of gulping it. Helpful how-to information gets to the nuts and bolts of everything from installing a cistern to using seep irrigation. Author Diana (Dee) Maranhao brings over 30 years of experience to help you create the garden of your dreams and save water at the same time.

Diana (Dee) Maranhao has been an active member of the horticulture and landscape industry for more than 35 years. She holds a degree in Ornamental Horticulture, and is a credentialed teacher in California, specializing in Ornamental Horticulture. Diana has spent much of her career in higher education, serving as a Horticulture Program Manager, Nursery Production Specialist, and educator specializing in Xeriscape-Low Water Use Landscaping, Nursery Production, and Plant Propagation. Diana has also been a horticulture editor and project editor for numerous educational texts, magazines, garden guides, and horticulture books. She has been a regularly featured garden columnist for more than ten years, authoring hundreds of gardening and horticulture articles for the public and the horticulture industry. Diana is also the author of Rocky Mountain Fruit and Vegetable Gardening (Cool Springs Press, 2014). She received the “2014 Professional of the Year” Award from Southwest Trees & Turf Magazine at Desert Green XVIII Conference in Las Vegas.