The Do-It-Yourself Rain Garden

When it rains all weekend and you’re stuck at home, you have time to notice a lot of things. Like that one spot where water runs off the roof a bit too fast, some of it disobediently making its way into the basement. Or maybe you watched as rainwater rushed down the driveway, straight into a storm drain, or into a depression in the lawn where it always seems to pool after a downpour.

A scaled-down version of the storm-water management tactics used in municipal planning can help solve those problems, slowing water flow and increasing infiltration. And if the solution is landscape-focused and involves planting native species, it will also support pollinators and other beneficial insects, promoting overall diversity.

Think of it as a do-it-yourself rain garden to the rescue — and then some.

At her home near Wilmington, Del., Carrie Wiles is making a plan to redirect the water that rushes off the roof over her kitchen and dining room, entering the cellar, during the new-normal downpours that follow extended dry spells. Ms. Wiles, a horticulturist and the marketing manager at North Creek Nurseries, in Landenberg, Pa., is no stranger to the idea of using plants to solve environmental issues.

North Creek, whose motto is “where horticulture meets ecology,” is a wholesale producer of what are called plugs or liners, many of them species native to the Eastern United States. These baby plants are sold not just to garden centers but to landscapers, park systems, universities and municipalities. Plants used to restore woodland habitats and create meadows are among the company’s specialties, as are those that address water challenges in smarter ways.

To solve her problem, Ms. Wiles knows that it won’t be enough to extend the lower end of her downspout or to add a longer splash block. She also knows that a stretch of turf grass beyond the downspout won’t help much. Compared with properly chosen native plants with far deeper root systems, she said, lawn is a poor substitute where water management is needed. That’s one more reason, among many, to reduce your expanse of mowed green.

‘Put the Water in the Landscape’

First, and maybe most important: A rain garden is not a water garden, although the main portion is shaped like a basin or a shallow pond.

“It’s meant to capture and hold that water for a short, specific amount of time — maybe 12 or 24 hours — as it infiltrates or seeps into the ground,” Ms. Wiles said.

In his book “Sustainable Stormwater Management: A Landscape-Driven Approach to Planning and Design,” Thomas W. Liptan, a landscape architect, explained the basic goals of landscape storm-water management, principles that are relevant whether a project is municipal-large or backyard-small.

“Put the water in the landscape,” Mr. Liptan writes — meaning not in the basement, or running down the driveway or across the garden in gully-washing fashion, where its path is lined with impervious surfaces like roofs and pavement.

You can help water move across the landscape, he suggests, by creating a design that is both functional and attractive. At some sites, that may mean a rectilinear bed; elsewhere, it may be something curvilinear. The garden must also be planned with aftercare in mind, for ease of maintenance.

Rain Garden Basics

A rain garden has three basic components.

The main feature is a shallow basin, usually about six inches deep, that is large enough to accommodate the runoff you’re trying to manage. If you plan to mulch around the perennials or shrubs that will be planted in the basin, excavate to eight or nine inches to allow for that layer (and be sure to use shredded mulch, not chips that will float to the surface).

To guide the water from its origin point toward the basin’s inlet, and to slow it a bit, Ms. Wiles recommends installing a dry stream bed of stone or coarse gravel.

And at the farthest or lowest end from the water source is the garden’s third component: a small lip or berm.

Where in your home landscape will a rain garden (or more than one) be most effective? Plan to spend the next rainstorm or two outdoors to figure that out, Ms. Wiles advised, watching the water and how it moves naturally — for better or worse — as things are currently configured.

“Look primarily at how the water flows away from the roof, and down your downspouts from the house,” she said. “Also watch for where it tends to pool.”

Look again a couple of hours after the storm ends, and then 12 and 24 hours later, to see how quickly it seeps into the ground. Make notes.

Finding a Site

Taking into consideration your observations, identify a relatively level potential site for your rain garden, Ms. Wiles said. Areas that have a 12 percent or steeper grade would require extensive excavation to create a level basin, with a substantial berm on the far side.

Spots where bedrock is close to the surface or those with seasonally high water tables won’t work, either. Nor will anywhere that water currently pools in a prolonged way after a storm (which indicates poor drainage). A sunny site is preferable to a shady one.

A rain garden should not be within 10 feet of any foundation, Ms. Wiles cautioned (and farther is better, if the property allows). Nor should it be near a septic system (make sure it’s at least 15 feet away) or a drinking well (allow for a distance of 25 feet or more).

Other underground realities to consider: Because digging will be involved, call the utility companies for information on underground lines, to confirm that the spot you’re considering is safe.

Once a suitable site is identified, the next requirement is a basic percolation test, to evaluate drainage. Dig a roughly cylindrical hole about a foot deep and six inches wide. A post-hole digger is ideal for this task. Fill the hole with water and let it drain thoroughly, which may take hours. Measure the depth of the hole, then refill it with water, and recheck the water depth every hour for the next four hours, to calculate the rate of infiltration. An inch or an inch and a half of drainage in an hour is ideal, Ms. Wiles said; less than a half-inch is not suitable without some soil amendment.

Calculating the Size

A rain garden must be matched to the potential volume of runoff it may be asked to accommodate. Most backyard rain gardens are in the 100- to 300-square-foot range; some homes have multiple problem spots, and need more than one.

“A typical home has two major impervious surfaces where you get runoff: a roof and a driveway,” Ms. Wiles said.

Most runoff starts with the roof, as you probably noticed during your rainstorm research, so the math of the rain garden size starts there, too.

Ms. Wiles uses the scenario of a one-inch rainfall event to begin her calculation, as 90 percent of the precipitation totals in her area in any 24-hour period are an inch or less. The other parts of the equation are the square footage of the portion of the roof feeding the trouble spot, the number of downspouts serving that part of the roof, and your soil type and slope.

Each region has its own rainfall patterns, and soil types vary widely, so Ms. Wiles recommends searching online for your state or region and the term “calculating rain garden size” for localized advice from a cooperative extension, state department of environmental conservation or other such organization. One that she has used: The Three Rivers Rain Garden Alliance calculator.

A Palette of Plants

In step with North Creek’s ecological philosophy, Ms. Wiles thinks of the rain garden she is planning at home as a habitat in miniature, with distinct zones. She visualizes each zone — the bottom of the basin, the sides, up top — as being best suited to plants from distinct habitats in nature, from wet-feet-tolerant species to upland ones.

“In the lowest portion of the basin go the ones that can handle the most water,” she said, citing examples like common rush (Juncus effusus), upright sedge (Carex stricta), white turtlehead (Chelone glabra) and golden ragwort (Packera aurea).

A search tool on the North Creek website allows users to filter for suitable plants, including obligate wetland species (those almost always found in wetlands) and facultative wetland plants (found in wetlands and non-wetlands). Choices with other relevant qualities, both functional and aesthetic, can also be identified.

Along the top of the rain-garden perimeter, Ms. Wiles said, plants “must be able to tolerate being really dry between storms.”

Tall garden phlox (Phlox paniculata) and coneflowers (Echinacea) are possibilities. For late-season color, she may work in an ironweed like Vernonia lettermannii Iron Butterfly.

And for the intermediate area? Great blue lobelia (Lobelia siphilitica) and obedient plant (Physostegia virginiana) are among the leading contenders.

Like the others, they are plants she looks forward to looking out at during a rainstorm, knowing that they are not just beautiful but also hard at work.

Margaret Roach is creator of the website and podcast A Way to Garden, and a book of the same name.

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