Build a Rain Garden

Rain gardens help filter pollutants out of our water supply. They’re also pretty and easy to make.

Gardeners need water, but we don’t think of ourselves as caretakers of water. In fact, within our gardens we have the ability to help improve the quality of our local streams, rivers, lakes, or bays. Here’s how: When rain runs from your roof directly across the sidewalk or driveway and into street drains, it flows directly into sewers, taking with it pollutants like oily residue from roofs and driveways and air particulates (which are tiny bits of material that may contain carbon monoxide or sulfur). From there these pollutants head straight to our natural waterways.

Rain gardens—depressions in the landscape that contain permeable soil and plants—intervene in this process. Rain is channeled to the depression, which has been designed to allow water to move into and through the ground rather than run over the top. The soil and plants in the rain garden filter the water, helping remove pollutants before the water finds its way to the water table.

Building a rain garden isn’t just environmentally friendly, though—it’s also a great way to add a low-maintenance bit of beauty to your yard, and it’s easy.

Rain gardens can be as beautiful as they are functional when properly built and planted. Guide rainwater from a downspout into a rain garden with a small channel. This one in Minneapolis, MN, runs underground, beneath the sidewalk.

Rain Garden Basics

A rain garden is shaped like a saucer. In the center, it’s usually about 4 to 10 inches lower than the surrounding area. The bottom is flat and the sides slope gradually. It can be shallow and large, or deep and small, depending on your desires and conditions. Water from your roof’s downspout is typically directed into it by a rock-filled channel or underground pipe.

Before building a rain garden, you need to determine how big it should be, where it will be located, whether your soil will need amending for proper drainage, and what kinds of plants to put in it.

Calculate Size

The size of the rain garden is largely dependent upon the area of your roof from which you want to divert water. It can also be influenced by the amount of rainfall you want to capture; generally, you should plan to capture 50 percent of the water from the surface.

Many local agencies (city, county, state, or extension services) can help you figure out the size; they often have developed their own standard calculations. Some city websites even have a rain garden calculator—you simply enter your address and property information to get the size. Other agencies have methods that are based on the amount of area you have in impervious surfaces (roof, driveway, patio, etc.). Still others are more complicated calculations that involve multiplying pervious and impervious surfaces with coefficients that reflect a runoff value.

The city of Springfield, Missouri, offers an easy way to calculate a rain garden size: It should be about one-third the square footage of the runoff surface. Here’s a sample calculation:
Runoff surface: Roof
Size: 200 square feet
Rain garden size: About 67 square feet (one-third of the roof area)

Determine Location

When you choose the spot to build a rain garden, look for a well-drained, sunny location near the downspout but at least 10 feet away from your home or garage and situated on a level area away from tree roots and underground utilities. If there’s a gentle incline that will guide water from the downspout toward the rain garden, that’s perfect; but if not, you can build a rock- or tile-filled channel that directs the water.

A rain garden is shaped like a saucer to capture water that’s directed into it and to allow the water to drain slowly through the soil. There are four keys to building one: Locate it 10 feet or more from the runoff surface, size it correctly, make sure the soil drains adequately, and choose native plants that can tolerate the varying moisture levels.

Check Drainage, Prepare Soil

Once you’ve decided on a location, test to make sure the soil drains in a timely fashion to accommodate the water from a rainfall. Dig a hole about 1 foot wide by 1 foot deep. Fill it with water, and time how quickly it soaks through. If the level goes down at least an inch an hour, you have good drainage. If it drains well, you only need to excavate about 10 inches deep, remembering to slope the sides gradually at an angle no greater than 45 degrees. Add a few inches of compost to the bottom of the garden. If your soil drains poorly, dig down another 10 inches and replace some of the original soil with a 60-40 mix of sand and compost. Don’t add sand alone to heavy clay soils because you’ll end up with concrete. If you have sandy soil, add some compost.

Go Native

When you’re selecting plants for a rain garden, choose native species. They do best in rain gardens because they’re accustomed to local rainfall patterns and should only need supplemental water during an extended drought.

Select and place plants according to how much water they can tolerate around their roots over certain lengths of time. To help you figure this out, rain gardens are divided into three zones, which refer to how wet the soil may be for the few days it takes for water to drain through.

Zone 1: the bottom of the depression. Choose plants that can tolerate standing water from a few hours up to a couple of days.

Zone 2: the middle layer. Plants in this zone must tolerate the extremes of either wet or dry soil.

Zone 3: the top, or rim, of the garden. This area is the driest, and plants here should be drought-tolerant. Be aware that some municipalities reverse these numbers (so that Zone 1 is dry and Zone 3 is wet), and others use “wet, medium, dry” instead of numbers.

Just in Case

When it’s completed, your rain garden will help keep the environment cleaner, and it will offer delight and interest in all seasons.

Plants for Rain Gardens

Native species are the champs here, because they’re adapted to the rainfall pattern where you live. Here are some choice selections for various regions. The zones refer to those of a rain garden, not hardiness zones.

Western sword fern

Baja fairy duster

Pacific Northwest
Zone 1: lady fern (Athyrium filix-femina), deer fern (Blechnum spicant), twinberry (Lonicera involucrata)
Zone 2: redtwig dogwood (Cornus sericea), Western sword fern (Polystichum munitium), snowberry (Symphoricarpos albus)
Zone 3: low Oregon grape (Mahonia nervosa), red-flowering currant (Ribes sanguineum), evergreen huckleberry (Vaccinium ovatum)

Dwarf fothergilla

Butterfly weed

Zone 1: prairie acacia (Acacia angustissima), Wright’s beebrush (Aloysia wrightii), indigo bush (Amorpha fruticosa)
Zone 2: Baja fairy duster (Calliandra californica), superb penstemon (Penstemon superbus), trumpet bush (Tecoma stans)
Zone 3: bladderpod (Cleome isomeris), desert rose mallow (Hibiscus coulteri), prairie zinnia (Zinnia grandiflora)

New England aster

Red-flowering currant

Zone 1: boltonia (Boltonia asteroides), blue flag iris (Iris versicolor), smooth witherod (Viburnum nudum)
Zone 2: false indigo (Baptisia spp.), inkberry (Ilex glabra), garden phlox (Phlox paniculata)
Zone 3: dwarf fothergilla (Fothergilla gardenii), wax myrtle (Myrica cerifera), black-eyed Susan (Rudbeckia fulgida)

Prairie zinnia

Blue flag iris

Zone 1: cardinal flower (Lobelia cardinalis), cinnamon fern (Osmunda cinnamomea), ninebark (Physocarpus opulifolius)
Zone 2: blue star (Amsonia tabernaemontana), threadleaf coreopsis (Coreopsis verticillata), American cranberry bush (Viburnum trilobum)
Zone 3: butterfly weed (Asclepias tuberosa), sweet pepperbush (Clethra alnifolia), calico aster (Symphyotrichum lateriflorum)

American cranberry bush

Rattlesnake master

Zone 1: swamp milkweed (Asclepias incarnata), Joe Pye weed (Eupatorium maculatum), meadowsweet (Spiraea alba)
Zone 2: New Jersey tea (Ceanothus americanus), purple coneflower (Echinacea purpurea), New England aster (Symphyotrichum novae-angliae)
Zone 3: rattlesnake master (Eryngium yuccifolium), beard-tongue (Penstemon digitalis), heath aster (Symphyotrichum ericoides)

U.S. Department of Agriculture’s Natural Resources Conservation Service:
The Lady Bird Johnson Wildflower Center’s plant database:
A consortium of rain garden information:
City of Springfield, Missouri:

Illustration: GARY HITTLE