How to Design a Rain Garden

Choosing the site, shape, plants, and more

Rain garden

 

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The term "rain garden" is something of a misnomer. A rain garden conjures up visions of perpetually moist landscape features: juicy basins teeming with thirsty plants and perhaps an amphibian or two. In fact, rain gardens experience a wide range of moisture conditions, ranging from boggy to bone-dry. According to the Soil Science Society of America, a rain garden is a feature "used to slow, collect, infiltrate, and filter stormwater." Rain gardens benefit the environment because they prevent stormwater (and the many pollutants it can contain) from running off of a property and into local waterways via the storm sewer system. Rain gardens also provide an effective solution for dealing with occasional flooding, in addition to providing attractive landscape features.

What Is a Rain Garden?

A rain garden is a planting that you install in a low spot in your landscape. This may be a natural depression on your property, or it may be the result of a planned change in grade that you implement in your landscape design. In addition to collecting rainwater, you can also direct downspouts into rain gardens.

A rain garden works somewhat like a retention pond: It collects water runoff and allows it to drain into the soil and be taken up by flood-tolerant plants. But, rather than being water-hungry types, the ideal plants for rain gardens are native plants, which are naturally adapted to a wide range of moisture conditions. Approximately 80 percent of a native plant's mass is underground. This gives you an idea of native plants' potential to trap and filter runoff water.

Benefits of Rain Gardens

Rain gardens prevent pollution. Runoff water isn't pure rainwater. It collects pollutants as it flows, especially excess nitrogen and phosphorus from lawn fertilizers as well as oils, salts, and other pollutants dropped onto driveways and walkways. These pollutants can harm rivers and ponds by causing algae blooms and fish kill. The deep roots of native plants readily absorb these nutrients, protecting groundwater quality.

Rain gardens also can reduce the effects of flooding. A mature stand of rain garden plants will take up excess water that might normally puddle in your yard, leading to plant or lawn damage, unpleasant odors, and possibly water intrusion issues in your home. When you get rid of standing water, you also reduce mosquito breeding problems around your home.

Plus, rain gardens create habitats for wildlife. The monoculture that is a lawn does little to nurture wildlife, but a mixed planting of native flowers will draw butterflies, bees, other beneficial insects, and birds. Finally, rain gardens help conserve water. Native plants thriving in a rain garden typically don't need supplemental irrigation because they're deep-rooted and have evolved to survive periods of drought between rains.

The Right Site, Size, and Shape for a Rain Garden

If you're planning to divert your roof gutters into your rain garden, you won't want a rain garden to be too far away from your home. However, it must be at least 10 feet away from the house to prevent the risk of water intrusion. Underground utilities and tree roots may also influence the placement of the rain garden. If your yard doesn't have a natural depression, you'll need to create one by digging either by hand or with an excavator. It isn't necessary to create a pit; a gentle basin no more than about five inches deep is sufficient. The basin should have a flat bottom and gently sloping sides, and you can use some of the fill dirt to create a berm to hold excess rainfall until the plants can absorb it.

How big should a rain garden be? A small rain garden is better than nothing, but you should plan on a garden of at least 150 square feet to provide enough room to include an interesting variety of plants. Choose a natural, rounded shape for your rain garden, like an oval, teardrop, or kidney shape. If you're starting from scratch with an excavated garden, outline the potential area with a hose or rope first, and view it from all angles as well as from inside your home to envision the finished product.

Choosing Plants for a Rain Garden

Proper rain garden design dictates that all plants must be perennial. This is due to the way a rain garden functions as a water abatement feature, which depends on the deep roots of perennial plants.

Plants that you install in the center of the garden will experience longer periods of wet soil and must be able to thrive in these moist conditions. Five hardy and long-blooming plants you can't go wrong with include false indigo; coneflower; hardy hibiscus; black-eyed Susan; hardy geranium (cranesbill); and hardy grasses, such as fountain grass or corkscrew rush

Plants around the edge of the rain garden will rarely have wet feet and won't mind occasional periods of drought. Some attractive flowering choices with big wildlife appeal include gaura, coral bells, yarrow, butterfly weed, and catmint

Trees and Shrubs for a Rain Garden

Although a rain garden isn't the place to grow a large shade tree, you can add a small ornamental tree or shrub to act as a focal point. The deep tap root of a pawpaw tree is particularly well-suited to a rain garden. A redbud tree has a pleasing vase shape and will reach a mature height of 15–20 feet. A hackberry or river birch specimen can work in a larger landscape and will greatly increase the water absorption ability of your rain garden.

For smaller rain gardens, shrubs provide those necessary bones that lend interest to the landscape in the winter months. An oakleaf hydrangea will thrive in a rain garden while providing three seasons of interest. Ninebark shrubs have some interesting cultivars to play with in rain garden design, including the chartreuse leaves of 'Nugget' and the moody purple foliage of 'Diablo.' Spicebush, named for its aromatic foliage, will draw swallowtail butterflies, which use it as a host plant.

Rain Garden Care and Maintenance

Although rain gardens require less maintenance than a typical flowerbed, they do benefit from some of the same care practices. Rain gardens may require more care in their first year as they establish their root systems. Water deeply but infrequently during times of drought to encourage healthy roots.

Rain gardens need to be kept free of weeds, both for plant health and aesthetics. The best time to weed is after a rain, when the soft soil allows weed roots to release their purchase easily. A three-inch layer of mulch will further suppress weeds. Use a fine hardwood mulch, as it's less likely to float away after a heavy rain.