How to Design a Rain Garden

Rain Garden

Montgomery County Planning Commission/Flickr/CC BY-SA 2.0

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 temporary flooded bog, 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 improve the environment, and look beautiful while doing it. Learn how to put rain gardens to work in your landscape.

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. Homeowners may choose to direct their downspouts into rain gardens, where plants can absorb and filter even more water. Rather than water-hungry plants, the ideal plants for rain gardens are native plants, which are naturally adapted to a wide range of moisture conditions. Did you know that approximately 80% of a native plant's mass is underground? That gives you an idea of the potential of native plantings 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. 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 can protect your home from floods. A mature stand of rain garden plants will take up excess water that might normally puddle in your yard, leading to water intrusion issues in your home. When you get rid of standing water, you also reduce mosquito breeding problems around your home.

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, beneficial insects, and birds.

Rain gardens help conserve water. The native plants that thrive in the rain garden won't need supplemental irrigation, as they are deep rooted and have evolved to survive periods of drought between rains.

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

If you're planning to divert your roof gutters into your rain garden, you won't want to install the feature too far away from your home, but you must place it at least ten feet away from your home. Underground utilities and tree roots may also influence the placement of the rain garden. If your yard doesn't have a natural depression, you will 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 at least 150 square feet in order to include an interesting variety of plants that can process the runoff associated with your landscape. Choose a natural, rounded shape for your rain garden, like an oval, teardrop, or kidney shape. 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.

Plants for a Rain Garden

Proper rain garden design dictates that only perennial plants be installed. 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 garden soil, and will thrive in these moist conditions. Five hardy and long-blooming plants you can't go wrong with here include false indigo, coneflower, hardy hibiscus, black-eyed Susan, and cranesbill. Include some hardy grasses as well, like 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 cat mint.

Trees and Shrubs for a Rain Garden

Although a rain garden is not the place to grow a large shade tree, you can add a small ornamental tree or shrub to act as a focal point in the landscape. 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 the 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 are lower in maintenance than a typical flowerbed, they do benefit from some of the same good 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.