Rain Gardens Work

Rain gardens allow water to percolate slowly into the soil

By Joan Kimmel ©2012
Penn State Master Gardener

Two words being heard around Allegheny County with increasing importance are ‘rain’ and ‘garden.’ To understand why, first a little background.

Most of Pittsburgh’s sewers were built during a period when the systems collected both the waste water (including sewage) that comes from homes, businesses and industry, as well as the rain water that drains off roofs, streets, sidewalks, driveways and parking lots.   These impermeable surfaces prevent water from slowly draining through soil layers where the salts, minerals and heavy oils in the water can be partially cleaned or absorbed by plant roots before entering our waterways. 

City of Pittsburgh

During normal periods this system works just fine, but during periods of heavy rain or melting snow it is simply overwhelmed.  It results in what’s called a Combined Sewer Overflow (CSO), and the untreated, polluted water is discharged directly into our rivers.  After the next heavy rain, check out some of the storm water culverts along the many streams that flow through our valleys towards the city; you will often see a red flag and sign posted that warns ‘CSO.’

Help with Rain Gardens

To try to alleviate some of the pressure on this system, many people are detaching their downspouts from the sewage system and directing them onto their lawns or into rain barrels where the water can be stored for use during dry periods.  Others are finding that installing a rain garden is an even better solution, as holds the water so it can slowly percolate through the soil.


What, exactly, is a rain garden?  Well, first, here’s what it is not.  It is not a pond, or a bog or a raised bed.  It is designed to be a depressed area planted with trees, shrubs or perennials that collects water and allows it to flow slowly into the ground, completely draining within one or two days.  (This tempo of drainage prevents the breeding of mosquitoes, whose eggs need at least 72 hours to mature.)

Sizes of Rain Gardens

The size of a rain garden is determined by how much rain run-off is expected to flow into it.  This can be determined by the size of the roof area that will drain into the downspout(s) and be directed to the rain garden, or the size of the driveway or other area where the water will be diverted to drain into the garden.  It also depends on the kind of soil in the proposed garden area.  (That’s easy to determine here in southwestern Pennsylvania – it’s most likely clay!) The Rain Garden Alliance of Allegheny County has a clever calculator on its website that can help you figure out what size rain garden you may need.

rain barrels

Of course, you’ll want to make sure the water actually reaches the rain garden.  This can be done by laying a pipe from the downspout to the garden, or building a gravel or grass-lined swale that will direct the water into the garden.  A swale will also help to slow down the water so that a very heavy flow doesn’t just wash the dirt and plants away!

If your yard is flat, the depression of the rain garden can be as little as 4 inches; if there is a gentle slope (a very steep slope would not be a good place to site the garden), the depth might need to be as much as 8 inches with a berm built at the lower end to hold in the water.  The base of the finished garden depression should be flat.  And note that, before digging, homeowners should take care to find out where utility lines and storm sewers are buried and position the garden at least two feet away from them.  PA One Call is a service that can tell you the location of utility lines for any property by phoning 811 or

Shape of a Rain Garden

The shape of the garden and the chosen plants are pretty much up to the whim of the designer.  There is a wonderful website developed by the Low Impact Development Center in Maryland that has a number of design templates for a variety of sizes, design goals and sun/shade conditions.  Although these are designed for plants appropriate to the Chesapeake Bay Watershed, the templates that refer to the Piedmont region all have plants that would be equally appropriate for our area.

Of course, plants that are native to southwestern Pennsylvania would be the best choices for a rain garden, being the hardiest and most resistant to insects and disease.  However, any plant that can withstand the shift of conditions from brief periods of standing water to extended periods of dryness will work.

There are many local landscapers, garden centers, and other resources that can help you plan for and install a rain garden.  Even if you can’t manage to install one that collects all the run-off from your property, a small rain garden will help the cause.


  • Big bluestem (Andropogon gerardii)

  • Swamp milkweed (Asclepias incarnata)

  • New England aster (Symphyotrichum novae-angliae)

  • Marsh marigold (Caltha palustris)

  • Tall tickseed  (Coreopsis tripteris)

  • Joe Pye weed (Eupatorium fistulosum)

  • Bottle gentian (Gentiana andrewsii)

  • Common sneezeweed (Helenium autumnale)

  • Tall Sunflower (Helianthus giganteus)

  • Oxeye (Heliopsis helianthoides)

  • Swamp Rose Mallow (Hibiscus moscheutos)

  • Blazing Star (Liatris spicata)

  • Cardinal flower (Lobelia cardinalis)

  • Great blue lobelia (Lobelia siphilitica)

  • Bee balm (Monarda didyma)

  • Switchgrass (Panicum virgatum)

  • Rough-stemmed goldenrod (Solidago rugosa)

  • New York Ironweed (Vernonia noveboracensis)


  • Marsh Marigold (Caltha palustris)

  • White Turtlehead (Chelone glabra)

  • Marginal Wood Fern (Dryopteris marginalis)

  • Wild Geranium (Geranium maculatum)

  • Woodland Sunflower (Helianthus divericatus)

  • Northern Blue Flag (Iris versicolor)

  • Monkey Flower (Mimulus ringens)

  • Sensitive Fern (Onoclea sensibilis)

  • Spreading Jacob’s Ladder (Polemonium reptans)

  • Blue-eyed Grass (Sisyrinchium angustifolium)

  • Meadowsweet (Spiraea alba)

  • Tall Meadow Rue (Thalictrum pubescens)


  • Silky Dogwood (Cornus amomum)

  • Wild Hydrangea (Hydrangea arborescens)

  • Spicebush (Lindera benzoin)

  • Ninebark (Physocarpus opulifolius)


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