PA Environment Digest

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Rain Gardens Can Help Slow The Flow Of Stormwater

By Jodi Sulpizio, Penn State Extension Natural Resources Educator

To improve water quality downstream, in a river or a bay, changes need to be made upstream in headwater streams and our backyards.

There is no denying that humans alter the landscape, leading to more stormwater runoff. More impervious surfaces such as roads, rooftops and sidewalks are created, increasing the amount of stormwater.

Stormwater picks up pollutants such as fertilizers, pesticides, road salt, trash, sediment, animal waste and grass clippings carrying them to local streams and rivers. Pollutants are major stressors to the fragile, aquatic ecosystem.

Increased stormwater also increases the risk of flooding, decreases infiltration and groundwater recharge and increases the cost of water treatment.

Everyone impacts the land, but property owners can make small changes to help curb the growing stormwater problem.

Homeowners can install best management practices (BMPs) on their property to help lessen the impact of stormwater. Rain gardens, bioswales, rain barrels, pervious pavement and green roofs are examples of various BMPs. Today’s focus is rain gardens.

Rain gardens are bowl-shaped gardens designed to capture water, allowing it to soak into the ground. They can drain water from roof downspouts, driveways, walkways or other impervious surfaces.

They slow and reduce the amount of runoff from your property, while adding a lovely garden feature that reaps the following benefits: flood reduction, pollutant removal, groundwater protection, enhanced wildlife habitat and improved aesthetics.

A rain garden must be designed and installed with careful consideration or you may create more problems.

First look at the topography, take a walk in the rain and study water flow on your property. Look for low, flat areas with a maximum slope of 10-12 percent. Choose a location that gets runoff from a roof downspout or other impervious surface and ask your municipality for specific ordinances.

A rain garden should be constructed at least 10 feet from a foundation, 50 feet from a septic system, and 100 feet from a well.

If you install a rain garden within 10 feet of your foundation, it should be underlined and underdrained.

Before you dig, always call PA One Call at 8-1-1 or 800-242-1776 to locate underground utility lines. Be sure to avoid tree roots, utilities and engineered stormwater management structures.

Know your soil type. Avoid areas with soils that are permanently or periodically waterlogged.

Once constructed, ponding depth should be less than 12 inches; 4-8 inches is best. Water should drain from the site in less than 24 hours after a rain event. Drainage in 2-4 hours is preferred.

A percolation test should be done to ensure adequate drainage. If the site does not drain well, the soil will need to be amended.

How do you determine the size of your rain garden? It’s important to place your garden where it will collect as much runoff as possible.

If you are directing a downspout into the garden, there is a simple calculation you can use. Measure the footprint of your house. Estimate how much or the roof area is actually draining into the downspout. Then, divide the area by six.

This sizes the garden to hold one inch of roof runoff into a garden that is six inches deep.

Visit this webpage for more detailed instructions on building the correct sized garden.

Before you plant, outline the shape of your garden. Remove or kill the turf grass and dig the garden to the desired depth. Hand digging is recommended so you don’t compact the soil.

Remove the topsoil and return it once you reach desired depth. The bottom of the garden should be flat. If the garden is on a slope, a berm should be built on the downhill side.

An overflow or small indentation should be added to the berm so excess water can flow out of the garden during large storm events. Protect the overflow with an erosion control net or mat topped with gravel or stone.

Choose native plants suitable to the area based on soil, sunlight and location in the garden. The plants must be able to tolerate variable moisture conditions. The lowest part of the garden is the settling basin. For this area, choose plants that can tolerate inundation.

The transition zone includes the lower and upper slopes of the garden. Soil dries out faster on the slopes, so plants in this zone should be able to tolerate drier conditions.

Select plants with a variety of shape, color, height and bloom time to maximize the benefits for wildlife, including pollinators. Also consider choosing a plant with some evergreen leaves to prevent soil erosion in winter months.

Check out our webpage for a list of native rain garden plants

Mulch the garden the first year to help retain moisture and limit weed growth. Water the garden until plants are established or during drought conditions and remove weeds and sediment as needed.

Vegetation should remain through winter months to provide both a seed source and shelter for wildlife. Cut vegetation back in early spring as new growth emerges. Continue to monitor the garden, provide necessary maintenance and replace plants as needed.

To learn more about creating and managing a rain garden to control stormwater on your property, go to Penn State Extension's video on rain gardens. By making changes upstream, we can all play a part in improving water quality!

(Reprinted from Penn State Extension Watershed Winds newsletter.)

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