“A rain garden is an artificial depression in the landscape that collects and stores stormwater runoff until it can infiltrate the soil.”

Justin Mechell and Bruce Lesikar, TX A&M

“Adding long lived, deep rooted, drought and wet tolerant plants adapted to local weather extremes and soil conditions increases the rain garden’s sustainability.” Adds Carrie Dubberley

Why have rain gardens?

To have an attractive landscape while saving on water and maintenance.  Sustainable landscapes featuring rain gardens require less maintenance then a traditional landscape and help reduce or eliminate the need for watering, mowing, fertilizers, pesticides, and herbicides.

Rain gardens are a fairly new concept for Texas but have proven value in other parts of the country especially east coast states such as Maryland and Virginia.  With North Texas’ recent severe drought many more people realize pure clean water is our most precious resource.  Whether we are in a drought or flood, we all benefit by capturing rain and holding it on our property.  With the drought we wanted to keep our rainwater to ourselves to use for watering our landscapes.  With the recent severe flooding, rain gardens can help ease the force of flooding and erosion by allowing storm water to infiltrate slowly into the ground.  Less storm water runoff means less severe flooding and less pollution and sediment entering our rivers and lakes while also recharging our ground water supplies.

Furthermore, urbanization has resulted in an increase of impervious surfaces (concrete, asphalt, patios, and lawns) and an increase in stormwater runoff. Once rainwater had the opportunity to infiltrate into the soil, now much of it runs off after storms directly into streams and lakes, contaminating them, preventing the recharge of groundwater, and losing water for landscape irrigation that is then replaced with treated municipal water. Rain gardens and sustainable landscapes reverse this process and allow rainwater to remain in a landscape long enough to be absorbed into the soil, supply the needs of surrounding plants and be transpired through the plant leaves, and to be naturally filtered to recharge groundwater systems.

Deep-rooted sod forming Buffalo grass withstands drought and flood much better than traditional sod forming grasses such as Bermuda and St. Augustine because its roots may extend into the soil to a depth of 7 feet.  With Buffalo grass there is no need for additional water other than rainfall, plus it will grow in a variety of soils.

Elements of a Rain Garden

The basic elements are source, collection, diversion, and storage. Rainwater that falls on roofs and lawns may be collected through downspouts and drains and directed (diversion) to a rain garden, where it is stored and slowly infiltrates through layers of mulch, compost and native soils, and gravel. Rain harvesting systems such as rain barrels complement the function of rain gardens.

Design Considerations

Rainfall and catchment area: The catchment area includes roof area and the grass drainage area. For each inch of rain, 100 sq.ft. of catchment area captures 62 gallons, or 8.3 cubic feet of water.

Location: A rain garden should be located at least 10 ft from a building or house. It should be on the lower end of a gentle slope.

Size of Rain Garden: There are many ways to calculate the size of a rain garden, or Rain Garden Surface Area (RGSA). A simple method is to multiply the catchment area by a size factor. The size factor depends on the soil type and on the distance from the downspout to the garden. Tables of size factors for different conditions are available through the Texas Cooperative Extension of the Texas A&M University System. Typically, if the RSGA is greater than 300 sq.ft. then multiple rain gardens are built. Rain gardens are usually 4 to 8 inches deep.

Soil Type: Soil types influence how fast water will infiltrate. Clayey soils such as those found in North Texas have the slowest infiltration, and gravel and sand the fastest. Impermeable layers and seasonal high water tables must also be taken into account when building a rain garden.

Slope: The greater the slope, the more soil needs to be brought in and the deeper the rain garden.

Shape: The shape of a rain garden depends on the site and the creativity of the landscape designer. Typically the width is about half the length. The top and bottom of the rain garden must be level, to facilitate absorption and prevent water drainage to one end only.

Plants: Native plants that are adapted to both droughts and rainy conditions usually do well on rain gardens, which supply all their water needs once they are established. Some of the plants that thrive in the southwest region include spring blooms plants such as yarrow, Bergamont, columbine, and blue false indigo; summer blooms such as Mexican milkweed; purple perilla, and daylilies; Fall blooms such as gray feather, cardinal flower, purple aster, and goldenrod. Native grasses such as Indian grass and Longspike tridens, because of the long root systems, can tolerate drought conditions well and also the moist seasons.

Addition information about Rain Gardens

As water becomes more scarce and expensive, people will need alternatives to highly maintained landscapes. Droughts and floods will create demand for rain gardens and these gardens will become more acceptable as earth-friendly landscape solutions multiply.

Rain gardens offer design and installation opportunities. Native, sustainable landscapes featuring rain gardens, coupled with rain harvesting systems, drip and soaker hose irrigation are the cutting edge for landscape design.

In the long run, rain gardens save money by saving water, maintenance, and energy. In addition the reduction or absence of chemical controls in gardens and sustainable landscapes make them a healthier environment for children and adults and provide a greater exposure to nature.

Resources

Dubberley Landscape, Inc. Carrie Dubberley, principal of Dubberley Landscape, Inc., works with landscape designers, landscape architects, home and commercial builders, and homeowners to design and build sustainable landscapes featuring rain gardens.

Rainwater Harvesting, Raingardens, Texas Cooperative Extension of the Texas A&M University System.

Texas-The State of Water, Finding a Balance (a documentary on the problem with ground water in Texas)

Rain Gardens, A How-To Guide For Homeowners, University of Wisconsin (A 32-page PDF for the do-it-yourself gardener. Many good tips and includes suggestions of plants for different soils and sun).

RainGardens.org, Rain Gardens of West Michigan (A website dedicated to rain gardens - click on the "Create a Rain Garden" link for a 10-page PDF with tips on building your own rain garden).

Rain Gardens, Taylor Creek Restoration Nurseries - Applied Ecological Services (Includes a very good 4 page PDF with great tips to help you Build you own Rain Garden).

Rainwater Gardens, Modeling Mother Nature, City of Maplewood, Minnesota (A very good website with lots of wonderful rain garden information, includes some great plant layout designs for different types of rain gardens).