RAIN GARDENS

 

What are rain gardens?

Rain gardens are depressional areas landscaped with perennial flowers and native vegetation that soak up rainwater.  They are strategically located to capture runoff from impervious surfaces, such as roofs and streets.  Rain gardens fill with a few inches of water after a storm then water filters into the ground, rather than running off to a storm drain and directly into the receiving water without treatment.

 

Why are rain gardens important?

As cities and suburbs grow, increased stormwater runoff from impervious surfaces becomes a problem.  As more impervious surfaces are added to our communities, it is more important than ever to help rainwater infiltrate.  This protects water quality and reduces stormwater runoff.  Stormwater runoff from developed areas increases flooding potential and carries pollutants from streets, parking lots, and lawns directly into local streams and lakes.  Rain gardens can absorb most rainfall events.

 

What about the lake level?

Capturing rainwater in a rain garden will not have an adverse effect on the lake level.  Although the water will not run through the storm sewer system to be directly discharged to the lake, the water will infiltrate and reach the lake through the groundwater system.  This is more desirable because the water enters the lake at a much more natural rate and helps reduce erosion at storm sewer outfall locations.  Infiltration through the groundwater system also helps remove pollutants that would otherwise be directly discharged to the lake.

 

How is a rain barrel designed and installed? 

 

Location.  It needs to go where the water will flow.

·        At least 10 feet away from building foundations, utilities, and septic systems.

·        Call Iowa One Call for locations of utilities before you begin digging.

·        Locate the rain garden downhill from your home so that water flows in the direction of the garden.

·        Do not install in areas where there is a shallow depth to the water table (when you dig a hole less than 4 feet deep and you find groundwater), or shallow soils over bedrock.

 

Size.  Rain gardens are typically 5 to 10 percent the size of the impervious surface generating the runoff entering the garden.  Measure the square footage of the impervious area (length x width); then multiply this by 0.07 (7 percent).  Determine a length and width of the rain garden that best fits the site.  For example, a 2,000 square foot roof, when multiplied by 7 percent, would call for a rain garden 140 square foot in size, or 14 feet long by 10 feet wide.

 

Soil Testing.  If it rains, how well will the soil drain?

·        Dig several holes and conduct a percolation test of the soils in the area where you wish to place a rain garden.  Dig the hole below the depth of the proposed rain garden depression (or bowl) to determine if the soil will soak up (or percolate) water at an adequate rate to support a rain garden.  12 inches of water should drain away in 12 to 24 hours for adequate percolation rates.

·        Dig several holes and conduct a ribbon test of the soils in the area where you wish to place a rain garden to estimate the amount of clay content and determine if there are adequate percolation rates.  Dig the hole below the depth of the proposed rain garden depression.  If the soil won’t ribbon and breaks off as you squeeze it, the soils should have low clay content and good percolation rates in order for the rain garden to drain properly.

 

Garden depth.  A typical rain garden is between four and eight inches dep.  A rain garden less than four inches deep will need too much surface area to provide enough water storage to infiltrate larger storms.  Stormwater runoff should spread evenly across the entire rain garden, to increase the opportunity for infiltration.

 

Plant selection.  While rain gardens are a highly functional way to help protect water quality, they can also be an attractive part of yards and neighborhoods.  Choose native plants based on site considerations for light, moisture, and soil.  Vary plant structure, height, and flower color for seasonal appeal and butterfly habitat.  Mowed grass borders are recommended around the garden.

 

Young plants, or plugs, are best for rain gardens because they are easier to establish and maintain.  When laying plants out, randomly clump individual species in groups of 3 to 5 plants to provide broader color.  Be sure to repeat these individual groupings to create repetition and cohesion in a planting.  It is a good idea to place plant labels next to each individual grouping.  This will help identify the young native plants from weds as the garden is maintained.

 

It is important to water rain gardens regularly throughout the first season.  Once established, they will thrive without additional watering.  A shredded wood mulch is an important part of a rain garden.  Mulch helps retain moisture and discourages weed seeds from germinating.

 

Where can I find additional information about rain gardens?

Visit www.iowastormwater.org (click on Iowa Rain Gardens) to view the "Iowa Rain Garden Design and Installation Manual," which is a detailed guide to designing and installing rain gardens.  A list of people that have attended a rain garden training workshop is also provided for those who may wish to hire someone to install the garden.  The local Soil and Water Conservation District may also be able to provide guidance and assistance (https://idals.iowa.gov/FARMS/index.php/districtMap)

 

Additional information can be found by visiting any of the following websites:

            www.iowasudas.org

            www.raingardens.org

            www.mninter.net/~stack/rain/

            www.lowimpactdevelopment.org

            www.cwp.org

            www.stormwatercenter.net

 

If you are interested in installing a rain garden at your residence and would like additional information or assistance, feel free to contact the Project Coordinator.