Science

Is having a rain garden a liability? Part 3 of the Vermont Green Infrastructure Initiative’s guide to stormwater misconceptions.

Is having a rain garden is a liability?  Part 3 of the Vermont Green Infrastructure Initiative’s guide to stormwater misconceptions.

This is the third post in a series about stormwater misconceptions identified by Green Infrastructure Initiative Roundtable.  We hope that these posts will foster greater understanding and integration of Green Stormwater Infrastructure (GSI) practices and Low Impact Development principles throughout the Vermont landscape.

The popular phrase “One bad apple can spoil the whole barrel,” immediate springs to mind when it is applied to emerging technologies like GSI.  The negative public reaction of one bad project that didn’t work as planned can take years to overcome.  Misconceptions take tireless work to correct and examples of successful projects may have to greatly outnumber the failures before widespread changes in public opinion occur.  In the arena of GSI, no practice carries the burden of misconceptions more than rain gardens.

For the purposes of this discussion, we’ll define rain gardens as small-scale bioretention best management practices that can retain, treat, evapotranspire, and infiltrate stormwater runoff from a residential property.  Rather than being popularly referred to as “bioretention cells” the more accessible term “rain garden” highlights the plants that, if chosen well for tolerance of wet soils, provide beauty that equals the functionality of the BMP.  But rain gardens are more than a shallow pool with attractive plants.  They are an engineered solution to a stormwater problem that require specific technical details be incorporated into their design and construction.  If built correctly, rain gardens can avoid some of the common misconceptions.  Here are the facts about rain gardens:

  1. Rain gardens are mosquito-free. Rain gardens should not hold water long enough to support the lifecycle of mosquitoes.  Design guidance for rain gardens require that infiltrate or drain stormwater in 2 days or less.  Development times for mosquitoes from egg to adulthood are a minimum of 6-8 days.  Even if a rain garden doesn’t quite meet the design guidance for drainage (or drains more slowly over the years after construction), it is unlikely to become a mosquito breeding site.  The depth of the water table and infiltration rate of the soil are two key elements that will determine the suitability of a potential rain garden location for proper drainage.  Sites where the water table is less than two feet below the bottom of the rain garden at the wettest time of year will not provide adequate drainage to support a rain garden.  Testing of the infiltration rate of a site’s soil is straight-forward (the sidebar on this information sheet is a good guide) and will guide a rain garden’s design.  If infiltration rates are low, a rain garden design should feature an underdrain to provide an adequate rate of dewatering.
  1. Rain gardens do not become hazardous waste sites. Because rain gardens retain and filter stormwater runoff (and the pollutants it carries) the fear over the fate of those pollutants (like heavy metals, petroleum products, bacteria, and nutrients) is understandable.  However, studies have considered the mechanistic pathway that pollution takes from its dilute concentration in runoff, through the rain garden’s efficiency at removing pollutants, and periodic removal of pollutants that are taken up into plant tissue to quantify the amount of pollution that accumulates in rain garden soils.  In short, pollutant accumulation in a residential rain garden over 20 years will result in concentrations that are three orders of magnitude lower than what is considered toxic waste.  A recent review of available data from Washington state is a good source of additional information on this topic.
  1. Rain gardens do not flood basements. Because rain gardens are designed to accept and infiltrate runoff from surfaces like roofs, it is important to build them at least 10 feet from a building’s foundation to prevent flooding problems.  In some instances rain gardens are paired with rain barrels to provide a system of disconnecting downspouts to both convey rooftop runoff away from a house and provide an additional source of water for irrigating flower beds and other landscape plantings.  In a system like this, a rain garden accepts excess rooftop runoff during the warmer months as well as accepts all runoff when the rain barrel is by-passed in colder months to prevent freezing.
  1. Rain gardens will support property values. If properly sited, sized, and maintained, a rain garden can provide many benefits beyond stormwater management.  They are aesthetically pleasing additions to home landscaping that can provide benefits to wildlife, including pollinators like bees and butterflies.  They may serve as demonstration sites, adding community educational benefits.  Research conducted by the Center for Economic Development at The University of Wisconsin-Milwaukee indicates that they may actually increase property values.  Lastly, rain gardens can always be modified to improve performance and aesthetics.

 In short, rain gardens are assets for homeowners.  Just like any other investment, rain gardens should be designed, built with an attention to detail and maintained.  In addition to the resources linked within this post, there are enumerable resources available to those interested in planning a rain garden on their own.  These include:

Let It Rain Stormwater Program

Vermont Rain Garden Manual “Gardening to Absorb the Storm” available from the Vermont Association of Conservation Districts

Smartwaterways.org

The BLUE® Program

Construction of a rain garden at a Montpelier residence in 2012.

Construction of a rain garden at a Montpelier residence in 2012.

The property owner at the same rain garden in 2015.

The property owner at the same rain garden in 2015.