Rain Gardens – A Primer

by Risa Edelstein on February 22, 2008

in Rain Gardens

It’s a simple idea.  Capture rain coming off the roof of your house or building, direct it to a landscaped area and let it soak slowly into the ground, replenishing the ground water.  Essentially what mother nature did before we got here. A rain garden allows about 30% more water to soak into the ground than a conventional lawn. 

Why is this so important?

Because of the increase in development, we have created a lot of impervious surfaces – like asphalt – which increases the flow of stormwater runoff.  This runoff can create flooding and more importantly carries pollutants from roads, parking lots and lawns into our local streams and lakes. According to the US EPA, polluted runoff is the #1 water quality problem in the US.  Pollutants include pesticides, fertilizers, oils from car, animal waste, toxic chemicals, pathogens and more.  By creating rain gardens, we try and capture water where it lands and reduce this runoff effect.  The plants in these rain gardens are able to filter pollutants and reduce the amount of pollution we send into our local water sources.

Rain gardens were first introduced in Prince George’s County, Maryland, in the late 1980s.  These "bioretention systems" were conceived by stormwater specialists as a way to soak up polluted runoff water and have it infiltrate back into the soil.  The first example was implemented by the Maryland Environmental Protective Department in a public parking lot. This rain garden approach was found to be more cost-effective than the traditional engineering approach and has since influenced the rain garden movement throughout the US.

I was first introduced to rain gardens in a class I took at the Landscape Institute of the Arnold Arboretum at Harvard University.  Fascinated and wanting to learn more, I created a hypothetical rain garden as one of my design projects.  It was to replace a parking lot right in the middle of Harvard Square (Cambridge, MA) and it was smack in the center of a group of building that would all direct their roof runoff to this rain garden that the public would then be able to enjoy.  Ahhhh…the joys of pretending when in design school!

In any event, I got to research rain gardens.  The most progressive initiative I could find in the country was the 10,000 Rain Gardens project, a joint initiative between the Kansas City government and its citizens to manage stormwater runoff.  Their goal is to create 10,000 rain gardens and their site is a wealth of information and even contains instructions on how to build a rain garden. 

While it seems like one rain garden cannot make a difference, think about the power of community recycling and apply it to rain gardens. It could be a very effective way to help manage stromwater runoff which is becoming a bigger and bigger issue for our state.  The real plus of rain gardens versus recycling is that is can add so much beauty to the surroundings.  Check out Seattle’s Street Edge Alternative, completed in 2001.    This was a public rain garden initiative that reduced stormwater runoff by 99% plus added a tremendous value to the neighborhood.

Some basics about rain gardens and where to put them - 

1. They should be 10′ from the building so water does not seep into the foundation.

2. Do not put a rain garden over a septic system.

3. Do not put a rain garden in an already wet area that does not drain well.

4. Most rain gardens should be build in full to part sun areas, not under a shade tree.

5. Put rain gardens on a flat or very gentle slope, preferably downslope from your impervious surfaces.

Use the information provided at the various sites I list below to figure out how large to make your rain garden.  Factors that come into play are: size of roof, slope, type of soil and depth of the garden.

2007_12_21_005 Plants to use include those that like their "feet" wet as well as those plants that can take it when it is dry.  There are a whole range of them including lots of native perennials like Joe pye weed, aster novae-angliae/new England aster, helenium auntumnale and boltonia asteroides.  Shrub examples include Cornus amomum/Silky Dogwood, Cornus sericea/Red osier dogwood, Ilex verticillata/Inkberry ( photo at left) and Clethra alnifolia/Summerweet.  The sources below provide a whole range.  Plant in bands and those that need more moisture should be closer to the center where more water will be retained. 


Rain Gardens – Brooklyn Botanic Gardens

Rain Gardens – A Design Guide for Homeowners in Connecticut – UCONN Cooperative Extension System

Rain Gardens of West Michigan

Rain Gardens – A How to Manual for Homeowners -from the University of Wisconsin Extension

Wisconsin Department of Natural Resources

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Bruce January 26, 2010 at 7:42 am

Your comment about not locating a raingarden in shade is not correct. It is correct that you should not locate them under a tree, but many areas are shaded because of buildings or other features (even trees at a distance can cause shady conditions many feet away, without causing the problems of trees growing where you actually want the garden). In these situations shaded rain gardens ARE appropriate.

Risa Edelstein January 28, 2010 at 8:16 am

Thanks for the comment. After two years since I posted this, I have seen more and more shade rain gardens so you are absolutely right.

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