Question

I need advice about creating a bioswale or rain garden in my yard where overflow rainwater collects.

Asked by Susan Cindrich
Utica, MI

I have an area in my yard that collects overflow rainwater due to an improper grade issue. Water sits there for about 24 hours and then dissipates into the ground. If I cut the grass and create a bioswale, or rain garden, do I need to install large grasses with deep roots, or is there a lower-profile plant I can use? I am in zone 6 in southeast Michigan.

Answer

Richard Heller

Answered by Richard Heller

New Rochelle, NY

Greener by Design

April 24, 2011

Managing stormwater efficiently and effectively is a problem with many solutions.  Rain gardens and bioswales are two possible solutions, and they're far more sustainable than redirecting water to sewage lines.

Both bioswales and rain gardens use plants to absorb excess water that your standard lawn cannot process. Both require soil amending and excavation, though in different ways, and both may be executed with lower-profile plants as needed.

Bioswales and rain gardens are similar but have different functions.  According to Wikipedia,

  • "Bioswales are landscape elements designed to remove silt and pollution from surface runoff water. They consist of a swaled drainage course with gently sloped sides (less than six percent) and filled with vegetation, compost and/or riprap. The water's flow path, along with the wide and shallow ditch, is designed to maximize the time water spends in the swale, which aids the trapping of pollutants and silt."
  • "A rain garden is a planted depression that allows rainwater runoff from impervious urban areas like roofs, driveways, walkways, parking lots, and compacted lawn areas the opportunity to be absorbed. This reduces rain runoff by allowing stormwater to soak into the ground (as opposed to flowing into storm drains and surface waters, which causes erosion, water pollution, flooding, and diminished groundwater)."

I'll assume it's a rain garden you're after

The key difference between the two is that bioswales are designed more for cleaning and processing water as well as redirecting it (usually away from a road or lot), while rain gardens are more strictly designed to absorb stormwater, particularly off rooftops and walkways.

I will assume that it is a rain garden you are after.

Constructing a rain garden

If water is sitting on top of your soil for 24 hours, there is a drainage problem. You probably have high clay content and therefore poor water penetration. 

For a rain garden to work, you will need to:

  • excavate a shallow pit (approximately 2'-3' deep),
  • install a 6" layer of ¾" to 1" stone,
  • amend the soil with some organic material and lay it over the stone, and then
  • plant it with water-loving plants.

You will find a list of native water-lovers for southeast Michigan in a PDF at the Southeastern Oakland County Water Authority website here.

Underground water storage is the key to success

The key to a successful rain garden is underground water storage, where the roots of the plants can reach it, but mosquitos can't.

The "Rain Gardens of the Rouge River" PDF recommended above is less aggressive about water storage, recommending sand, which you can try.

However, my feeling is "better safe than sorry," so if you have to dig the pit, why not engineer it to the max? 

 

For more information:

Read "How can I improve my outdoor area to avoid getting water in my basement and near-flooding in my backyard?" a Q&A answered by Ryan Flegal.

Tagged In: native plants

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