Results of Severn River Rain Garden Survey

What Is A Rain Garden

Environmental Benefits

Keeping A Rain Garden Healthy

How To Conduct A Watershedwide Rain Garden Audit

Further Information

Watershed Audit

 

 

 

 

 

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Rain Garden Audit

Maximizing the Benefits of Rain Gardens and Other Bioretention Facilities

Rain Garden colorful

Rain Gardens are among THE most effective practices for preserving aquatic resource health.  When planted to create color most seasons of the year, Rain Gardens can greatly enhance neighborhood quality of life and property value.  While Rain Gardens are definitely low-maintenance, they do need some care.  Assessing maintenance needs and keeping Rain Gardens healthy is the purpose of the Audit.  For other aspects of aquatic resource health care visit the CEDS Watershed Audit webpage.

What Is A Rain Garden

The purpose of a Rain Garden is to treat the pollutants washed by rain from rooftops, streets, parking lots and other impervious surfaces before they reach a waterway.  To achieve this purpose Rain Gardens are placed where impervious surface runoff can be captured, such as below a roof downspout, along a street or downhill of a parking area. 

Next you look for soils that are permeable avoiding those with lots of clay, rock or a shallow water table.  A pit is excavated two- to four-feet deep into these permeable soils.  The pit might range from three feet wide and maybe ten- or twenty-feet long or it could be oval-shaped, round, whatever.

rain garden plan

The pit is filled with planting soil just like that used in flower pots and other gardens.  Two- to three-inches of hardwood mulch is placed on the soil surface.  But the pit is filled in a way that leaves the surface six- to twelve-inches below the adjoining land.  This six- to twelve-inch depression is where runoff is stored for the few hours it takes to percolate down through the mulch layer and into the planting soil before eventually moving into those permeable soils.

A bioretention facility is a larger version of a Rain Garden.  The design and function is essentially the same as is the maintenance.  However, while Rain Gardens usually serve a single home or other small area, whereas bioretention facilities treat runoff from multiple homes or businesses, like that pictured below.

Rain Garden in parking lot

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Environmental Benefits

Rain Gardens serve to counter the effects of development upon aquatic resources.  Homes, shopping centers and other growth begins causing a negative effect on streams, lakes, etc. when 2% or more of a watershed is covered by impervious surfaces.  That's about one house for every eight acres.  For 70% of all Marylanders development is the leading factor making their nearest waters unfit for swimming or fishing.

Development degrades waterways through four impacts:

polluted stream

Rain Gardens can retain 60% to 90% of the pollutants, maintain recharge at 100%, while substantially reducing flooding.  If every home, business, street and parking lot in a watershed drained to a Rain Garden then water quality would be fit for swimming and fishing.

Additionally, Rain Gardens can improve the attractiveness of a neighborhood without causing problems such as  mosquitoes or other nuisances associated with many runoff control practices.

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Keeping A Rain Garden Healthy

Keeping a Rain Garden healthy is as simple as the following five steps. 

  1. Replace any dead or dying plants. A list of suitable plants can be found in Rain Gardens Across Maryland.
  2. Remove trash washed or blown into the Rain Garden.
  3. Once a year take up the old mulch, scarify the underlying soil with a garden rake, then lay down a new layer of well aged (6- to 12-month old) shredded or chipped hardwood mulch.  The mulch layer is a critical component accounting for a large part of the pollutant removal.  The mulch layer also prevents clogging at the soil surface, which is the most common cause of failure.
  4. Watch for indications of clogging such as water standing in the Rain Garden when a day or more has passed since the last storm ended. If clogging does occur then see Rain Gardens Across Maryland for advice.
  5. The surface depression in a Rain Gardens should contain all runoff from up to an inch of rain.  This ensures 90% of all runoff is treated.  If a Rain Garden overflows from less than an inch of rain in 24 hours, then it likely needs cleaning.  For a simple way to check a Rain Garden for adequate capacity click: Rain Gage & Float Method

For a printed version of this information click: Rain Garden Factsheet.

Auditing Rain Gardens Watershedwide

CEDS assists groups in auditing Rain Gardens throughout a watershed and across the nation.  These Audits go into additional maintenance factors which are described in our Rain Garden & Bioretention Facility Audits guidebook.  Typically, volunteers meet on a Saturday morning for a one-hour training session.  They form into teams to evaluate several facilities and return to the meeting place by noon to report their findings.  The survey results identify shortcomings in the existing programs to keep these facilities working at their best.  To see an example of what such an Audit can produce click: Results of Severn River Rain Garden Survey.

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Further Information

The best source of information on Rain Gardens is Rain Gardens Across Maryland.

For bioretention facilities see:

 

 
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