Rain Garden

Also Known As: Bioretention, Bioretention (Lined)


Bioretention areas, or rain gardens, are landscaping features adapted to provide on-site capture and treatment of stormwater runoff. They are commonly located in parking lot islands or within small pockets of residential land. The surface stormwater runoff is directed into the shallow, vegetated depressions, which increase stormwater storage, filtration, and pollutant reduction. These bioretention depression areas are designed to mimic many of the pollutant removal mechanisms that operate in natural ecosystems. An excellent resource for rain gardens, including design and rain garden plants recommended for Western Pennsylvania is the Three Rivers Rain Garden Alliance website, http://raingardenalliance.org/.

During storm events, stormwater runoff travels as sheet flow or via a curb cut to the bioretention treatment area, which usually consists of a grass buffer strip, sand bed, ponding area, organic layer or mulch layer, planting soil, and plants. As the runoff passes over or through a sand bed, the runoff's velocity is slowed allowing the stormwater to be distributed evenly along the length of the ponding area. The bioretention area is graded to divert excess runoff away from itself. Water stored in the bioretention area planting soil exfiltrates over a period of days into the underlying soils. Runoff from larger storms is generally diverted past the facility to the storm drain system. The remaining runoff filters through the mulch and prepared soil mix. The filtered runoff can be collected in a perforated underdrain and returned to the storm drain system.

Bioretention is effective for removing:

  • Sediments
  • Trash
  • Bacteria
  • Organics
  • Metals
  • Oil and Grease


Bioretention Lined

Siting and Design:

Drainage AreaSoil Infiltration RateWater Table SeparationDepth to bedrockFacility slopeInflow rate
<5 acres> 0.5 in/hr if < 0.5 in/hr, install UD)> 10 ft(if > 2 but < 10 ft, install UD)> 10 ft(if > 2 but < 10 ft, install UD)< 2%Mulch: 1 cfs,
Grass: 3 cfs



TaskFrequencyMaintenance Notes
Pruning1-2 times/yrNutrients in runoff often cause bioretention vegetation to flourish.
Mowing2-12 times/yrFrequency depends on location and desired aesthetic appeal.
Mulching1-2 times/yrBetween 1"–3" of mulch depth is ideal.
Mulch removal1 time/2–3 yearsMulch accumulation reduces available water storage volume. Removal of mulch also increases surface infiltration rate of fill soil.
Watering1 time/2–3 days for first 1–2 months; Sporadically after establishmentIf drought conditions exist, watering after the initial year might be required.
Fertilization1 time initiallyOne time spot fertilization for first year vegetation.
Remove and replace dead plants1 time/yearWithin the first year, 10% of plants can die. Survival rates increase with time.
Inlet inspectionOnce after first rain of the season, then monthly during the rainy seasonCheck for sediment accumulation to ensure that flow into the bioretention is as designed. Remove any accumulated sediment.
Outlet inspectionOnce after first rain of the season, then monthly during the rainy seasonCheck for erosion at the outlet and remove any accumulated mulch or sediment.
Miscellaneous upkeep12 times/yearTasks include trash collection, plant health, spot weeding, removing invasive species, and removing mulch from the overflow device



Homeowner-installed rain gardens will probably cost less than the costs reflected here (up to 20% less).

BMPRetrofit ($/sq ft)O&M ($/sq ft/yr)Retrofit with Underdrain ($/sq ft)New Construction ($/sq ft)New Construction with Underdrain ($/sq ft)

27.33 (national average)

17 - 21 local contractor installed

8 - 12 homeowner installed


20.5 (national average)

17 - 21 local contractor installed

8 - 12 homeowner installed
Bioretention - Lined 1.658.3343.7543.75