Fact Sheet: Bioretention Areas

Bioretention is an important technique that uses soil, plants and microbes to treat stormwater before it is infiltrated or discharged. Bioretention “cells” are shallow depressions filled with sandy soil, topped with a thick layer of mulch, and planted with dense vegetation. Stormwater runoff flows into the cell and slowly percolates through the soil (which acts as a filter) and into the groundwater; some of the water is also taken up by the plants. Bioretention areas are usually designed to allow ponded water 6-8 inches deep, with an overflow outlet to prevent flooding during heavy storms. Where soils are tight or fast drainage is desired, designers may use a perforated underdrain, connected to the storm drain system.

A “rain garden” bioretention cell for rooftop and driveway runoff. Photo courtesy of University of Connecticut.

Management Objectives

  • Provide water quality treatment; remove suspended solids, metals, nutrients
  • Increase groundwater recharge through infiltration.
  • Reduce peak discharge rates.
  • Reduce total runoff volume.
  • Improve site landscaping

Bioretention areas can provide excellent pollutant removal and recharge for the “first flush” of stormwater runoff. Properly designed cells remove suspended solids, metals, and nutrients, and can infiltrate an inch or more of rainfall. Distributed around a property, vegetated bioretention areas can enhance site aesthetics. In residential developments they are often described as “rain gardens” and marketed as property amenities. Routine maintenance is simple and can be handled by homeowners or conventional landscaping companies, with proper direction.

Applications and Design Principles

Bioretention systems can be applied to a wide range of development in many climatic and geologic situations; they work well on small sites and on large sites divided into multiple small drainages. Common applications for bioretention areas include parking lot islands, median strips, and traffic islands. Bioretention is a feasible “retrofit” that can be accomplished by replacing existing parking lot islands or by re-configuring a parking lot during resurfacing. On residential sites they are commonly used for rooftop and driveway runoff.

A parking lot bioretention cell the day after a storm. Bioretention areas are designed to store water for no more than 72 hours after a storm. Photo courtesy of Rich Claytor.

Bioretention cells are usually excavated to a depth of 4 feet, depending on local conditions. Generally, cells should be sized (based on void space and ponding area) to capture and treat the water quality volume (the first 0.5” or 1” of runoff, depending on local requirements.) Some manuals suggest a minimum width of 15’, though much narrower bioretention cells have been installed in parking lot islands and are functioning well. Regardless of size, some type of filter should cover the bottom of the excavation. Filter fabric is commonly used but can be prone to clogging; consequently some engineers recommend a filter of coarse gravel, over pea gravel, over sand.

The cell should be filled with a soil mix of sandy loam or loamy sand. The area should be graded to allow a ponding depth of 6-8 inches; depending on site conditions, more or less ponding may be appropriate. The area should be planted with a mix of herbaceous perennials, shrubs, and (if conditions permit) understory trees that can tolerate intermittent ponding and occasionally saline conditions (due to road salt.) The soil should be covered with 2-3” of fine-shredded hardwood mulch.

Schematic bioretention plan

A schematic bioretention plan.

In very permeable soils, some bioretention areas can be designed as “off-line” treatment structures (no overflow necessary), but in most situations they will be an “on-line” component of the stormwater management system, connected to downstream treatment structures through an overflow outlet or an overflow drop inlet installed at the ponding depth and routed to the site’s stormwater management system. Ideally, overflow outlets should be located as far as possible from runoff inlets to maximize residence time and treatment. In general, bioretention area should be designed to drain within 72 hours. In slowly permeable soils (less than 0.3 inches/hour) a perforated underdrain can be installed at the bottom of the excavation to prevent ponding.

Bioretention areas work best if designed with some pretreatment, either in the form of swales or a narrow filter strip. A stone or pea gravel diaphragm (or, better yet, a concrete level spreader) upstream of a filter strip will enhance sheet flow and better pre-treatment.

Benefits and Effectiveness:

  • Bioretention areas remove pollutants through filtration, microbes, and uptake by plants; contact with soil and roots provides water quality treatment better than conventional infiltration structures. Studies indicate that bioretention areas can remove 75% of phosphorus and nitrogen; 95% of metals; and 90% of organics, bacteria, and total suspended solids. Bioretention areas qualify as an organic filter according to the Massachusetts Stormwater Policy.
  • In most applications, bioretention areas increase groundwater recharge as compared to a conventional “pipe and pond” approach. They can help to reduce stress in watersheds that experience severe low flows due to impervious coverage.
  • Low-tech, decentralized bioretention areas are also less costly to design, install, and maintain than conventional stormwater technologies that treat runoff at the end of the pipe. The use of decentralized bioretention cells can also reduce the size of storm drain pipes, a major driver of stormwater treatment costs.
  • Bioretention areas enhance the landscape in a variety of ways: they improve the appearance of developed sites, provide wind breaks, absorb noise, provide wildlife habitat, and reduce the urban heat island effect.


  • Because bioretention areas infiltrate runoff to groundwater, they may be inappropriate for use at stormwater “hotspots” (such as gas stations) with higher potential pollutant loads. On these sites, the design should include adequate pretreatment so that runoff can be infiltrated, or else the filter bed should be built with an impermeable liner, so that all water is carried away by the underdrain to another location for additional treatment prior to discharge.
  • Premature failure of bioretention areas is a significant issue that results from lack of regular maintenance. Ensuring long-term maintenance involves sustained public education and deed restrictions or covenants for privately-owned cells.
  • Bioretention areas must be used carefully on slopes; terraces may be required for slopes >20%.
  • The design should ensure vertical separation of at lease 2’ from the seasonal high water table.


Bioretention requires careful attention while plants are being established and seasonal landscaping maintenance thereafter. In many cases, maintenance tasks can be completed by a landscaping contractor working elsewhere on the site.

  • Inspect pretreatment devices and bioretention cells regularly for sediment build-up, structural damage, and standing water.
  • Inspect soil and repair eroded areas monthly. Re-mulch void areas as needed. Remove litter and debris monthly.
  • Treat diseased vegetation as needed. Remove and replace dead vegetation twice per year (spring and fall.)
  • Proper selection of plant species and support during establishment of vegetation should minimize—if not eliminate—the need for fertilizers and pesticides.
  • Remove invasive species as needed to prevent these species from spreading into the bioretention area.
  • Replace mulch every two years, in the early spring.
  • Upon failure, excavate bioretention area, scarify bottom and sides, replace filter fabric and soil, replant, and mulch.


Bioretention areas require careful design and construction, the price of which will depend on site conditions and design objective. Generally, the cost of bioretention areas is less than or equal to that of a catch basin and underground chambers intended to treat the same area. Additionally, bioretention areas treat and recharge stormwater thereby reducing the amount/size of piping needed and the size of downstream basins and treatment structures.

Design Details

  • Where bioretention areas are adjacent to parking areas, allow 3” of freeboard above ponding depth to prevent flooding.
  • Determine the infiltrative capacity of the underlying native soil through an infiltration test using a double-ring infiltrometer. Do not use a standard septic system percolation test to determine soil permeability.
  • Soil mix should be sandy loam or loamy sand with clay content less than 15%. Soil pH should generally be between 5.5-6.5, which is optimal for microbial activity and adsorption of nitrogen, phosphorus, and other pollutants. Planting soils should be 1.5-3% organic content and maximum 500ppm soluble salts.
  • Planting soils should be placed in 1’-2’ lifts, compacted with minimal pressure, until desired elevation is achieved. Some engineers suggest flooding the cell between each lift placement in lieu of compaction.
  • Planting plan should generally include one tree or shrub per 50 s.f. of bioretention area, and at least 3 species each of herbaceous perennials, shrubs, and (if applicable) trees to avoid a monoculture. Plants should be able to tolerate intermittent flooding, occasionally saline conditions, and extended dry periods.
  • The bioretention landscaping plan should meet the requirements of any applicable local landscaping requirements.
  • During construction, avoid excessive compaction of soils around the bioretention areas and accumulation of silt around the drainfield.
  • In order to minimize sediment loading in the treatment area, only runoff from stabilized drainage areas should be directed to bioretention areas; construction runoff should be diverted elsewhere.

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