Bioretention components

The following provides a description and suggested guidelines and specifications for the components of bioretention cells and swales. Some or all of the components may be used for a given application depending on the site characteristics and restrictions, pollutant loading, and design objectives.

Flow entrance
Flow entrance design will depend on topography, flow velocities, and volume entering the pretreatment and bioretention area, adjacent land use, and site constraints. Flows entering a rain garden should be less than 1.0 foot per second to minimize erosion potential. Five primary types of flow entrances can be used for bioretention cells:
  • Dispersed, low velocity flow across a landscape area: Landscape areas and vegetated buffer strips slow incoming flows and provide an initial settling of particulates and are the preferred method of delivering flows to the bioretention cell. Dispersed flow may not be possible given space limitations or if the facility is controlling roadway or parking lot flows where curbs are mandatory.
  • Dispersed or sheet flow across pavement or gravel and past wheel stops for parking areas.
  • Curb cuts for roadside, driveway or parking lot areas: Curb cuts should include a rock pad, concrete, or other erosion protection material in the channel entrance to dissipate energy. Minimum curb cut width should be 12 inches; however, 18 inches is recommended. Avoid the use of angular rock or quarry spalls and instead use round (river) rock if needed. Removing sediment from angular rock is difficult. The flow entrance should drop 2 to 3 inches from curb line (see Figures 4.4.7 and 4.4.8) and provide an area for settling and periodic removal of sediment and coarse material before flow dissipates to the remainder of the cell (Prince George’s County, Maryland, 2002, and U.S. Army Environmental Center and Fort Lewis, 2003).
  • Curb cuts used for bioretention areas in high use parking lots or roadways may require higher level of maintenance due to increased accumulation of coarse particulates and trash in the flow entrance and associated bypass of flows. Recommended methods for areas where heavy trash and coarse particulates are anticipated:
    • Make curb cut width a minimum of 18 inches.
    • At a minimum the flow entrance should drop 2 to 3 inches from gutter line into the bioretention area and provide an area with a concrete bottom for settling and periodic removal of debris.
    • Anticipate relatively more frequent inspection and maintenance for areas with large impervious areas, high traffic loads, and larger debris loads.
    • Catch basins or forebays may be necessary at the flow entrance to adequately capture debris and sediment load from large contributing areas and high use areas. Piped flow entrance in this setting can easily clog, and regular maintenance of catch basins is necessary to capture coarse and fine debris and sediment.
  • Piped flow entrance: Piped entrances should include rock or other erosion protection material in the channel entrance to dissipate energy and disperse flow.
  • Trench drains: Trench drains can be used to cross sidewalks or driveways where a deeper pipe conveyance creates elevation problems. Trench drains tend to clog and may require additional maintenance (see Figure 4.4.9).
Woody plants can restrict or concentrate flows, be damaged by erosion around the root ball, and should not be placed directly in the entrance flow path.

Forebays and pre-settling are recommended for concentrated flow entrances (curb-cuts, trench drains, and pipes) to reduce accumulation of sediment and trash in the bioretention area and maintenance effort. Catch basins or open forebays can be used for pre-settling.
  • Catch basins: In some locations where road sanding or higher than usual sediment inputs are anticipated, catch basins can be used to settle sediment and release water to the bioretention area through a grate for filtering coarse material (see Figure 4.4.10).
  • Open forebays (pre-settling areas specifically designed to capture and hold flows that first enter the bioretention area): The bottom of the pre-settling area should be large rock (2- to 4-inch streambed or round cobbles) or concrete pad with a porous berm or weir that ponds the water to a maximum depth of 12 inches.

Bottom area and side slopes

Bioretention areas are highly adaptable and can fit various settings, such as rural and urban roadsides, ultra urban streetscapes, and parking lots by adjusting bottom area and side slope configuration. Recommended maximum and minimum dimensions:
  • Maximum planted side slope if total cell depth is greater than 3 feet: 3H:1V. If steeper side slopes are necessary, rockeries, concrete walls, or soil wraps may be effective design options (see Figure 4.4.11). Local jurisdictions may require bike and/or pedestrian safety features, such as railings or curbs with curb cuts, when steep side slopes are adjacent to sidewalks, walkways, or bike lanes.
  • Minimum bottom width for bioretention swales: 2 feet recommended. Carefully consider flow depths and velocities, flow velocity control (check dams), and appropriate vegetation or rock mulch to prevent erosion and channelization at bottom widths less than 2 feet.
Bioretention areas should have a minimum shoulder of 12 inches between the road edge and beginning of the bioretention side slope where flush curbs are used. Compaction effort for the shoulder should be 90 percent standard proctor (see Figure 4.4.12).

Ponding area

Ponding depth recommendations:
  • Maximum ponding depth: 12 inches.
  • Maximum surface pool drawdown time: 24-48 hours.
The ponding area provides surface storage for storm flows, particulate settling, and the first stages of pollutant treatment within the cell. Pool depth and draw-down rate are recommended to provide surface storage, adequate infiltration capability, and soil moisture conditions that allow for a range of appropriate plant species. Soils must be allowed to dry out periodically in order to restore hydraulic capacity to receive flows from subsequent storms, maintain infiltration rates, maintain adequate soil oxygen levels for healthy soil biota and vegetation, and provide proper soil conditions for biodegradation and retention of pollutants.

Maximum surface pool drawdown time is also influenced by the location of the facility. For highly visible locations with denser populations, a 24-hour drawdown may be appropriate for community acceptance, while a 48-hour drawdown may be appropriate for less visible and dense settings.

Surface overflow
Surface overflow can be provided by vertical stand pipes that are connected to under-drain systems, horizontal drainage pipes, or armored overflow channels installed at the designed maximum ponding elevations (see Figure 4.4.13). Overflow can also be provided by a curb cut at the down-gradient end of the bioretention area to direct overflows back to the street. Overflow conveyance structures are necessary for all bioretention facilities to safely convey flows that exceed the capacity of the facility and to protect downstream natural resources and property.

The minimum freeboard from the invert of the overflow stand pipe, horizontal drainage pipe, or earthen channel should be 6 inches unless otherwise specified by the local jurisdiction’s design standards.

Bioretention soil media

The soil media and plants must work together to provide effective flow control and water quality treatment in bioretention areas. Soil mixes for bioretention areas need to balance four primary design objectives to provide optimum performance:
  • Provide high enough infiltration rates to meet desired surface water drawdown and system dewatering.
  • Provide infiltration rates that are not too high in order to optimize pollutant removal capability.
  • Provide a growth media that supports long-term plant and soil health.
  • Balance nutrient availability and retention to reduce or eliminate nutrient export during storm events (Hinman, 2009).

Bioretention soil media recommendations often have a topsoil component that generally does not have a grain size distribution specification and is highly variable depending on the source. As a result, the BSM can have higher than desired fines which may result in lower than desired infiltration rates.

The percent fines (aggregate passing the 200 sieve) in a BSM is important for proper system performance and requires particular attention. Presence of some fine material improves water retention, nutrient exchange and, as a result, the growing characteristics of soils. Smaller aggregate also increases receptor sites for adsorbing pollutants. In contrast, fine material strongly controls hydraulic conductivity and a small increase as a percentage of total aggregate can reduce hydraulic conductivity below rates needed for proper system drawdown (Hinman, 2009).

Overall gradation is important for BSM performance as well. The soil mix will likely infiltrate too rapidly if the aggregate component is a uniform particle size. Specifically, a uniformly graded, fine-grained material will have relatively low hydraulic conductivity (K). A uniformly graded, coarse-grained material will have a relatively high K. However, a well-graded material that appears coarse grained (BSM sand) can have relatively lower K in ranges suitable for BSM used without control structures.

The following provides guidelines for Ecology-approved BSM. If the BSM is verified to meet the mineral aggregate gradation and compost guidelines below then no laboratory infiltration testing is required. If a different aggregate gradation and compost guideline is used, laboratory infiltration tests (ASTM methods given below) are required to verify that the BSM will meet infiltration requirements.

Infiltration rates
  • When using the approved BSM guidelines provided below, enter a Ksat of 6 inches per hour with appropriate correction factor into the sizing model.
  • If using a different BSM guideline, laboratory Ksat testing is required. The Ksat determination should be no less than 1-inch per hour after a correction factor of 2 or 4 is applied (see Section Determining subgrade and bioretention soil media design infiltration rates) and a maximum of 12 inches per hour with no correction factors applied. Enter the laboratory-determined Ksat with appropriate correction factor into the sizing model.
Mineral aggregate
Percent fines
  • A range of 2-4 percent passing the 200 sieve is ideal and fines should not be above 5 percent for a proper functioning specification according to ASTM D422-63.
Aggregate gradation
  • The aggregate portion of the BSM should be wellgraded. According to ASTM D 2487-11 (Classification of Soils for Engineering Purposes (Unified Soil Classification System)), well-graded sand for BSM should have the following gradation coefficients:
    • Coefficient of Uniformity (Cu = D60/D10) equal to or greater than 4; and
    • Coefficient of Curve (Cc = (D30)2/D60 x D10) greater than or equal to 1 and less than or equal to 3.

Table 4.4.2 provides a gradation guideline for the mineral aggregate component of a BSM specification in western Washington (Hinman, 2009). The sand gradation below is often provided by vendors as a well-graded utility or screened sand. With compost, this blend provides enough fines for adequate water retention, hydraulic conductivity within the recommended range (see below), pollutant removal capability, and plant growth characteristics for meeting design guidelines and objectives. If compost is not available or desired for use on the project, other locally available materials may be used as long as the blend achieves the desired properties as described in this Section. Some experimentation with locally available materials and mix specifications may be needed as demand for bioretention grows in eastern Washington and suppliers learn how best to provide mixes that meet designer specifications and provide desired benefits, such as supporting healthy plants and treating and retaining runoff on-site.

Existing soils

  • Where existing soils meet the above aggregate gradation, those soils may be amended rather than importing mineral aggregate.
  • For small projects that do not trigger Core Elements #5 or #6, the native soil may be amended according to guidance in the Rain Garden Handbook for Western Washington (WSU, 2013) to build rain gardens.
For information on using compost, compost benefits, a list of soil laboratories, and more, visit www.soilsforsalmon.org or www.buildingsoil.org and the Washington Stormwater Center website at www.wastormwatercenter.org/low-impact/.

Cation exchange capacity
  • Cation Exchange Capacity (CEC) must be ≥ 5 milliequivalents/100 g dry soil (S-10.10 from Gavlak et. al. 2003).
CEC is a measure of how many positively charged elements or cations (e.g., magnesium (Mg+2), calcium (Ca+2), and potassium (K+1)) soil can retain. Clay and organic material are the primary soil constituents providing receptor sites for cations and to a large degree determine CEC. One of the parameters for determining site suitability for stormwater infiltration treatment systems is CEC. Site Suitability Criteria #5 in the 2004 SWMMEW requires that soil CEC must be ≥ 5 milliequivalents/100 g dry soil. Bioretention soil mixes easily meet and exceed the Site Suitability Criteria #5 requirement.

BSM depth
  • Typical BSM depth is 12 to 24 inches.
  • For metals treatment, the recommended minimum depth is 18 inches.
  • A minimum depth of 24 inches should be selected for improved phosphorus and nitrogen (Total Kjeldahl Nitrogen [TKN] and ammonia) removal where underdrains are used.
Deeper BSM profiles (> 24 inches) may enhance phosphorus, TKN and ammonia removal (Davis, Shokouhian, Sharma and Minami, 1998). Nitrate removal in bioretention cells can be poor and in some cases cells can generate nitrate due to nitrification (Kim, Seagren, and Davis, 2003). See under-drain section for design recommendations to enhance nitrate removal. Deeper or shallower profiles may be desirable for specific plant, soil, and storm flow management objectives.

Infiltration rates and water quality treatment considerations
Bioretention soil media provide the necessary characteristics for infiltration facilities intended to serve a treatment function. To meet Ecology’s current criteria for infiltration treatment, the maximum initial infiltration rate should not exceed 12 inches per hour, the soil depth should be at least 18 inches, the CEC at least 5 meq/100 grams of soil, and the soil organic content at least 1.0 percent.

Bioretention soil media have high organic matter content and cation exchange capacities exceeding the above CEC criteria. Additionally, recent water quality treatment research for bioretention soils suggests that capture of metals remains very good at higher infiltration rates. Nitrate and ortho-phosphate retention and removal is likely influenced by plants, organic matter, and soil structure as well as soil oxygen levels, soil water content, and hydraulic residence time. Infiltration rate is, therefore, one of several factors that likely play an important role for nitrate and phosphate management in bioretention systems. More research is needed examining the influence of these various factors and to develop defensible infiltration rate guidelines for nutrient management. See below for nutrient management guidelines given current research.

Phosphorus management recommendations
These recommendations are applicable to any bioretention installation, but are critical for bioretention areas that have under-drains and direct release to fresh water or eventually drain to water bodies with TDMLs for nutrients or are specifically designated as phosphorus (P) sensitive by the local jurisdiction. Levels of P in bioretention areas are generally not a concern with groundwater unless there is groundwater transport of P through soils with low P sorption capability and close proximity to surface freshwater. Note that additional research is needed on P management in bioretention; however, current research indicates the following:
  • Mature stable compost: reduces leaching of bioavailable P.
  • Healthy plant community: provides direct P uptake, but more importantly promotes establishment of healthy soil microbial community likely capable of rapid P uptake.
  • Aerobic conditions: reduce the reversal of P sorption and precipitation reactions.
  • Increasing BSM column depth: to 24 or 36 inches may provide greater contact time with aluminum, iron, and calcium components, and sorption in the soil.
  • Relatively neutral pH.
  • Metal oxides: iron, aluminum, and calcium are metals that can be added to adsorb or precipitate P. Aluminum is the most applicable for bioretention systems with appropriate adsorption reaction time, relative stability, and pH range for reaction (Lucas, 2009). Water treatment residuals (WTRs), used for settling suspended material in drinking water intakes, is a waste product and source for aluminum and iron hydroxides. More research is needed in this area, but current trials indicate that WTRs can be added at a rate of 10 percent by volume to the BSM for sorption of P. WTRs are fine textured and, if incorporated into the BSM, laboratory analysis is required to verify appropriate hydraulic conductivity (see Section Determining subgrade & bioretention soil media design infiltration rates). If using WTRs at a rate of 10 percent by volume, add shredded bark at 15 percent by volume to compensate for the fine texture of the WTRs (e.g., 60 percent sand, 15 percent compost, 15 percent shredded bark, 10 percent WTRs).
  • Available P: the molar ratio of ammonia oxalate extracted P in relation to ammonia oxalate extracted Fe and Al in the BSM should be < 0.25.
  • Sandy gravel filter bed for under-drain: provides a good filter for fine particulates and additional binding sites for P (see below for more details on underdrains).
Nitrogen management recommendations
Nitrogen (N) levels in bioretention areas are generally not a concern with groundwater unless there is groundwater transport of N in close proximity to a drinking water aquifer. Note that additional research is needed on N management in bioretention; however, current research indicates the following:
  • Mature stable compost: Reduces leaching of bioavailable nitrate (NO3-N).
  • Healthy plant community: Provides direct NO3-N uptake, but more importantly promotes establishment of healthy soil microbial community likely capable of rapid NO3-N uptake.
  • Increasing BSM column depth: to 24 or 36 inches may provide greater contact time with small anoxic pockets within the soil structure and denitrification in the soil column.
  • Elevated under-drain: Research suggests that N capture and retention in bioretention areas varies from good retention to export of nitrate. Where nitrate is a concern, various under-drain designs can be used to create a fluctuating anoxic/aerobic zone below the drain pipe. Denitrification within the anaerobic zone is facilitated by microbes using forms of N (NO2 and NO3) instead of oxygen for respiration. A suitable carbon source provides a nutrition source for the microbes, enables anaerobic respiration, and can enhance the denitrification process (Kim, Seagren, and Davis, 2003). Dissolved and particulate organic carbon that migrates from the BSM to the aggregate filter and bedding layer likely provides adequate carbon source for microbes.
Biosolids and manure composts can be higher in bioavailable P and N than compost derived from yard or plant waste. Accordingly, biosolids or manure compost in bioretention areas are not recommended in order to reduce the possibility of exporting bio-available P and N in effluent.

Under-drain (optional)
Under-drain systems should typically only be installed when the bioretention area is:
  • Located near sensitive infrastructure (e.g., unsealed basements) and potential for flooding is likely.
  • Used for filtering storm flows from gas stations or other pollutant hotspots (requires impermeable liner).
  • Areas with contaminated groundwater and soils.
  • In soils with infiltration rates below the minimum rate allowed by the local jurisdiction or that are not adequate to meet maximum pool and soil column drawdown time.
  • In an area that does not provide the minimum depth to a hydraulic restriction layer.
  • In an area where the under-drain discharge would not be routed to a phosphorus-sensitive water body (Ecology, 2013).
The under-drain can be connected to a downstream open conveyance (such as a bioretention swale), to another bioretention cell as part of a connected treatment system, day-lighted to a dispersion area using an effective flow dispersion practice, or to a storm drain.

Under-drain pipe
Under-drains should be slotted, thick-walled plastic pipe. The slot opening should be smaller than the smallest aggregate gradation for the gravel filter bed (see underdrain filter bed below) to prevent migration of material into the drain and clogging. This configuration also allows for pressurized water cleaning and root cutting if necessary. Under-drain pipe recommendation:
  • Minimum pipe diameter: 4 inches (pipe diameter will depend on hydraulic capacity required, 4 to 8 inches is common).
  • Slotted subsurface drain PVC per ASTM D1785-12 SCH 40.
  • Slots should be cut perpendicular to the long axis of the pipe and be 0.04- to 0.069-inch by 1-inch long and be spaced ¼-inch apart (spaced longitudinally).
  • Slots should be arranged in two rows spaced on 45-degree centers and cover ½ of the circumference of the pipe.
  • The under-drain can be installed with slots oriented on top or on bottom of pipe.
  • Under-drains should be sloped at a minimum of 0.5 percent unless otherwise specified by an engineer.
Perforated PVC or flexible slotted HDPE pipe cannot be cleaned with pressurized water or root cutting equipment, are less durable, and are not recommended. Wrapping the under-drain pipe in filter fabric increases chances of clogging and is not recommended (Low Impact Development Center, 2012). A 6-inch rigid non-perforated observation pipe or other maintenance access should be connected to the under-drain every 250-300 feet to provide a clean-out port as well as an observation well to monitor dewatering rates (Prince George’s County, 2002).

Under-drain aggregate filter and bedding layer
Aggregate filter and bedding layers and filter fabrics buffer the under-drain system from sediment input and clogging. When properly selected for the soil gradation, geosynthetic filter fabrics can provide adequate protection from the migration of fines. However, aggregate filter and bedding layers, with proper gradations, provide a larger filter surface area for protecting under-drains and are preferred if available locally (see Table 4.4.3).


Drain position
For bioretention areas with under-drains (see Figure 4.4.14), elevating the drain to create a temporary saturated zone beneath the drain promotes denitrification (conversion of nitrate to nitrogen gas) and prolongs moist soil conditions for plant survival during dry periods.

Under-drains rapidly convey water out of the bioretention area and decrease detention time and flow retention. Properly designed and installed bioretention have shown very good flow control performance on soils with low infiltration rates (Hinman, 2009). Accordingly, when under-drains are used, orifices or other control structures are recommended to improve flow control. Access for adding or adjusting orifice configurations and other control structures is also recommended for adaptive management and optimum performance.

Orifice and other flow control structures
The minimum orifice diameter is an important consideration in cold climates, where ice formation could restrict flows if the under-drains are not maintained during freezing periods. Consult the local jurisdiction standards for minimum orifice diameters to be used in design and consider long-term maintenance when selecting any type of flow control structure.

Check dams and weirs
Check dams may be necessary for reducing flow velocity and potential erosion as well as increasing detention time and infiltration capability on sloped sites. Typical materials include concrete, wood, rock, compacted dense soil covered with vegetation, and vegetated hedge rows. Design depends on flow control goals, local regulations for structures within road right-of-ways, and aesthetics. Optimum spacing is determined by flow control benefit (through modeling) in relation to cost considerations. Some typical check dam designs are included in Figure 4.4.15.

Hydraulic restriction layers
Adjacent roads, foundations or other infrastructure may require that infiltration pathways are restricted to prevent excessive hydrologic loading. Two types of restricting layers can be incorporated into bioretention designs:
  • Clay (bentonite) liners are low permeability liners. Where clay liners are used, under-drain systems are necessary.
  • Geomembrane liners completely block infiltration to subgrade soils and are used for groundwater protection when bioretention facilities are installed to filter storm flows from pollutant hotspots or on sidewalls of bioretention areas to restrict lateral flows to roadbeds or other sensitive infrastructure (see Figure 4.4.16). Where geomembrane liners are used to line the entire facility, under-drain systems are necessary. The liner should have a minimum thickness of 30 mils and be ultraviolet (UV) resistant.


Plant roots aid in the physical and chemical bonding of soil particles that is necessary to form stable aggregates, improve soil structure, and increase infiltration capacity. In arid environments, plants can provide significant transpiration of stormwater runoff during the summer growing season. In cold climates, plants can help maintain infiltration through the bioretention section by developing macropores in the soil around roots. See Appendix D for a list of recommended plants for bioretention.

The primary design considerations for plant selection include:
  • Arid climates: Plants should tolerate sustained drought (EPA, 2013).
  • Cold climates: In cold climates, bioretention can be used for snow storage. If used for this purpose, or if used to treat runoff from a surface where salt is used as a deicer, the bioretention area should be planted with salt-tolerant, non-woody plant species (EPA, 2013). Other cold climate considerations include rooting depth and season of growth.
  • Soil moisture conditions: Plants should be tolerant of summer drought, ponding fluctuations, and saturated soil conditions for the lengths of time anticipated by the facility design.
  • Sun exposure: Existing sun exposure and anticipated exposure when bioretention plants mature is a primary plant selection consideration.
  • Above- and below-ground infrastructure in and near the facility: Plant size and wind firmness should be considered within the context of the surrounding infrastructure. Rooting depths should be selected to not damage underground utilities if present. Slotted or perforated pipe should be more than 5 feet from tree locations (if space allows).
  • Expected pollutant loadings: Plants should tolerate typical pollutants and loadings from the surrounding land uses.
  • Adjacent plant communities and potential invasive species control: Consider planting hearty, fast growing species when adjacent to invasive species and anticipate maintenance needs to prevent loss of plants to encroachment of invasive species.
  • Habitat: Native plants and hardy cultivars attract various insects and birds, and plant palettes can be selected to encourage specific species.
  • Site distances and setbacks for safety on roadway applications.
  • Location of infrastructure: Select plants and planting plan to allow visual inspection and easy location of facility infrastructure (inlets, overflow structures and other utilities).
  • Expected use: In higher density settings where foot traffic across bioretention areas is anticipated, elevated pathways with appropriate vegetation or other pervious material that can tolerate pedestrian use can be used. Pipes through elevated berms for pathways across bioretention areas can be used to allow flows from one cell to another.
  • Visual buffering: Plants can be used to buffer structures from roads, enhance privacy among residences, and provide an aesthetic amenity for the site.
  • Aesthetics: Visually pleasing plant designs add value to the property and encourage community and homeowner acceptance. Homeowner education and participation in plant selection and design for residential projects should be encouraged to promote greater involvement in long-term care.
Note that the BSM provides an excellent growth media and plants will often attain or surpass maximum gowth dimensions. Accordingly, planting layouts should consider maximum dimensions for selected plants when assessing site distances and adjacent uses. In general, the predominant plant material utilized in bioretention areas are facultative species adapted to stresses associated with wet and dry conditions (Prince George’s County, 2002). Soil moisture conditions will vary within the facility from saturated (bottom of cell) to relatively dry (rim of cell). Accordingly, wetland plants may be used in the lower areas, if saturated soil conditions exist for appropriate periods, and drought-tolerant species planted on the perimeter of the facility or on mounded areas (see Figure 4.4.17). See Appendix D for recommended plant species.

Planting schemes will vary with the surrounding landscape and design objectives. For example, plant themes can reflect surrounding wooded or prairie areas. Monoculture planting designs are not recommended. As a general guideline, a minimum of three small trees, three shrubs, and three herbaceous groundcover species should be incorporated to protect against facility failure due to disease and insect infestations of a single species (Prince George’s County, 2002) (see Figure 4.4.18).

Native and hardy cultivar plant species, placed appropriately, tolerate local climate and biological stresses and usually require no nutrient or pesticide application in properly designed soil mixes. Natives can be used as the exclusive material in a rain garden or in combination with hardy cultivars that are not invasive and do not require chemical inputs. In native landscapes, plants are often found in associations that grow together well, given specific moisture, sun, soil, and plant chemical interactions. Native plant associations can, in part, help guide planting placement. To increase survival rates and ensure quality of plant material, the following guidelines are suggested:
  • Plants should conform to the standards of the current edition of American Standard for Nursery Stock as approved by the American Standards Institute, Inc. All plant grades should be those established in the current edition of American Standards for Nursery Stock (Low Impact Development Center, 2012).
  • All plant materials shall have normal, well developed branches and a vigorous root system. Plants should be healthy and free from physical defects, plant diseases, and insect pests. Shade and flowering trees should be symmetrically balanced. Major branches should not have V-shaped crotches capable of causing structural weakness. Trunks should be free of unhealed branch removal wounds greater than a 1-inch diameter (Low Impact Development Center, 2012).
  • Plant size: For installation, small plant material provides several advantages and is recommended. Specifically, small plant material requires less careful handling, less initial irrigation, experiences less transplant shock, is less expensive, adapts more quickly to a site, and transplants more successfully than larger material (Sound Native Plants, 2000). Typically, small herbaceous material and grasses are supplied as plugs or 4-inch pots and small trees and shrubs are generally supplied in pots of 3 gallons or less.
  • Plant maturity and placement: Bioretention areas provide excellent soil and growing conditions; accordingly, plants will likely reach maximum height and width. Planting plans should anticipate these dimensions for site distances, adjacent infrastructure, and planting densities. Shrubs should be located taking into account size at maturity to prevent excessive shading and ensure establishment and vigor of bioretention area bottom plants.
  • All plants should be tagged for identification when delivered.
  • Optimum planting time is during April or May; although, fall planting between September 15th and October 31st is acceptable.

Mulch layer
Bioretention areas can be designed with or without a mulch layer; however, there are advantages to providing a mulch application. Properly selected mulch material reduces weed establishment (particularly during plant establishment period), regulates soil temperatures and moisture, and adds organic matter to soil. When used, mulch should be:
  • Arborist wood chips consisting of shredded or chipped hardwood or softwood trimmings from trees and shrubs. Wood chip operations are also a good source for mulch material and provide good control of size distribution and consistency.
  • Free of weed seeds, soil, roots, and other material that is not bole or branch wood and bark.
  • Coarse compost in the bottom of the facility and up to the ponding elevation (compost is less likely to float when the cell is inundated).
  • Arborist wood chips on side slopes above ponding elevation and rim area.
  • Free of shredded wood debris to which wood preservatives have been added.
  • A maximum of 2 to 3 inches thick. Thicker applications can inhibit proper oxygen and carbon dioxide cycling between the soil and atmosphere (Prince George’s County, 2002).
Mulch should not be:
  • Grass clippings (decomposing grass clippings are a source of N and are not recommended for mulch in bioretention areas).
  • Pure bark (bark is essentially sterile and inhibits plant establishment).
If planting bioretention areas is delayed (e.g., BSM is placed in summer and plants are not installed until fall), mulch should be placed immediately to prevent weed establishment.

Dense groundcover enhances soil structure from root activity, does not have the tendency to float during heavy rain events, inhibits weed establishment, provides additional aesthetic appeal, and is recommended when high heavy metal loading is not anticipated. Mulch is recommended in conjunction with the groundcover until groundcover is established.

Research indicates that most attenuation of heavy metals in bioretention cells occurs in the first 1 to 2 inches of the mulch layer. That layer can be removed or added to as part of a standard and periodic landscape maintenance procedure. No indications of special disposal needs are indicated at this time from older bioretention facilities in the eastern U.S. (personal communication between Curtis Hinman and Larry Coffman).

In bioretention areas where higher flow velocities are anticipated, aggregate mulch may be used to dissipate flow energy and protect underlying BSM. Aggregate mulch varies in size and type, but 1- to 1½-inch gravel (rounded) decorative rock is typical (see Figure 4.4.19).

Figure 4.4.7

Curb cut inlet with drop to prevent clogging at flow entrance. Source: Photo by Curtis Hinman. Illustration courtesy of the Bureau of Environmental Services, City of Portland, OR.

Figure 4.4.8

Typical curb cut details. Source: Photo by Curtis Hinman. Illustration courtesy of the Bureau of Environmental Services, City of Portland, OR.

Figure 4.4.9
Typical trench drain details. Source: Illustration courtesy of the Bureau of Environmental Services, City of Portland, OR.

Fig4-4-10_6.1.13 catch basin inlet
Figure 4.4.10
Catch basin inlet. Source: Photo by Curtis Hinman

Figure 4.4.11
Bioretention rockery wall detail. Source: AHBL, Inc. courtesy of Low Impact Development Technical Guidance Manual for Puget Sound (2012).

Fig4-4-12_6.1.16 flush curb and shoulder
Fig 4.4.12
Bioretention area with flush curb and shoulder. Source: Photo by Curtis Hinman

Figure 4.4.13
Bioretention outlet structure photo and drawing providing elevation drop. Source: AHBL, Inc. courtesy of Low Impact Development Technical Guidance Manual for Puget Sound (2012). Photo by Curtis Hinman

Figure 4.4.14
Upturned under-drain to create a saturated zone for denitrification. Source: AHBL, Inc. courtesy of Low  Impact Development Technical Guidance Manual for Puget Sound (2012)

Figure 4.4.15
Check dam and berms.  Source: AHBL, Inc. courtesy of Low Impact Development Technical Guidance Manual for Puget Sound (2012). Photo by Curtis Hinman

Figure 4.4.16
Bioretention planter section with liner. Source: AHBL, Inc. courtesy of Low Impact Development Technical Guidance Manual for Puget Sound (2012)

Figure 4.4.17
Bioretention soil moisture zones. Source: AHBL, Inc.

Figure 4.4.18
Example planting scheme for a bioretention swale in Spokane. Source: AHBL, Inc.

Fig4-4-19_6.1.27 aggregate mulch
Figure 4.4.19
Aggregate mulch is used in the high gradient bioretention swale. Plants are installed through the aggregate mulch and into the BSM below. Source: Photo by Curtis Hinman