Biofiltration Systems for Stormwater Management

The Vegetation Component

by Julie Whitacre, Fourth Corner Nurseries

As our landscape becomes more developed, management of stormwater runoff is a crucial part of maintaining the health of natural water systems. Diminishing natural streams and wetlands are increasingly important for beleaguered wildlife populations, and we should do all we can to ensure that clean water enters these systems in natural flow patterns. Stormwater runoff often contains high sediment loads and many types of pollutants, including oil and grease, chemicals, pesticides, heavy metals, bacteria, viruses, and oxygen-demanding compounds (Interagency Workgroup on Constructed Wetlands, 2000). Treatment facilities are engineered to capture and transform pollutants in water running off roads, parking lots, and roofs so that they will not reach natural wetlands and other ecologically important habitats. The time over which water from a storm event enters streams can be extended to prevent flooding and, depending on the engineering design and site conditions, groundwater recharge is also possible. Over time, however, pollutants will concentrate in the sediment and vegetation in these facilities, creating an unhealthy environment for aquatic life. Wildlife exclusion devices may be necessary. The loss or damage to wetland habitat incurred during development should be replaced with mitigation wetlands, providing the same functions and harboring the same species diversity and biotic richness as the wetlands they replace. Mitigation wetlands require different designs and may not be used as stormwater filters due to pollution concerns.

Biofiltration refers to the “simultaneous processes of filtration, infiltration, adsorption and biological uptake of pollutants in stormwater that take place when runoff flows over and through vegetated treatment facilities”(III-6.2.2, WA Dept or Ecology, 1992). Design options include filter strips, biofiltration swales, wetponds, detention ponds, treatment wetlands or combinations of these. Choice of design is mainly an engineering/permitting issue dealing with square footage of impervious surfaces, flow rates, and site conditions, and requirements may vary considerably by county. The choice of vegetation is also important for proper functioning of the system, since species have different tolerances for moisture regimes, different filtration and uptake qualities, and different maintenance needs.

Runoff management in land development begins with appropriate site design. The Stormwater Management Manual for Western Washington, Vol. V states that at least 65% of natural vegetation on a site should be maintained (p. 91). Also, the square footage of impervious surfaces directly connected to storm drains should be minimized (p. 93). An engineered soil/landscape system, with permeable soil (not compacted), high in organic matter can help water infiltrate into the groundwater. Pollutants are removed in the soil (p. 97).

Bioswales and filter strips are used to remove low concentrations of pollutants such as sediments, heavy metals, nutrients, and petroleum hydrocarbons (p. 255, WA Dept of Ecology, 2000) by means of sedimentation, filtration, soil sorption, and/or plant uptake (p. 253, WA Dept of Ecology, 2000). Bioswales are open channels vegetated with grasses and other herbaceous plants through which runoff is directed during storm events. Filter strips are flat strips of grass with no side slopes, and stormwater is distributed as sheet flow along the width of the strip. Water flow rate is slowed by plants, thereby encouraging particulates and their associated pollutants to settle. The pollutants are then incorporated into the soil where they may be immobilized and/or decomposed (WA Dept of Ecology, 1992).

Typically bioswales and filter strips are hydroseeded with an appropriate grass mix and kept mowed to a 4-9 inch grass height. Grass shorter than 4 inches will not perform the intended functions. Non-native, turf-forming grasses including fescue (tall or meadow) and bentgrass (redtop, spike or colonial) are most often used, since the dense growth is considered a more effective sediment trap than plants with a clumping habit (Mazer, 1998). The most commonly observed problems leading to vegetation failure are: “Prolonged inundation, erosional damage due to long periods of flooding, high flow velocity, wide water depth and soil moisutre fluctuations, excessive shade, poor soils, and improper installation.” (p. 15 Mazer, 1998) While some of these are design issues, proper soil preparation and plant species selection for the moisture and light conditions are crucial for the system to function well.

Wetponds and detention ponds. A wetpond is “a constructed stormwater pond that retains a permanent pool of water… at least during the wet season.” The main purpose of a wetpond is pollutant removal from runoff. “As an option, a shallow marsh area can be created within the permanent pool volume to provide additional treatment for nutrient removal” (p. 208, WA Dept of Ecology, 2000). In contrast to a wetpond, detention ponds are designed to slow peak flow, releasing water more slowly to prevent flooding. A retention or infiltration pond collects stormwater and allows the water to soak into the soil. This infiltration process helps recharge groundwater.

Emergent vegetation should be established in shallow areas of wetponds to stabilize settled sediments. Species recommended include (from Stormwater Management Manual for Western Washington, Vol V):

Agrostis exarata (1)
Spike bent grass; 2 feet max water depth
Carex stipata 
Sawbeak sedge; Wet ground
Eleocharis palustris 
Spike rush 2 feet max water depth
Glyceria occidentalis 
Western mannagrass; 2 feet max water depth
Juncus tenuis 
Slender rush; Wet soils, wetland margins
Oenanthe sarmentosa 
Water parsley; Saturated soils all summer
Scirpus atrocinctus (formerly S.cyperinus) 
Woolgrass; Tolerates shallow water; tall clumps
Scirpus microcarpus 
Small-fruited bulrush; Wet ground to 18 inches depth
Sagittaria latifolia 
Arrowhead; 18 inches
Agrostis exarata (1)
Spike bent grass
Alisma plantago-aquatica 
Water plantain
Eleocharis palustris 
Spike rush
Glyceria occidentalis 
Western mannagrass
Juncus effusus 
Soft rush; Wet meadows, pastures, wetland margins
Scirpus microcarpus 
Small-fruited bulrush; Wet ground to 18 inches depth
Sparganium emmersum 
Bur reed Shallow standing water, saturated soils
Carex obnupta 
Slough sedge; Wet ground or standing water to 3 feet
Beckmania syzigachne (1)
Western sloughgrass
Scirpus acutus (2)
Hardstem bulrush; 3 feet max water depth
Scirpus validus (2)
Softstem bulrush
Nuphar polysepalum 
Spatterdock; Deep water 3 to 7.5 feet
Nymphaea odorata (1)
White waterlily; Shallow to deep ponds to 6 feet
(1) Non-native species. Beckmania syzigachne is native to Oregon. Native species are preferred.
(2) Scirpus tubers must be planted shallower for establishment, and protected from foraging waterfowl until established. Emerging aerial stems should project above water surface to allow oxygen transport to the roots.


Where water levels are expected to fluctuate greatly, but stay wet most of the year, Scirpus acutusS microcarpusSparganium and Veronica are recommended (p.244, WA Dept of Ecology, 2000.) Water level fluctuations are common, and problems with plant survival can occur when the system is completely dry in the late summer, or when actual water levels are different from designed levels.

Trees and shrubs can be planted around the edges to provide shade and discourage waterfowl use of the pond. Recommended shrubs include vine maple (Acer circinatum), wild cherry (Prunus emarginata), red osier dogwood (Cornus sericea), California myrtle (Myrica californica), Indian plum (Oemeleria ceraciformis), and pacific yew (Taxus brevifolia). Ornamental shrubs may also be used (p. 215, WA Dept of Ecology, 2000.) Do not plant trees or shrubs on berms, since water “piping” by roots or tree blow-down can compromise berm integrity.

Different plant species take up different toxins from water. Excess nutrients (nitrogen and phosphorous) are best removed by species of Juncus and Typha, with Scirpus species being a little less effective. Phosphorous taken up by emergent vegetation is re-released in the winter when top-growth dies back, so mowing and removal of growth may be necessary. As for toxins, Juncus and Scirpus species remove oil most effectively, andTyphaCarex sppSparganiumScirpus acutusScirpus validus, and Lemna sequester metals most effectively.Typha will also increase pH in water that is too acidic. Pathogens such as E coli, Enterococci, Salmonella are reduced by Alisma plantago-aquaticaMentha aquaticaJuncus effususScirpus acutus, and Iris pseudoacoris(Kulzer, 1990). This iris, yellow flag iris, may have some water-cleansing properties, but it is also a Class C noxious weed. It is not appropriate to use in any area near or with hydrologic connectivity to natural wetlands, lakes or streams due to its invasive nature. Cattails also, although they are native and very helpful to water quality, are rarely recommended to plant because they tend to establish on their own.

For more information on emergent vegetation and wildlife habitat, see “Recommendations for Using Bare-Root
Wetland Plants
.” Whether you need wetland plants for a water treatment pond, a development mitigation project, a salmon habitat restoration planting, or for an ornamental pond planting, please refer to our menu of beautiful, functional, seed-grown emergents.


Azous, A and Horner, R (eds). 2001. Wetlands and Urbanization: Implications for the Future. Final Report of the Puget Sound Wetlands and Stormwater Management Research Program. Lewis Publishers.

Campbell, C. and M. Ogden. 1999. Constructed Wetlands in the Sustainable Landscape. John Wiley & Sons, Inc.

Interagency Workgroup on Constructed Wetlands. 2000. Guiding Principles for Constructed Treatment Wetlands: Providing Water Quality and Wildlife Habitat. US EPA publication #843-B-00-003.

King Co. DNR. 1998. The Integrated Pond. Viewed at on July 11, 2005.

Kulzer, L. 1990. Water Pollution Control Aspects of Aquatic Plants: Implications for Stormwater Quality Management. Office of Water Quality, Municipality of Metropolitan Seattle.

Mazer, G. 1998. Environmental Limitations to Vegetation Establishment and Growth in Vegetated Stormwater Biofilters. Center for Urban Water Resources Management, University of Washington.

Puget Sound Water Quality Action Team. 2001. Low Impact Development in Puget Sound, Innovative Stormwater Management Practices. Viewed at

Schultz, D, 1998. Current Status of Vegetation Management in Roadside Ditches and Stormwater Management Facilities: Implications for Stormwater Quality. Center for Urban Water Resources Management, University of Washington.

WA Dept. of Ecology. 2000. Stormwater Management Manual for Western Washington, Vol V. Runoff Treatment BMPs. Publication # 99-15. Viewed at on June 15, 2005. 2001 amendments available at the same web address.