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The original bioretention concept was modeled from the hydrologic characteristics and properties of a terrestrial forest ecosystem. The forest community model for stormwater management was selected based upon a forest's documented ability to cycle and assimilate nutrients, pollutants, and metals through the interactions among plants, soil, and the organic layer. This theme is appropriate where the facility is located at wooded edges, in the rear of residential lots, or where a wooded buffer is desired. Where space is at a premium, large shrubs may be used for the canopy layer with smaller shrubs and an assortment of perennials underneath.  In areas expected to have high metal loadings (industrial and institutional land uses) mulching is preferred over a herbaceous groundcover.

Site trees on the perimeter of bioretention areas, to maximize the shading and sheltering of bioretention areas to create a microclimate which will limit the extreme exposure from summer solar radiation and winter freezes and winds. The final plant material layout should resemble a random and natural placement of plants rather than a standard landscaped approach with trees and shrubs in rows or other orderly fashion. A minimum of three species of trees and three species of shrubs should be selected to ensure diversity. This will protect the system against collapse from insect and disease infestations and may ensure a more constant rate of evapotranspiration and nutrient and pollutant uptake throughout the growing season. Perennials can be planted along the edge of the facility where color and seasonal interest are desired, and shade tolerant perennials can be planted as an underlying herbaceous layer throughout the rest of the facility. Utilize native plants in the design. Native species demonstrate a greater ability of adapting and tolerating physical, climatic, and biological stresses (Metropolitan Washington Council of Governments, 1992). Select herbaceous ground covers to prevent erosion of the mulch and the soil layers. Select at least 3-4 species of herbaceous groundcover. Two to three shrubs should be specified for each tree (2:1 to 3:1 ratio of shrubs to trees). Where the plant material for the facility is utilized to meet the residential landscape requirements or street tree requirement trees may be 2.5 inches in caliper. All other trees should be specified as >1.0 inch caliper. Shrubs should be 3 to 4 feet in height or 18 to 24 inches in spread at installation.  Mixed trees and shrubs can be planted as close as 7 feet; and shrubs alone from 4 to 7 feet apart. Ground cover may be as seed or, preferably, plugs.

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The information on this website was compiled by T.E. Scott & Associates, Inc. from various sources.

We make no guarantees or warranties to the applicability of this material to specific applications. 
Last modified: 04/27/08