Soil, Media, & Plantings

Selecting the appropriate soil media(s) and plant species is critical for successful bioretention facilities. Soil characteristics should be chosen to support drainage, pollutant removal rates, and plant health and viability. Plantings should include a diverse community of native species. 

Soil Media

The type(s) of soil media used in a bioretention facility varies based on region-specific conditions, water quality treatment requirements, flow attenuation, and regulations. The soil must:

• Have proper permeability to drain;
• Absorb the targeted pollutants and capture suspended solids;
• Include the proper materials to avoid exporting pollutants such as nutrients or pathogens;
• Sustain vegetation.

The characteristics of the bioretention soil media will affect the infiltration rate. Slow draining media infiltrates less than 1 inch/hour, while faster draining media can infiltrate 10 inches/hour or more.¹

Plants & Trees

Plants and trees used in the bioretention cross-section provide a stormwater function and also improve the green aesthetic experience in the right-of-way. Trees, plants, and their root structures help with absorption of stormwater, uptake of nutrients and pollutants, soil stabilization, and sustaining the bioretention soil media infiltration rate over the long term. 

The type of plants and trees suitable for a bioretention facility varies across regions. When selecting plants, consider regional or local climate conditions and the species’ ability to withstand varying wet and dry conditions. Plantings should include a diverse community of species appropriate for each specific site. Native plantings are preferred, but vegetation appropriate to each climate enhances the aesthetic and performance of the infrastructure. Consider salt tolerance in winter climates and flooding tolerance in regions with heavy storms.

The type of bioretention soil media will affect the type of plant species that can thrive. Media with higher concentrations of organics will allow for a wider selection of plants. Bioretention soil media that drains quickly (in cells with an underdrain or with native soils with high infiltration rates) may limit the types of native plants that can be used. 

Plants and trees in green stormwater infrastructure can provide habitat for bees and other pollinators. Select a diversity of species to improve ecological health and minimize susceptibility to infestation or disease.

Consider tree placement carefully at the beginning of the project design phase. Trees should be sited adjacent to and/or within the bioretention facility so that adequate soil is provided for healthy long term tree growth. During the siting and selection of bioretention facilities, review the placement of trees along the street in coordination with other infrastructure and setback requirements. 

Trees can be planted within the bottom of cells or on the side slopes of the bioretention facility. Consider the branching form as the tree grows to ensure that there is adequate height clearance on both the sidewalk and roadside.

When planting trees on the side slopes of bioretention swales, in order to have space for the tree pit and watering ring, the bottom width of the bioretention swale can narrow and the side slopes for tree planting can increase to 2H:1V to transition to the bioretention swale’s sides. 

Narrow bioretention planters or other facilities that are lined can limit the ability to plant medium to large trees, and smaller trees may not have a branching structure for sidewalk clearance.  In cases where the bioretention facility type and/or location limits the ability to plant trees, consider widening the gaps between bioretention cells to make space for street trees, which help with stormwater absorption. Coordinate bioretention cell types and locations to optimize stormwater management and vegetation health. 

Consider operations and maintenance needs and available resources when selecting soil media and plant species. Choose low maintenance plants that minimize the need for mowing, pruning, weeding, and irrigation. Avoid using fertilizers or pesticides.

Critical

In bioretention swales with graded sides, vegetation should be planted in a contour pattern with regard to each plant’s role in infiltration; plants at the lowest grade of the cell that will be submerged during storm events must be able to withstand flooding well, while plants along the graded side slopes must be effective at stabilizing soil and controlling erosion. 

When selecting vegetation, review the plant species’ mature height. For bioretention facilities near intersections, pedestrian crossings, and driveways, use a maximum mature plant height of 24 inches for sight clearance and visibility. If plants are planted in the bottom of the cell, the mature plant height can be greater so long as the height above street grade does not exceed 24 inches near crossings. 

1. Maimone, Mark, D. E. O’Rourke, J. O. Knighton, and C. P. Thomas. “Potential impacts of extensive stormwater infiltration in Philadelphia.” Environmental Engineer 14 (2011): 29-39.  ↩︎