Jeremy Person

by Jeremy Person, PLA, ASLA, with co-authors Ann English, PLA, ASLA, Ted Shriro, Andy Szatko, John Watson, and Jim Cooper, ASLA

Lanark Way bioretention, Montgomery County, Maryland Department of Environmental Protection retrofit. Aromatic aster blooming. / image: reproduced with permission from Montgomery County, MD Department of Environmental Protection

In January of 2020, The Field published an article on Performance-Based Plant Selection for Bioretention that sought an approach for planting design that prioritizes the functional attributes that plants provide in bioretention stormwater treatment facilities. In 2021, the Green Infrastructure Leadership Exchange (the Exchange) awarded a Collaborative Grant to a multi-disciplinary team from Chicago, Maryland, Omaha, and Oregon to explore these questions further and complete a first phase towards building a Bioretention Plant Selection Tool (BPST). The effort was focused specifically on functions plants provide in bioretention and the vegetative attributes to optimize overall bioretention performance. Biohabitats, a multidisciplinary consulting firm specializing in ecological restoration, conservation planning, and regenerative design, was hired to survey stormwater professionals, complete a review of research on plant functions in bioretention installations, and develop an outline of how stormwater practitioners could evolve planting design to improve facility performance.

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by Jeremy Person, PLA, ASLA, Brian Wethington, Donna Evans, and Irene Ogata, ASLA

Bioretention planter in Portland, OR, planted with large native plants. Oversized plantings cause visibility issues for pedestrians, cyclists, and motorists and create maintenance liabilities. / image: City of Portland

For more than two decades landscape architects and stormwater professionals have been utilizing vegetated bioretention systems to help address complex stormwater and climate change-related issues. Bioretention systems use a combination of soil and plants to collect, detain, treat, and infiltrate runoff from roads, roofs, and other impervious surfaces. It is becoming apparent that plant health is one of the major drivers of increasing life-cycle costs, and that improper plant selection is partially to blame.

Landscape architects, horticulturalists, and designers are beginning to better define which characteristics make a plant ideal for use in bioretention. Understanding the site-specific needs for plants and identifying project goals allow designers to address performance issues up front and reduce long-term maintenance liabilities. The following three issues should be considered as early in a project as possible:

1. Project Goals and Facility Design

The two major goals for most bioretention projects are pollution reduction and flow control. Projects may serve one goal or both, and this may vary across a city or region. Bioretention facilities are designed for project-specific hydraulic regimes with controlled flooding and hydroperiods that affect plant viability. This affects plant selection in several ways:

Hydroperiod: Understanding the flooding cycle of the facility, its frequency, and how it relates to the growing cycle of the plants is critical. Smaller plants often fail because they are routinely flooded during the growing season, depriving them of needed oxygen. Designers should prioritize plants that grow taller than the high-water level and take cues from native wetland plants that have evolved to tolerate similar hydroperiods.

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