by Jeremy Person, PLA, ASLA, with co-authors Ann English, PLA, ASLA, Ted Shriro, Andy Szatko, John Watson, and Jim Cooper, ASLA
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.
While this article highlights the first phase and the BPST has yet to be further developed, the underlying questions guiding plant selection were explored in detail. The accompanying literature review made clear what we as practitioners know and, perhaps more importantly, what we do not know. It is the team’s hope that this work can spur questions, discussions, and research to better understand how plants perform in bioretention so we can best select plants, appropriate to their region, that will work to solve the unique challenges of each site.
Practitioner Survey
A questionnaire was developed to determine how planting goals are defined, what functions are desired, how cultural suitability for a community or neighborhood is determined, and what post-installation management and maintenance considerations are needed. The survey was distributed to Exchange members (municipal green stormwater infrastructure professionals including landscape architects, engineers, and planners) as well as the ASLA and other professional associations.
Plant performance metrics, mainly with respect to pollutant removal and peak flow reduction were clearly emphasized in Survey responses. Other emphases included ramifications of underlying engineered soil media (i.e., not in situ soils) on plant selection and survival; preferences for plant material that is adaptable to a wide range of conditions (soil drainage, sunlight exposure, soil moisture, drought tolerance, etc.); preference for installing larger plant stock sizes; and considerations relating to planting establishment including proper installation, irrigation, and maintenance requirements.
Literature Review
Survey results guided the Biohabitats team in a literature review, producing an annotated bibliography. Key takeaways were both unsurprising and interesting at similar turns. Higher root mass, depth, and plant growth rates are important plant traits driving nitrogen removal from the soil solution (Dagenais, Brisson, and Fletcher 2018). Some species may be better suited than others for nitrogen removal, but in general, the larger the plant (both above- and below-ground biomass) the more effective they are in nitrogen processing and removal (Muerdter, Wong, and LeFevre 2018; Dagenais, Brisson, and Fletcher 2018).
Deeper or more extensive root systems, which are associated with robust bacterial colonies and associated enzymes, are more effective at hydrocarbon removal than plants with shallower root systems (LeFevre, Hozalski, and Novak 2012). Translocation of hydrocarbons from root systems to above-ground biomass indicates that plant maintenance/pruning may be an effective tool in hydrocarbon removal from GSI facilities (Muerdter, Wong, and LeFevre 2018). Further, when it comes to hydrocarbon and heavy metal remediation, plants most suitable for phytoextraction of heavy metals tend to have high growth rates, production of more above-ground biomass, ease of harvest (pruning), and highly branched root systems (Ali, Khan, and Sajad 2013).
In terms of developing more robust planting schemes, it is best to develop a planting palette that accounts for a wide variety of climate and soil media factors, thereby exercising “plasticity” (Hunter 2011). It is recommended that planting designs account for current United States Department of Agriculture (USDA) hardiness zones during the design process as well as how these hardiness zones may shift with climate change. In summary, the most resilient planting designs account for a wide variety of planting media and environmental conditions, emphasizing flexibility for proactive management and replanting.
Example Plant Selection Criteria:
Plant Selection
Plasticity – perform across a range of environmental conditions
(i.e. Temp, Soil Moisture, Urban Pollution, Flood/ Drought)Ecological Resilience – maintain function in the face of environmental disturbances + functional redundancy
(ex. Overlapping bloom times, flood and drought tolerant species)Structural Diversity – Describes the spatial complexity offered by various plant forms
Source: Hunter, M. (2011) Using Ecological Theory to Guide Urban Planting Design: An adaptation strategy for climate change, Landscape Journal, 30(2), 173-193.
BPST Outline
Biohabitats developed an outline detailing what a BPST framework could look like and what the ‘top line’ selection criteria might be. The outline is a starting point for development of an eventual BPST, and the criteria are meant to establish a baseline foundation, rather than be exhaustive.
1. Project goals
- MS4/programmatic performance (regulatory-driven) requirements
- Environmental/ecological objectives
- Plant life-cycle projections and maintenance needs
- Cultural and aesthetic interests
2. Site location and context
- Broader scale location
- Finer scale location
- Socio-cultural context
- Physical and ecological context
3. Facility design considerations
- Facility type (based on the location, proximity to specific types of land cover and built infrastructure, and thus the amount and quality of water to be received, filtered, and infiltrated)
- New facility vs. retrofit
- Site stormwater drainage dynamics (separate storm and sanitary sewer system or combined sanitary-and-stormwater system)
- Size and configuration (including where the facility lies within a treatment train)
- Planting media (use of existing or engineered soils)
- Facility plumbing and/or overflows
- Plant material provenance and availability
- Regulatory requirements and/or other criteria
- Budget considerations (related to implementation and maintenance)
- Post-construction monitoring prospects and maintenance concerns
4. Planting functions
- Performance (filtering and transforming pollutants, and infiltrating water)
- Ecological (specific habitat and water quality functions and values; nearly full coverage for erosion control and soil-plant-water interactions)
- Cultural and aesthetic value (including potential use of clipped and removed materials for human use and enjoyment)
- Durability
5. Plant community life-cycle and management
- Adaptive management and care of plantings as facilities and their adjacencies (e.g., contributing land uses and land cover) evolve over time
- Pushing regulatory boundaries to enable a more adaptive maintenance regime vs. static regulatory guidance
- Consideration of how plant species and plant communities respond to facility maintenance activities and associated disturbance types and intensities
- Design to simplify irrigation and maintenance needs and frequency
- Clear identification (and training as needed) of parties responsible for maintenance/upkeep
- Recognition of facilities as organisms themselves
- Emphasis on adaptive management/maintenance (e.g., planned weeding, clipping, soil surface coverage by plants or mulch, and future plantings as particular facility conditions and plant communities change and evolve)
Next Steps
The work summarized above is just the first phase of a multi-phased effort to develop better tools to improve bioretention facilities by focusing on plants, soils, neighborhood beautification, and how they help build community. The project team is actively looking at what the second phase looks like and how we might be able to collectively move this effort forward.
As landscape architects, part of our role is to synthesize the goals of a project and propose a planting design that can best meet those needs. Plants improve infiltration, sequester and break down pollutants, and improve water quality, among many other benefits. Maximizing their performance requires us to understand how a plant’s function meets those goals so we can utilize them in the best way. This tool will help do that and be a resource for all stormwater practitioners, designers, contractors, and the public.
References
Ali, H, E Khan, and MA Sajad. 2013. Phytoremediation of heavy metals – concepts and applications. Chemosphere 91(7):869-81.
Dagenais, Danielle, J. Brisson, and TD Fletcher. 2018. The role of plants in bioretention systems; does the science underpin current guidance? Ecological Engineering 120:532-45
Hunter, M. 2011. Emerging landscapes: using ecological theory to guide urban planting design: an adaptation strategy for climate change. Landscape Journal 30(2):173-93.
LeFevre, GH, RM Hozalski, and PJ Novak. 2012. The role of biodegradation in limiting the accumulation of petroleum hydrocarbons in raingarden soils. Water Research 46(20):6753-62.
Muerdter, C, C Wong, G LeFevre. 2018. Emerging investigator series: the role of vegetation in bioretention for stormwater treatment in the built environment: pollutant removal, hydrologic function, and ancillary benefits. Environmental Science: Water Research & Technology 4(5) DOI:10.1039/C7EW00511C.
Person, J, B Wethington,D Evans, and I Ogata. 2020. Performance-Based Plant Selection for Bioretention. The Field. ASLA.
This post was a collaborative effort supported by the Green Infrastructure Leadership Exchange. The Exchange is a highly connected peer learning network of 60 organizations across North America that is building a practical playbook for implementing green stormwater infrastructure that any city can adopt. The Exchange seeks to use this playbook to accelerate implementation of green stormwater infrastructure affordably and equitably throughout North America. Learn more about the Exchange on their website.
This post was reviewed and edited by Ecology & Restoration Professional Practice Network (PPN) leader Lee Skabelund, ASLA.
Jeremy Person, PLA, ASLA, is a Landscape Architect in the Surface Water Restoration Design team within the Bureau of Environmental Services. The Surface Water Restoration team designs projects that restore rivers, streams, wetlands and uplands to manage stormwater, reduce flooding, improve water quality, and restore habitat. Jeremy has been with BES since 2007 and has over 20 years’ experience in the field of landscape architecture spanning bioretention, restoration, private development, and residential design. Jeremy holds a BS in Landscape Architecture from the Ohio State University, and a Masters of Landscape Architecture from California State Polytechnic University, Pomona.
Ann English, PLA, ASLA, manages the RainScapes Program for the Watershed Restoration Division of the Montgomery County MD Department of Environmental Protection. RainScapes supports watershed health goals through installation of small-scale green infrastructure projects that reduce runoff and improve water quality and habitat. Ann has been with DEP since 2009 and has over 30 years of experience focused on ecological design, non-profit project management and teaching design at the university level. Ann holds a BA in American History (U. Pennsylvania), Masters of Regional Planning (Pennsylvania State University), and a Masters of Landscape Architecture (University of Georgia).
Ted Shriro is a Stormwater Specialist for the City of Eugene, Oregon, managing 725 private vegetated stormwater sites, providing inspection for new projects and support to property owners. Ted has been with the City of Eugene for 8 years and has 20 years of experience in architecture, landscape architecture and ecological restoration, specializing in sustainable design. Ted is a licensed architect and landscape architect with a masters of architecture and landscape architecture from the University of Oregon, and a BA in fine art from the University of California, Berkeley.
Andy Szatko is a plan reviewer and project manager for green infrastructure maintenance contracts with the City of Omaha Public Works Department and has been with the City of Omaha since 2011. Prior to that, he was in the landscaping industry for over 15 years as a landscape designer, installer, owner, and manager. He is a graduate of the University of Nebraska in Lincoln with a Bachelor’s degree in Horticulture-Landscape Design and a Masters in Community and Regional Planning, with a specialization in Water Resource Planning and Management. He is passionate about understanding, implementing, and educating the public about stormwater management and green infrastructure.
John Watson is the Senior Civil Engineer of Water Resources at the Forest Preserve District of Cook County, IL, which includes Chicago and its suburbs, where he manages nature-based solutions for stormwater concerns, water quality, erosion issues, kayak and canoe launches and sustainability goals. Before that, he had 10 years of experience at the Metropolitan Water Reclamation District of Greater Chicago, where he co-managed the green stormwater infrastructure program and helped to develop the Green Infrastructure Plan to meet the EPA Consent Decree. He earned his Bachelor’s degree in Civil & Environmental Engineering from Valparaiso University and his Master’s degree in Environmental Hydrology and Hydraulic Engineering from University of Illinois – Urbana Champaign. Today, John is a Professional Engineer, a NGICP-professional, and a Certified Floodplain Manager, with over 15 years of experience, working with green stormwater infrastructure and nature-based solutions.
Jim Cooper, ASLA, is a senior landscape architect at Biohabitats with 23 years of experience in ecological resource restoration and low impact development (LID) design. He is primarily interested in the way that the landscapes we restore, create, and enhance affect the ecological and social confluences we share as humans with the wild. He believes that vibrant communities, both natural and anthropogenic, achieve their full potential when design proactively engages in compassionate understanding of the factors that influence our knowledge of both ecological and cultural landscape phenomena. Jim holds a BS in Natural Resources from North Carolina State University, a MEM in Resource Ecology from Duke University, and a MLA from North Carolina State University.