Performance-Based Plant Selection: Developing a Bioretention Plant Selection Tool

For over 20 years, vegetated bioretention systems have been designed by stormwater professionals to solve complex stormwater and climate change related issues. For the government agencies managing these facilities, it has become clear that poor plant performance is a major driver of increasing lifecycle costs. Diminished plant performance is a loss of treatment and is further exacerbated by our changing climate and changing precipitation patterns. Healthy, high-functioning plants are critical components of green infrastructure and without them, there is widespread decline of stormwater treatment success. At the same time, cities have fewer spaces in which to address stormwater treatment needs, thus asking us to do more with less. While we understand how to engineer bioretention facilities, we have much less knowledge of how biotic systems, including plants and soils, react and affect facility performance. 

Engineering design determines how much water is to be detained, for how long, and the rate at which it is released. However, how do we optimize treatment of stormwater runoff once it flows into a vegetated bioretention feature? What are we asking the plants to do? Are they removing pollutants? If so, which ones and how much? Are certain plants better for promoting infiltration? Why? What traits improve nutrient uptake and other performance metrics? What factors, other than survivability, should be part of plant selection? As stormwater professionals, can we use the site-specific needs of each project to specify plant material selection to optimize performance?  For instance, would using Juncus patens promote infiltration better than Liriope muscari?  Which plants can remove and sequester hydrocarbons? Ultimately, how can facility planting design be orchestrated to target specific performance parameters? 

In 2021, the Exchange awarded a collaborative Grant to a multi-disciplinary team from Chicago, Maryland, Omaha, and Oregon to explore these questions and complete a first phase towards building a Bioretention Plant Selection Tool (BPST) that would focus on the functions plants provide in bioretention and the specific traits one should look for. Biohabitats, a values-based consulting firm with a focus on ecological restoration and stewardship, was hired to survey stormwater professionals, complete a review of research on plant functions in bioretention, and develop an outline of how stormwater practitioners could evolve plant selection to optimize the values that specific plants provide.     

This article will summarize Biohabitats’ approach and their findings. While this is just the first phase, and the BPST does not exist, this work explored the underlying questions guiding plant selection and made clear what we 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 administered by the Exchange and distributed to Exchange members as well as outside professional associations, like the American Society of Landscape Architects (ASLA). Survey questions pertained to a variety of categories regarding planting function and utility including site location/planting design goals, existing plant selection resources, existing site conditions, planting installation and establishment challenges, desired vegetation attributes, the planting installation stage, and post-installation maintenance and management. 

Survey responses reflected the state of the practice when it comes to bioretention plant selection design, installation, performance, and maintenance. After the Survey was administered, the Exchange and Biohabitats compiled responses and identified trends.

There was a clear emphasis on expectations for plant performance metrics, mainly with respect to pollutant removal and peak flow reduction. 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 

Biohabitats performed a literature review tailored to specific areas of interest identified by the Survey results and compiled an annotated bibliography. Key literature review takeaways were both unsurprising and interesting at similar turns. In order to affect the highest degree of nitrogen removal, higher plant biomass and plant growth rates decrease nutrient effluent concentrations. Higher root mass, depth, and plant growth rates are important plant traits driving nitrogen removal from the soil solution. In summary, some species may be better suited than others for nitrogen removal, but in general, the larger the plant (both above and belowground biomass), the more effective they are in nitrogen processing/removal. 

In terms of phosphorous removal, interestingly, one study found that herbaceous species may be more effective than woody species at removing phosphorus. Others found that removal of phosphorus and associated total suspended solids (TSS) (phosphorus adsorbs to particular soil material) is not necessarily determined by specific plant species and that differentiation between species was not especially important. There is an apparent and under-researched nexus between phosphorus retention and mycorrhizal fungi. Phosphorus removal is more related to how soil media and facilities behave from a resistance to erosion standpoint and settlement of imported sediment within GSI facilities than plant removal, though it is important to note that plants provide the structural mechanisms through above-ground and root biomass that holds media in place and helps in retaining imported sediments/TSS. Essentially, the more robust the root systems, the better plants function at retaining soil media and thus phosphorus in bioretention facilities. 

Deeper root systems, which are associated with robust bacterial colonies and associated enzymes, are more effective at hydrocarbon removal than plants with shallow root systems. 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. 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. 

Unsurprisingly, plants with higher above- and below-ground growth rates have higher transpiration demand and thus result in lower water volumes within bioretention areas. Trees and shrubs may be more efficacious than herbaceous vegetation in peak flow/volume reduction due to higher evapotranspiration rates. Woody vegetation consistently has higher evapotranspiration rates than native, deep rooting herbaceous species. 

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.”  It is recommended that planting designs account for United States Department of Agriculture (USDA) hardiness zones during the design process as well as how they 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. 

BPST Outline 

Biohabitats was asked to draft an outline of what a BPST could look like and what the ‘top-line’ selection criteria might be. Survey responses, the ensuing literature review, and the overarching project goals informed the development of BPST criteria outline (below). It is important to note that the outline is a starting point for development of an eventual BPST. Outline criteria are not intended to be exhaustive and definitive; rather, they are meant to establish a baseline foundation upon which an eventual BPST is based. 

1. Project goals
2. 1. MS4/programmatic performance (regulatorily driven)
   2. Environmental/ecological
   3. Plant life cycle/maintenance needs
   4. Cultural/aesthetic

2. Site location and context

1. 1. Broader scale location
   2. Finer scale location
   3. Cultural context
   4. Physical/ecological context 

3. Facility design considerations 

1. 1. Facility type 
   2. New facility vs. retrofit
   3. Site stormwater drainage dynamics (separate storm/sanitary sewer? Combined sanitary/stormwater) 
   4. Size/configuration
   5. Planting media
   6. Facility plumbing 
   7. Plant material availability 
   8. Regulatory requirements/criteria
   9. Budget considerations
   10. Post-construction monitoring /maintenance concerns

4. Planting functions

1. 1. Performance
   2. Ecological
   3. Cultural/aesthetic
   4. Durability

5. Plant community lifecycle and management

1. 1. Adaptive management and care of plantings as facilities and their adjacencies evolve over time
   2. Pushing regulatory boundaries to enable a more adaptive maintenance regime vs. static regulatory guidance
   3. Consideration of how plants/plant communities respond to facility maintenance activities and associated disturbance
   4. Design to simplify maintenance needs and frequency
   5. Clear identification of parties responsible for maintenance/upkeep
   6. Recognition of facilities as organisms themselves. 
   7. Emphasis on true adaptive management/maintenance (e.g., planned future plantings/replantings as facility conditions and plant communities change/evolve)

The intent of the outline and the BPST is not to serve as the basis for a deterministic, prescriptive plant selector. The vision of the BPST is of a dynamic tool with built-in feedback loops (describing site conditions and desired planting functions) will help define plant typologies which can then be matched up with locally available plant species. 

Next Steps 

The work summarized above is just the first phase of what the project team hopes will be a multi-phased effort to develop better tools to guide plant selection in bioretention – focusing on plants that can optimize facility performance, beautify neighborhoods, and therefore help build community.  The project team is actively looking at what the second phase looks like and how we might be able to partner with other practitioners, educators, and researchers to collectively move this effort forward. 

When plantings succeed, there is generally a good likelihood that facilities are functioning as designed and that they are aesthetically acceptable and regarded as assets in the neighborhoods where they reside. When they fail, there is a perception that the facilities they are installed in are also failing, and an expectation and reality of the high costs associated with replantings. In many cases planting success, from the public’s standpoint, defines GSI facility success. 

Project Team and Authors:

Jeremy Person – Landscape Architect, City of Portland, Oregon

Ann English – RainScapes Manager, Montgomery County, Maryland

Ted Shriro – Stormwater Services Technical Assistant, City of Eugene, Oregon

Andy Szatko – Stormwater & Green Infrastructure Coordinator, City of Omaha, Nebraska

John Watson – Engineer, Cook County Forest Preserve District, Illinois

Jim Cooper – Senior Landscape Architect, Biohabitats, Baltimore, Maryland

The State of Public Sector GSI, 2022 is a research report created to establish a baseline to better understand the state of local public sector green infrastructure implementation as well as create a shared blueprint for how we go forward together. The report offers an unprecedented look at implementation, best practices, investment trends and drivers for increasing the use of GSI in stormwater management.

Several media outlets have covered the report’s release. Executive Director, Barbara Hopkins, was interviewed by  Katie Johns, Editor of Stormwater Magazine, in April. You can find that interview in the Stormwater Solutions video library.

Two great articles, one from Governing titled Cities Are Learning to Manage Urban Stormwater the Way Nature Would and the other from Stormwater Report titled Public-Sector Green Infrastructure on the Rise, New Report Finds speak to the rise in green infrastructure adoption and how the recently released The State of Public Sector GSI report can help facilitate this upward swing in GI interest and implementation. “Investment in GSI has been growing, and projections for the next five years show even greater growth,” says Barbara Hopkins, Executive Director of Green Infrastructure Leadership Exchange.

The Governing article highlights SPPC members, Kate England, Green Infrastructure Director for the city of Boston and Torrey Lindbo, Water Resources Science and Policy Manager for the City of Gresham, Oregon.

Torrey discusses GI implementation such as streetside planters, stormwater tree wells, bioswales, rain gardens and pond facilities in Oregon and how it can prevent pre-spawn mortality in Coho salmon. Green infrastructure can reduce runoff from urban areas by mimicking natural processes, but flooding is also a natural process. Development standards also need to take the functions of streams and floodplains into account, says Lindbo. “We need to stay out of spots that we shouldn’t be in.”

Kate England notes that, “a comprehensive citywide plan will only be successful if every agency prioritizes the use of green infrastructure in every investment.” Kate is working across city departments such as public works and transportation, city’s housing and school and public facilities. As the Director of GI she requires every transportation or public works project that changes curb geometry to choose from one of five green infrastructure design alternatives.

Hopkins discusses our Learning Circles: “Peer-to-peer learning is open to any public-sector practitioner who wants to participate. Six learning circles meet monthly, and grants are available for them to work with consultants to work out approaches to especially thorny problems. Through peer learning, we’re trying to come up with innovations in GSI practice that will advance the whole field,” says Hopkins. “It’s a very collegial network of people.”

The Exchange is very happy to announce the hiring of three new staff members, Tashalee Cruz (Program Manager), Yanina Salerno (Office Manager), and Veronica Hotton (Education Manager)

Tashalee (“Tasha”) Cruz, Program Manager

Hello, Members and Friends of the GI Leadership Exchange,

I am elated to join the GI Leadership Exchange team as the Program Manager. The GI Leadership Exchange mission speaks to several of my passions – community, improving livability in disadvantaged communities, and equity.  It is an honor to be able to dedicate my time and efforts to such a unique organization.

Originally from the South Bronx, I graduated from college with a Bachelor’s degree in Sociology and an Associate’s degree in Baking & Pastry Arts. I now live in Hartford, Connecticut with my two fur babies: two 1-year old cats that I rescued.

Over the course of my 16-year career, I have learned the ins and outs of running an office in different roles. I am the founder of Velia’s Virtual Solutions, a Virtual Assistant Agency that empowers businesses and teams by streamlining processes and establishing company standards. With a solid background in running a successful business, I have honed my skills in analyzing and creating processes to improve efficiency. As Program Manager, I will continue to leverage my knowledge and leadership skills to drive positive change for the GI Leadership Exchange in the membership space as well as planning and running events. Please reach out to me at tashalee@giexchange.org.

Yanina (“Nina”) Salerno, Office Manager

Dear Members and friends of the Exchange,

I am honored and delighted to join the Exchange team as the Office Manager. The mission and vision of the Exchange is deeply aligned with one of my passions which lies at the intersection of nature and environmental justice for all (including other-than-human persons).

Originally from Montevideo, Uruguay, I now spend my time in Asheville, North Carolina sharing my knowledge as a Certified North Carolina Environmental Educator, Herbalist and Naturalist. In 2019 I founded Activated Earth, a Traditional Healing Space that offers environmental and cultural education, earth-honoring ceremony and ritual, and ancestral folk medicine consultations and services. I also enjoy creating Medicine Wheel Pollinator Gardens to restore native plant species as well as traditional ecological knowledge and practices.

I have been working and volunteering with environmental nonprofit organizations for the past eight years and find that being in community in this way is deeply meaningful and fulfilling. I hope that I can be of service to the Exchange team in a way that is mutually reciprocal, valuable, and enjoyable. I hope to meet you in person sometime soon but in the meantime, please feel free to email me at yanina@giexchange.org

Veronica Hotton, Education Manager

Last, but not least, we welcome Veronica Hotton, Education Manager. Since Veronica starts this Friday, there wasn’t sufficient time before publication of our newsletter to ask her to draft her own introduction. But we thought it would be important for you to know something about her, as you will be seeing her in attendance at virtual events beginning next week.

Veronica holds a Ph.D in Education from Simon Fraser University and an MA in Geography from the University of Hawaii, among other degrees. Her cover letter speaks to why she is a good match for the position:

I have been passionate about the intersection of environment, people, and professional development since I was a young adult. One of the times the link between the environment and people became clear to me was while I was a graduate student at the University of Hawai‘i at Mānoa studying environmental geography. Before moving to Hawai’i, I read From a Native Daughter by Haunani-Kay Trask and her words were always with me–I was a guest of the islands. In Honolulu, I studied local stream-bed sediment contamination in highly channelized rivers. Since then and for over a decade, I channeled my passion into project management, organizing, and education for nonprofits, professional associations, higher education, and unions. After several years teaching in higher education, I transitioned into environmental education nonprofit work, and I am excited to continue in a nonprofit Education Manager role that can combine my passion for environment and people that supports a “peer learning network” that can “share experiences, circulate ideas, and solve problems together toward [a] more sustainable water infrastructure.”

Veronica was previously the Program and Learning Manager for Ecochallenge.org, a nonprofit that focuses on sustainability and climate change education for employee engagement and post-secondary education audiences through peer discussion learning approaches. Veronica lives in Portland, Oregon and previously taught at Portland State University, developing courses in interdisciplinary social justice, natural sciences, and resource management. Please reach out to welcome Veronica at veronica@giexchange.org.

On March 15th Exchange members attended the third of four members-only community engagement workshops. Facilitated by Sam Novak, Co-founding Director of Loam, the workshop set out to teach participants about facilitation mindsets, techniques, and skills that engage diverse communities, animate equitable collaboration, and inspire collective dreaming, decision-making, and action.

Facilitation for social change requires a different way of being. In a recent conversation, participants highlighted the need to reflect on facilitation techniques and explore the intersections of facilitation, social justice, and systems thinking. The meeting identified anti-oppressive facilitation as a key approach to facilitating social change, consisting of five characteristics: relational, adaptive, emergent, fractal, and complex. The importance of humility was also emphasized, as it allows everyone to feel heard and promotes deep collaboration and engagement.

Creating a safe space where everyone can voice their opinions and be heard is crucial for success in community meetings. When people feel understood and heard, they are more likely to participate in promoting and practicing social change. The meeting also stressed the importance of setting boundaries and expectations early on to ensure a productive and safe meeting space. As Atlanta member Amanda Hallauer noted, “If you set the expectation early on, it sets the tone.” Overall, the discussion underscored the need for a different approach to facilitation and the critical role it plays in driving social change efforts.

A number of exercises were shared for continued exploration into the world of facilitation for social change through the lens of anti-oppression and systems thinking. An additional resource, the Anti-Oppression Framework (chart), can be found here.

You can view the full slide deck from the Community Engagement Workshop: Facilitation for Social Change here.

This workshop was built on the foundations and frameworks of anti-oppression, systems thinking, and emergent strategy. It includes two free resources for continued, self-guided reflection that explore inspiring concepts, make deeper meaning, and find a sense of solidarity in the joys and challenges of facilitating social change.

This work would not be possible without the generous support of the Kresge Foundation that has enabled us to deepen our commitment to building the leadership skills of current and future local government leaders. If you’re an Exchange member, please be sure that you’re subscribed to our newsletter so that you don’t miss out on future workshops. To sign up, use the form embedded in the footer of any page on our website.