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

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