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Table 1 Weighted criteria for selecting test plant species based on suitability in field-scale biofilters and known antimicrobial production

From: Rise of the killer plants: investigating the antimicrobial activity of Australian plants to enhance biofilter-mediated pathogen removal

Selection criteria Description Reference Weighting of importance
Primary considerations for species selection
Availability in nurseries Available from ≥1 of 9 major native nurseries in Melbourne. [29]
Accessible to stakeholders for purchase
Adaptation to biofilter conditions Species were scored (1–3) based on their ability to maintain healthy growth under south-eastern Australian biofilter conditions (i.e. survival in sandy soil with temporary inundation and extended hot, dry periods). Plants with scores of ≥2 were considered for selection. [11, 68]
Plants must be adapted to harsh conditions in biofilters for effective performance
Antimicrobial activity of plants Species were assigned an antimicrobial score based on the number of positive Google Scholar search results associating plant genus with antimicrobial-associated terms*. Species with an antimicrobial score of > 30 and < 1 were considered for selection. [43]
Parameter essential to answer key objectives of study
Extensive root system Species were scored (1, 2 or 3) based on root structure characteristics, with 3 representing “very good” (deep, dense, extensive, fine roots), 2 representing “average” and 3 representing “poor” roots (shallow, thick, minimal root systems). Candidates with scores ≥2 were considered for selection. [22, 29, 68]
Extensive root systems correspond with high pollutant and faecal microorganism removal
Invasive species Species deemed to have a high risk of becoming invasive, even if native to Australia, were excluded from selection. [29]
Necessary to avoid ecological damage to surrounding ecosystems in field applications
Plant size Species typically growing ≤10 m in height and ≥ 1 m in canopy diameter (sedges excepted) were considered for selection. Tall trees are generally impractical or unpopular in streetscape biofilters, while slender shrubs/trees with sparse above-ground biomass have diminished treatment capacity [71] due to their generally lower litterfall and weaker root systems [68]. [29, 68]
Size constraints necessary for successful field application
Secondary considerations for species selection
Woody plants (shrubs and trees) Woody species were preferentially selected. Compared with herbaceous species, woody species tend to live longer, root more extensively, grow taller and produce more biomass and leaf litter for improved treatment capacity [72, 73]. [68, 72, 73]
Woody plants have associations with multiple criteria that improve suitability and performance in biofilters
Indigenous to Melbourne Species indigenous to Melbourne were preferentially selected over other Australian natives. Indigenous species are likely to have superior survival rates and provide greater ecological benefits over non-native species. Indigenous plants are also less likely to become invasive or cause environmental harm. [29, 68]
Plants indigenous to Melbourne are preferred, although other Australian natives are suitable depending on biofilter location and treatment context
High past success in biofilters Species with high past performance in biofilters were preferentially selected. [11, 29, 68]
Multiple pollutant removal for enhanced field application performance
High growth rate Species with high growth rates were preferentially selected due to associations with improved nutrient removal. [29, 68]
Multiple pollutant removal for enhanced field application performance
Nitrogen fixation Species lacking nitrogen-fixing root systems were preferentially selected to avoid compromised nitrogen removal. [29, 70]
Multiple pollutant removal for enhanced field application performance
Lifespan Plants with lifespans > 20 years were preferentially selected over shorter-lived species requiring frequent replacement. [74]
Reduced maintenance costs and disturbance to biofilter function
  1. The weighted/relative importance of each plant selection criterion (right-most column) was denoted by a number of ticks (), with five ticks indicating “very high importance”, four ticks “high importance”, three ticks “moderate importance” and two ticks “relatively low importance”
  2. *Details on antimicrobial score assignment are outlined in Table S.1 description. Secondary metabolite publications were incorporated in the overall antimicrobial score for each plant species, owing to secondary metabolite production providing an indication of antimicrobial activity in plants where antimicrobial testing has not yet been conducted [75]. A Spearman correlation rank of 0.86 (p < 0.0001) indicates there is a strong correlation between “Antimicrobial” and “Secondary metabolite” Google Scholar publication count for each filtered species (as determined by GraphPad Prism version 7, GraphPad Software, USA)