The role of plants in bioretention systems; does the science underpin current guidance?

This paper examines the scientific evidence underpinning four hypotheses about the role of plants in bioretention systems: (i) Planted systems are more effective than unplanted systems, (ii) Plant species differ in their effectiveness, (iii) Native species are more effective than exotic ones, (iv) Diverse systems are more efficient than monocultures. [Display omitted] •Manuals are often based on hypotheses on plant role in bioretention, not evidences.•Plants have a positive effect on permeability and nitrogen removal.•Right plant species is crucial for nitrogen not TSS, phosphorus, metals removal.•Native plants may help biodiversity, but not hydrologic or treatment performances.•Diverse plantings can be more resilient, but will not necessarily perform better. Plants are essential components of bioretention systems, with bioretention design-guides around the world providing extensive advice on the role of selection of plants to maximize system performance and sustainability. Four principal hypotheses regarding the role of plants have been identified in bioretention design manuals: (i) Planted systems are more effective than unplanted systems, (ii) Plant species differ in their effectiveness, (iii) Native species are more effective than exotic ones, (iv) Diverse systems are more efficient than monocultures. This paper examines the extent to which these hypotheses are supported by the scientific literature. Comparison of planted and unplanted systems show that increased permeability and hydraulic conductivity, as well as removal of nitrogen, are the main benefits of the presence of plants in bioretention. Knowledge on their positive effect on hydrocarbons remains fragmented, although there is evidence from phytoremediation studies in other plant-based technologies. Choosing the right species makes a difference in hydraulic performance and nitrogen removal, with root traits being identified as important predictors of performance. No scientific results can support the hypothesis that native plants or diversely-planted systems offer better performance than systems planted with fewer species or with exotic species. Questions remain regarding the plant-microbe interaction in the bioretention context, the role of biomacropores in pollutant migration or the differential impact of plant choice on performance.