Flooding Tolerance and Phosphorus Uptake of Southeastern Native Plants in Bioretention Gardens

Abstract

Bioretention gardens restore hydrologic function of urban landscapes and capture stormwater runoff pollutants, such as phosphorus, a main pollutant in urban cities and residential neighborhoods. Monoculture plantings are common in bioretention gardens; however, polyculture plantings can improve biodiversity and ecosystem resilience. Thus, objectives of this study were to evaluate phosphorus removal from simulated stormwater in two bioretention substrates, and evaluate four landscape plant species, alone and in monoculture and polyculture plantings, for phosphorus uptake and tolerance of bioretention garden conditions. Part 1: Liners of Ilex vomitoria Ait. ‘Schillings dwarf’, Andropogon tenarius Michx., Echinacea purpurea L. Moench. ‘Magnus Superior’, and Coreopsis verticillata L.‘Zagreb’ were planted in containers in either 85:15 sand:organic matter or 50:50 sand:organic matter substrates. Plants were irrigated or flooded with 0.0, 0.4, 0.8, or 1.6 mg∙L-1 P solutions of phosphorus. Part 2: Four planting combinations, C. verticillata ‘Zagreb’ monoculture, A. ternarius monoculture, I. vomitoria ‘Schilling’s Dwarf’ monoculture, and a polyculture of C. verticillata ‘Zagreb’, A. ternarius, and I. vomitoria ‘Schilling’s Dwarf’ were planted into 94 L nursery containers in a substrate of a 50:50 sand : organic matter. Containers were irrigated or flooded with 1.6 mg∙L-1 P solution. Overall, plant growth across species was lower in flood treatments, and higher in organic than sand substrate. Leachate and substrate P was higher in flood treatments and substrate P was higher in sand, however, plant tissue P in all treatments was higher than either leachate or substrate P. Polyculture plantings had the lowest leachate P, suggesting a polyculture planting may be more effective in preventing excess P from entering waterways from bioretention gardens. In the simulated gardens, two species had higher growth in the fall and another in the spring, suggesting that with a monoculture, there may be vegetation gaps across seasons. Thus, while all species tested (excluding E. purpurea) were tolerant of bioretention conditions, growth was higher in organic substrates and non-flood conditions, and P was highest in plant tissue.