Nutrient Uptake and Plant Selection in Southeastern Rain Gardens
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A bioretention rain garden is an innovative stormwater management practice that integrates stormwater infiltration and storage to improve the quality of runoff. Part1. Two bioretention rain garden designs constructed at the Donald E. Davis Arboretum on the Auburn University campus were compared, one using conventional (CONV) treatment through filtration and adsorption, and the other incorporating an internal water storage (IWS) layer. Contaminant removal from stormwater was observed in warm and cool seasons. The CONV rain garden (RG) provided improved removal of nutrients when compared to a rain garden with an IWS layer. Even when rain gardens included evergreen and perennial plants, a total net loss (release) of nutrients occurred during perennial die back during the cool season. Warm season plant uptake of nutrients was greater than cool season plant uptake in both rain gardens, indicating that evergreen species used in place of perennial plants may provide increased winter uptake. Part 2. Because rain garden plants must be able to tolerate repeated short intervals of flooding, research was conducted to screen five native landscape shrub taxa for tolerance to repeated flooding events. Fothergilla ×intermedia L. ‘Mt. Airy’ (dwarf witchalder), Ilex verticillata (L.) A. Gray ‘Winter Red’ (winterberry), Clethra alnifolia L. ‘Ruby Spice’ (summersweet), Callicarpa dichotoma (Lour.) K. Koch ‘Early Amethyst’ (purple beautyberry), and Viburnum nudum L. Brandywine™ (possumhaw) were flooded in 11.3 L (3 gal) containers in a greenhouse. Root zones of plants were flooded to the substrate surface for 0 (non-flooded), 3, or 6 days followed by 6 days of draining with no additional irrigation. Run 1. Flooding duration did not affect growth index (GI), shoot dry weight (SDW), or root dry weight (RDW) of I. verticillata ‘Winter Red’ and V. nudum Brandywine™. Growth index was lower in plants flooded for 6 days for C. alnifolia ‘Ruby Spice’, but flooding duration did not affect SDW or RDW. Growth index and RDW of C. dichotoma ‘Early Amethyst’ were not affected by flooding treatments, however SDW decreased. Growth index of F. ×intermedia ‘Mt. Airy’ decreased with increasing flood duration, and SDW and RDW were lower in plants flooded for 6 days. Generally, the lowest values for photosynthesis (Pn) and stomatal conductance (SC) occurred in plants flooding or draining for 6 days. Photosynthesis and SC were lower at 5th flood cycle of the experiment. Stem water potential (SWP) was higher in plants at 5th flood cycle of the experiment. Run 2. Growth index and RDW of F. ×intermedia ‘Mt. Airy’ were lowest in plants flooded for 6 days. The SDW of I. verticillata ‘Winter Red’ was highest in plants flooded for 6 days and in V. nudum Brandywine™ the GI was highest in plants flooded for 6 days. Overall, in both runs, all taxa, with the exception of F. ×intermedia ‘Mt. Airy’, maintained good visual quality, did not have any reduction in RDW, and exhibited minimal effects of flooding on shoot growth. F. ×intermedia ‘Mt. Airy’ exhibited poor visual quality, with growth adversely affected by flooding. Conversely, all other taxa appeared tolerant of flooding and would be appropriate native shrub selections for rain gardens. Part 3. Rain gardens are recommended as a way to remove pollutants from runoff. A pollutant of particular concern for waterways is phosphorus. Research was conducted to evaluate phosphorus (P) uptake by and growth of the native grass Muhlenbergia capillaris in flooded and non-flooded conditions. Muhlenbergia capillaris (Lam.) Trin. (gulf muhly grass) was placed into 3.8 L (1 gal) containers and root zones were flooded for 0 or 3 days and drained for 6 days. Run 1. Height (HT), SDW, and RDW were higher in non-flooded plants than flooded plants. Shoot dry weight increased linearly with an increasing P irrigation rate, while RDW changed cubically with increasing P irrigation rates. Phosphorus concentration in leachate increased linearly with an increasing P irrigation rate in non-flooded plants at the 1st flood cycle of the experiment. At the 5th flood cycle of the experiment, phosphorus concentration in the leachate of non-flooded plants increased linearly with increasing P irrigation rate and in flooded plants the phosphorus concentration in leachate changed quadratically with increasing P irrigation rates. Run 2. Plant HT, SDW, and RDW were higher in non-flooded plants than flooded plants, and plants with phosphorus fertilizer added to substrate had higher HT, SDW, and RDW than plants without phosphorus fertilizer added to substrate. Phosphorus concentration in leachate was highest in non-flooded or flooded plants with phosphorus fertilizer added to the substrate at the 1st, 3rd, or 5th flood cycles of the experiment. All plants exhibited growth during flooding and appeared tolerant of flooding and would be appropriate native shrub selections for rain gardens.