PS 48-249 - Compost addition greatly increases the number of flowers and pollinators in suburban native plant landscapes

Abstract: Pollinator decline is an important concern globally due to the role of pollinators in crop production and plant reproductive success. Urbanization can harm pollinators due to habitat loss and introduction of non-native plants, but urban areas can also support pollinators by providing diverse floral resources, habitats, and nesting sites. There is increasing interest in using native plants to enhance pollinator habitat in urban landscapes. However, there is a lack of information about how different urban growing conditions affect native plant floral resources and associated pollinators. In this study we examined the influence of two key aspects of sustainable landscape design, soil fertility and irrigation, on flower production and pollinators in native landscape plantings. The study was located at the Sunbridge development near Orlando, Florida. Sixteen 55.5 m² plots were established January 2022 in a randomized complete block design consisting of four blocks of four plots each. The treatments included two compost treatments (compost incorporated before planting at 33 L m^-2 or no compost added), and two irrigation treatments (weekly irrigation versus irrigation only during the first 6 months) applied in all combinations. Each plot was planted with the same number of 26 different native plant species being tested for use in residential landscapes. Flower production and pollinator diversity and abundance in the plots were assessed manually in June and September 2022.



Compost increased both flower and pollinator abundance, but irrigation had no significant effects. Floral abundance was more than two times greater in plots with compost than in plots without compost in June (57.6±11.6 vs 25.1±9.2 flowers m^-2, P< 0.001) and more than four times greater in September (66.5±19.7 vs 14.5±3.1 flowers m^-2, P< 0.001). Likewise, pollinators were more abundant in plots with compost than in those without in both June (17.4±5.9 vs 8.75±2.2 individuals per plot) and September (26.7±8.4 vs 16.1±9.1 individuals per plot). The top pollinator plant in June was Eryngium yuccafolium and in fall was Monarda punctata. Pollinator species composition varied among seasons with 43 total species in June and 47 in September. The most abundant pollinators included bees (Bombus pensylvanicus, Apis mellifera, Agapostemon splendens), flower wasps (Scolia nobilitata, Pygodasis quadrimaculata), and a flower beetle (Trigonopeltastes delta). Results show the benefit of soil compost amendment to growth and reproduction of native plants and associated pollinators in urban landscapes. The lack of an irrigation affect may reflect the lack of water stress during the study period.