Green Roofs Affect the Floral Abundance and Phenology of Four Flowering Plant Species in the Western United States

Abstract

This study investigates the potential for green roofs to support pollinator diversity and abundance in urban ecosystems through the altered floral phenology and floral abundance of plants. Floral phenology and the floral abundance of green roof plants are compared to plants grown at grade on the Front Range in Fort Collins, Colorado, and how these changes may affect pollinator biodiversity in urban ecosystems. An independent block design is employed, within one green roof and one ground-level garden, approximately 120 m apart, with replicate plants of 4 species in each garden. Pollinator observations were made weekly during the bloom period for each species. Blue vane traps were used to passively measure pollinator diversity along a transect between the green roof sites and the sites at grade. The total number of flowers per plant is variable between site types, depending on the plant species. However, all species of plants tested bloomed earlier when grown on the green roof than when grown at grade. Pollinator abundance and diversity were low at both site locations. Green roofs may provide foraging opportunities earlier in the season in temperate regions, which can extend the duration of floral foraging opportunities when supported by green infrastructure at grade.