Municipalities worldwide are showing substantial interest in urban greening, defined here as the introduction or conservation of flora in cities. Encompassing innovative policies, designs, and initiatives that are vegetating the urban landscape, this bloom of activity may be unlike anything since street trees and large parks transformed the fabric of cities in the 19th century. Yet, there has been little scholarship on the historical and contemporary contours of these practices, which are emerging amidst two important phenomena: global urbanization; and increasing awareness of human-induced alteration of the biosphere, described here as the Anthropocene Awakening. This dissertation strives to make meaning of urban greening at this significant inflection point through a chronicle of trees in U.S. cities, assessment of city planning scholarship, review of scientific literature addressing human health benefits of urban vegetation, and a survey of municipal tree planting practitioners. Longitudinal study reveals that the rationale for urban trees has recently shifted from civic improvement and beautification to ecosystem services. Research on urban ecosystem services is an open frontier; and there is a pressing need for a definition and conceptual framework that reflects municipal greening practice. Human health is a central aspect of ecosystem services, and scientific literature reveals a psychosocial orientation to the human health benefits of urban flora. This suggests that cultural ecosystem services are especially important in urban settings; and that research and practice should address the socioecological dimensions of flora in cities. This represents an opportunity for urban planners and designers, despite a lack of attention to greenery in city planning scholarship. Literature also suggests that urban trees may, depending on many factors, be a minor component in mitigating local and global air pollution; and arguments based on this rationale may divert attention from the problem – fossil fuel emissions. A survey of municipal tree planting programs and practitioners supports this reasoning. Findings also suggest a planning and design norm described as proximal greening for multifunctional urban landscapes. Finally, as municipal leaders and residents grapple with the profound implications of the Anthropocene and seek to enhance the livability and sustainability of cities, urban greening may contribute more to the former than the latter.