Sky gardens: The potential of urban balconies as sites for nanoscale nature-based solutions

Space for greening in cities is limited. Although individually small, residential gardens bring influential aggregate effects, especially considering their additional wellbeing outcomes. Balconies represent one such type of residential space, but despite their growing prevalence in compact urban cores, there has been little empirical investigation of their potential as sites for nature-based interventions at a very small scale – the nano-scale. In this study, we quantify the amount of balcony space across England and Wales and use a detailed urban case (Greater Manchester, UK) to determine greening extents and to assess local air quality, temperature, and humidity impacts. For Greater Manchester we estimate that there are around 57,000 balconies equivalent to 23.5 ha. ENVI-Met modelling and empirical monitoring of eight balcony pairs (one planted i.e. green, and one bare) revealed that green balconies were on average 0.5 o C cooler and 2% more humid than adjacent bare balconies with lower particulate air pollution concentrations occurring 98% of the time (mean difference -4.75 μg/m 3 ) on green balconies. We also observed 152 different plant species from 132 genera across 60 sampled balconies, with implications for biodiversity and pollinator support. Nevertheless, only 5% of observed balconies contained plants and only 25% of the area of greened balconies were planted. Across England and Wales, we estimate that there are 432,424 (±152,502) balconies with a total area of 177.3 ha. Even recognising physical restrictions in some balcony types, there is considerable opportunity for increasing residential participation in balcony greening by providing the skills, knowledge, and motivation to create vertical green gardens.