Relationships between socio-demographic / socio-economic characteristics and neighborhood green space in four Nordic municipalities – results from NORDGREEN

Urban green space governs important ecosystem services including providing health promoting facilities. However, unequal distribution of green space among socio-demographic and socio-economic groups may create environmental injustice among inhabitants and between neighborhoods. This paper reports on access to different types of green space for residents in four Nordic cities. More precisely it explores relationships between socio-demographic and socio-economic characteristics and access to green space at the neighbourhood level. We selected the following green space variables: distance between inhabitants’ dwellings and parks, forest, and cemeteries; percentages and area per inhabitant of parks, forest, and cemeteries; and vegetation cover derived from satellite images and Urban Atlas. These were linked to income, education, age, and percentage of the population born outside their country of residence at a neighbourhood level. The results show that inhabitants in the four Nordic cities have great access to green space, but the percentage of different types of green space, such as forest and parks, varies between the cities and within the cities. We found that most inhabitants have less than 300 m to the nearest green space ranging from 92% in Stavanger (Norway) to 99.2% in Täby (Sweden). We also found differences in access depending on inhabitants' age, education, immigration status, and median income. Espoo (Finland) had the highest total green space coverage (45.3%), while Täby had the highest proportion of park coverage (2.3%). By compensating with green spaces in less affluent areas, and focusing on equal access to green space, planners can contribute to reduce health inequalities in the population.     

Global patterns in urban green space are strongly linked to human development and population density

Urban green space is important for alleviating high temperatures, pollution, and flooding in cities. Furthermore, it is becoming increasingly clear that urban green space is important for the mental and physical health of humans residing in cities and that urban green space may harbor unique biodiversity. Understanding the extent and drivers of urban green space is thus important. While urban green space has been mapped and studied at local to national scales, the global patterns and drivers of urban green space remain unknown, potentially hampering effective planning and allocation of resources toward reaching sustainable development goals. Here, we quantified the effect of environmental and socio-economic drivers (temperature, precipitation, human development, and population density) on urban green space globally by focusing on national capital cities. We used satellite imagery to map urban green space using two measures: the Normalized Difference Vegetation Index (NDVI), and the fractional cover of “green” land cover classes. NDVI is useful as it includes all vegetated surfaces, also small ones like gardens. However, land cover classes allow the exclusion of certain classes such as sports fields or cropland. We used boosted regression trees to show that climatic variables accounted for 75 % of the relative influence in urban green space, with a positive effect of precipitation and a negative effect of temperature. Importantly, socioeconomic variables accounted for 25 % of the influence on global urban green space, with a positive effect of human development index (HDI) and a negative effect of population density. HDI in relation to urban green space has not previously been tested globally, and our study shows that significantly affects urban greenspace. The results demonstrate that cities where development status is low and population densities are high, typically in the Global South, have less urban green space than the climate would predict. The results therefore suggest that human wellbeing does not only benefit directly from increasing human development and decreasing population densities in urban areas, but that these effects may be compounded by also improving nature’s contribution to people.     

Environmental inequities in terms of different types of urban greenery in Hartford,Connecticut

Urban greenery has long been recognized as an important component of urban ecosystem and provides many benefits to urban residents. However, different types of urban greenery provide different kinds of natural experiences to people. In this study, green metrics calculated based on multisource spatial datasets were used to quantify the spatial distribution of different types of urban greenery in Hartford, Connecticut. Geo-tagged Google Street View images, which capture the profile view of cityscape, were used to quantify street greenery by considering the time information. Land cover map and urban parks map were used to measure residential yard greenery and proximity to urban parks, respectively. We analyzed the associations of the calculated green metrics with socio-economic variables derived from census data. Statistical results show that: (1) people with higher income tend to live in neighborhoods with more street greenery; (2) census block groups with a higher proportion of owner-occupied units tend to have more yard vegetation and yard tree/shrub coverage; (3) Hispanics tend to live in block groups that have less yard vegetation but African Americans mostly live in block groups with more yard greenery; and (4) there are no significant environmental disparities among racial/ethnic groups in terms of proximity to urban parks. In general, this study provides an insight into the environments of urban residents in terms of urban greenery, and a valuable reference data for urban planning.     

Local habitat characteristics have a stronger effect than the surrounding urban landscape on beetle communities on green roofs

Green roofs are a promising tool to return nature to cities and mitigate biodiversity loss brought about by urbanization. Yet, we lack basic information on how green roofs contribute to biodiversity and how their placement in the urban landscape affects different taxa and community composition. We studied the effects of local and landscape variables on beetle communities on green roofs. We expected that both local roof characteristics and urban landscape composition shape communities, but that their relative importance depends on species characteristics. Using pitfall traps, we collected beetles during two consecutive years from 17 green roofs in Basel, Switzerland. We evaluated the contribution of six local and six landscape variables to beetle community structure and to the responses of individual species. Communities on the roofs consisted of mobile and open dry-habitat species, with both local and landscape variables playing a role in structuring these communities. At the individual species level, local roof variables were more important than characteristics of the surrounding urban landscape. The most influential factors affecting the abundances of beetle species were vegetation, described as forb and grass cover (mainly positive), and roof age (mainly negative). Therefore, we suggest that the careful planning of green roofs with diverse vegetation is essential to increase their value as habitat for beetles. In addition, while beetle communities on green roofs can be diverse regardless of their placement in the urban landscape, the lack of wingless species indicates the need to increase the connectivity of green roofs to ground level habitats.     

Economic development and natural amenity: An econometric analysis of urban green spaces in China

Rapid urbanization and spontaneous economic development has brought about profound changes in urban landscapes throughout the world. The task of managing transforming urban landscapes, particularly urban green spaces, so as to provide sufficient natural amenity for increasing urban populations, is one of the critical challenges facing policy makers. However, little empirical evidence exists about the evolving path of urban green spaces along with economic development and urbanization. This study attempts to fill in this knowledge gap through an econometric analysis of panel data across 285 Chinese cities during a period of rapid urbanization and economic growth (2001–2010). The results point to the existence of an N-shaped environmental Kuznets curve (EKC) for an important aspect of environmental quality: urban green spaces. Urban green space coverage increases at the initial stage of economic development, and then it starts to decrease as GDP per capita exceeds RMB50,855 and then increases again at a high GDP per capita level (RMB107,558). Large elasticity (>1) is expected as GDP per capita grows to a higher level (beyond RMB128,095). By the end of 2010, 30% of Chinese cities are still located on the downward-sloping path and only four cities have attained elasticities greater than one. The findings present a challenging and pressing call for policy makers to effectively manage the tradeoffs between continuous economic development and better natural amenities.     

The value of vegetation cover for ecosystem services in the suburban context

Latin-American cities can be characterized by dynamic processes of urbanization that encroach upon the natural and semi-natural surrounding landscapes. Our study presents the effects of landscape development, transformed from semi-natural conditions into a mostly disperse suburban settlement. We explore the impact that this transformation has had on this context by three ecosystem services that regulate rainwater runoff, enhance microclimate conditions and help to improve air quality by monitoring vegetation cover. We have designed a spatio-temporal hierarchical analysis which employs remote sensing techniques to capture the structural changes of this landscape over long, medium and short term scales on two spatial levels. This methodological approach was tested in the Metropolitan Area of Santiago (MAS) as case study area. Despite of the increase in impervious surfaces due to urban processes, there has also been an increase in vegetation cover, which has led to an improvement in the provision of the above-mentioned ecosystem services. Hence, if diverse urbanization processes continue and they are coupled with an increase in vegetation cover, the provision of ecosystem services could also expand. This phenomenon can be observed in some areas, where public and private green spaces are created and maintained. Our data analyses give evidence that certain types of suburban areas which increase the share of vegetation cover can provide daily ecological benefits for urban neighborhoods, and beyond, for adjacent areas. Moreover, suburban development can successfully provide ecological benefits to citizens. Such processes can only be ecologically sustainable if the composition of vegetation is well-adapted to the regional climatic conditions.     

The inequity of distribution of urban forest and ecosystem services in Cali,Colombia

Little is known about urban forest planning, management and its benefits in emerging countries. The uneven distribution of tree canopy cover and parks in urban area is related to environmental justice, especially with disadvantaged socio-economic and marginated communities. However, the inequity of urban forest in many cities of emerging countries where often found irregular and unregulated land use patterns and social and socio-economic inequities, is hardly highlighted. This study explores the inequity of distribution of tree canopy cover and public park in Cali, Colombia. Utilizing the traditional socio-economic indices, the stratification, linear regression analysis is conducted to describe relationship between total tree canopy cover, tree canopy cover of various land use types, number of parks and park area per capita. The result demonstrates that lower income communities have lower tree canopy cover, fewer parks and smaller park area than higher income communities. This paper discusses importance of accounting for urban forests and ecosystem service in city planning efforts and better strategies of reducing inequity in emerging countries. Addressing the inequity of urban forest could be a better strategy to create resilient, sustainable, safe and livable cities in emerging countries.     

Too hot to handle? On the cooling capacity of urban green spaces in a Neotropical Mexican city

Urban areas are particularly vulnerable to climate change due to the Urban Heat Island (UHI) effect, which can be mitigated by urban vegetation through shading and evapotranspiration. Nevertheless, there is still a lack of spatially explicit information on the cooling capacity of green infrastructure for most Latin American cities. In this study, we employed Land Surface Temperature (LST) of the Neotropical Mexican city of Xalapa to (1) analyze its Surface UHI (SUHI) compared to its peri and extra-urban areas, (2) to assess the cooling capacity of urban green spaces larger than 1 ha, and (3) to evaluate the role of green spaces’ size, shape and their surrounding tree cover percentage (Tc) on green spaces cooling range. We evaluated the cooling range of green spaces and their relationships with green spaces metrics and Tc via a linear mixed-effect model and identified threshold values for the variables at 25 m, 50 m, 100 m, and 200 m from the borders of green spaces through Classification and Regression Trees. Xalapa exhibits a SUHI of 1.70 °C compared to its peri-urban area and 4.95 °C to the extra-urban area. Green spaces > 2 ha mitigated heat at ~2 °C and the cooling range was influenced by the size of green spaces ≥ 2.8 ha and Tc > 21% at 50 m and only by Tc surrounding the green spaces at 100 m and 200 m. This shows that the size threshold of urban green spaces should be complemented with the presence of Tc starting at least 50 m to maximize the cooling capacity provided by the green infrastructure. Planning agendas should account for the interaction between the size of green spaces and the cumulative cooling effect of scattered vegetation inside urban areas towards compact green cities to cope with urban warming.     

Predictors of the distribution of street and backyard vegetation in Montreal,Canada

Urban vegetation is shown to be unevenly distributed across cities and there is evidence of disparities in benefits provided by vegetation and of public health problems induced by urban heat islands. In order to improve vegetation cover, it remains crucial to understand the underpinning of such unevenness. In this paper, we investigate in Montreal (Canada) how the built environment, sociodemographic factors and administrative boroughs influence tree and lawn cover in public and residential land. The analysis was conducted at the dissemination area (DA) level, a Canadian census unit containing about 400–700 people. Six vegetation indicators were used as dependent variables: the proportion of a DA covered by trees/shrubs, lawn and total vegetation; the proportion of streets covered by trees/shrubs and the proportion of residential yards covered by trees/shrubs and total vegetation. Three sets of independent variables were studied: the built environment, sociodemographics and borough names. We used spatial autoregressive models to control for dependence and the spatial autoregressive term explained a large amount of variability in vegetation cover. The built-environment variables tend to have higher effects than the socio-demographic variables when predicting the three DA vegetation indicators, backyard vegetation, and to a lesser degree, street tree/shrub cover. In particular, population density is associated negatively to all indicators but positively to street tree cover. Socio-demographics are substantial in the explanation of the distribution of street trees, especially the presence of recent immigrants (negative effect) and of university degree holders (positive effect). These findings call for appropriate greening programs adapted to the local socio-demographic profile. The significance of boroughs also suggests the need for further research on the impact of within-city administrative hierarchies on the unevenness of urban vegetation.     

Land use, land cover changes and coastal lagoon surface reduction associated with urban growth in northwest Mexico

Coastal land use and land cover changes, emphasizing the alterations of coastal lagoons, were assessed in northwest Mexico using satellite imagery processing. Supervised classifications of a Landsat series (1973–1997) and the coefficients Kappa (K) and Tau (τ), were used to assess the area and verify the accuracy of the classification of six informational classes (urban area, aquatic systems, mangrove, agriculture, natural vegetation, and aquaculture). Pixel-by-pixel change detection among dates was evaluated using the Kappa Index of Agreement (KIA). Besides the overall estimation of the aquatic systems class, variations in the three lagoons present in the study area were analyzed individually. Measures of agreement between the classification and reference data indicate that the accuracy for the classification ranked from moderate to high (K = 0.76 ± 0.07; τ = 0.77 ± 0.06). From 1973 to 1997 urban area has doubled, growing to the north and the northeast, extending mainly over natural vegetation and agricultural land. La Escopama and El Sabalo, two of the lagoons studied, reduced their size to less than half that estimated in 1973, but the main estuarine system in the study area, Estero de Urias - El Infiernillo, has maintained its area without noticeable changes. However, the surrounding landscape in Estero de Urias - Infiernillo is changing from natural vegetation and agriculture to urban land use. Consequently, to limit as much as possible changes in the area to natural causes, some management measures must be considered to design urban development plans and to recover and preserve the natural areas, on a broad scale rather than a local spatial scale. This revised version was published online in August 2006 with corrections to the Cover Date.     

Do perceived walking distance to and use of urban blue spaces affect self-reported physical and mental health?

A cross-sectional analysis among 1041 urban residents was done to examine associations between self-reported walking distance to and use of freshwater urban blue space and health-related quality of life in two German cities: a city with “poor” urban blue space supply (Bielefeld; 0.8% blue space) and one with “better” urban blue space supply (Gelsenkirchen; 3.0% blue space). Health-related quality of life was assessed with the SF-12v2, which measures residents’ self-reported mental and physical health. Results showed a significant association between use frequency and perceived walking distance to blue space. Blue space use was a better indicator of health outcomes than perceived walking distance. After controlling for green space, socio-economic and demographic variables, use frequency was associated with higher mental health in Gelsenkirchen. These cross-sectional findings suggest that blue space use increases the probability of being healthier in highly urbanised areas in cities with an appropriate amount of blue space.     

Citizens’ perception of and satisfaction with urban forests and green space: Results from selected Southeast European cities

In this paper the results of the first comprehensive study on perception of and satisfaction with urban forests and green space in seven Southeast European cities are presented. The aims of the paper are to analyse 1) citizen perceptions of the current state of urban forests and green space in their cities, 2) to what extent current urban forests and green space meet their needs and how this can be improved. A cross-sectional study was conducted by using a common face-to-face survey questionnaire. Respondents were selected based on census data though a stratified sampling procedure by taking into account age, gender and city district (n = 384 in each city). The results showed that citizens genuinely care for urban forests and green space in their cities, but are not satisfied with their current state. The respondents found issues related to misbehaviour of other users, the presence and quality of facilities, as well as the presence and quality of management or maintenance the most pressing. There were more statistically significant differences than similarities between cities. Socioeconomic variables explained perceptions only to some extent. Citizens were very supportive of educational campaigns about the importance of urban forests and green space as well as of better enforcement of the existing regulations though having more community wardens that were expected to tackle current unsatisfactory situations. Urban planning and urban forest and green space management in these cities are facing many problems characteristic for post-socialist countries. Study findings are expected to contribute to decision making in urban planning and natural resource management.     

Primary productivity in cities and their influence over subtropical bird assemblages

Changes in ecosystem structure caused by urbanization produce a reduction in photosynthetic productivity, which can lead to reductions in resource availability for birds. Here, we analyzed the relation between photosynthetic productivity and bird assemblages in a subtropical urban ecosystem, in North-Western Argentina. We used Generalized Linear Models to assess the responses of bird abundance, richness and diversity to photosynthetic productivity, vegetation cover and distance to main natural forest. We found higher bird richness and diversity with increasing photosynthetic productivity and vegetation cover, and with decreasing distance to forests; while total bird abundance was positively related to vegetation cover. When we classified bird species in different groups, based on their use of the environment, we found that species adapted to urban environments were more dependent on photosynthetic productivity, while species related to native forests were more dependent on the distance to source forests. Understanding the factors that affect bird assemblages in cities is important for the development of strategies for urban planning and conservation.     

The role of green roofs in mitigating Urban Heat Island effects in the metropolitan area of Adelaide,South Australia

Changing an urban environment and replacing vegetated surfaces with low albedo materials is one of the reasons for increasing temperatures in an urban environment and consequently also one of the key causes of urban heat island effects. In this study, an experimental investigation at the micro-scale and also a numerical simulation at the macro-scale of a typical urban environment in Adelaide were conducted to estimate the potential for mitigating the UHI effect. The results showed that existing low albedo materials such as asphalt, metal roofs and brick pavements contribute to the heat island potential. Also, urban development and a lack of natural vegetation contribute to increased temperatures in cities. The ability of two types of extensive and intensive green roofs to reduce the surrounding micro-climate temperature were monitored. The results showed that they have significant cooling effects in summer time and could behave as an insulation layer to keep buildings warmer in the winter. Furthermore, different scenarios of adding green roofs to the Adelaide urban environment were investigated using the Envi–MET model. The scenario modelling of adding green roofs in a typical urban area in Adelaide, Australia, supported the hypothesis that this can lead to reductions in energy consumption in the Adelaide urban environment. Also an increased use of other water sensitive urban design technologies such as green walls and street trees together with the adoption of high albedo materials is recommended for achieving the optimum efficiency in terms of reducing urban temperatures and mitigating urban heat island effects.     

Increased home size and hardscape decreases urban forest cover in Los Angeles County’s single-family residential neighborhoods

Single-family residential neighborhoods make up large areas within cities and are undergoing change as residences are renovated and redeveloped. We investigated the effects of such residential redevelopment on land cover (trees/shrubs, grass, building, and hardscape) in the 20 largest cities in the Los Angeles Basin from 2000 to 2009. We identified spatially stratified samples of single-family home lots for which additional square footage was recorded and for which additional construction was not recorded by the tax assessor. We then digitized land cover on high-resolution color imagery for two points in time to measure land cover change. Redevelopment of single-family homes in Los Angeles County resulted in a significant decrease in tree/shrub and grass cover and a significant increase in building and hardscape area. Over 10 years, urban green cover (trees/shrubs and grass) declined 14–55% of green cover in 2000 on lots with additional recorded development and 2–22% of green cover in 2000 for single-family lots for which new permits were not recorded. Extrapolating the results to all single-family home lots in these cities indicate a 1.2 percentage point annual decrease in tree/shrub cover (5.6% of existing tree/shrub cover) and a 0.1 percentage point annual decrease in grass cover (2.3% of existing grass cover). The results suggest that protection of existing green cover in neighborhoods is necessary to meet urban forest goals, a factor that is overlooked in existing programs that focus solely on tree planting. Also, changing social views on the preferred size of single-family homes is driving loss of tree cover and increasing impervious surfaces, with potentially significant ramifications for the functioning of urban ecosystems.     

Tree canopy change and neighborhood stability: A comparative analysis of Washington,D.C. and Baltimore,MD

Trees provide important health, ecosystem, and aesthetic services in urban areas, but they are unevenly distributed. Some neighborhoods have abundant tree canopy and others nearly none. We analyzed how neighborhood characteristics and changes in income over time related to the distribution of urban tree canopy in Washington, D.C. and Baltimore, MD. We used stepwise multiple regression analysis to identify strong predictors of UTC, from variables found in neighborhoods with different patterns of wealth-stability over time. We then built spatial lag models to predict variation in UTC cover, using the results of a Principal Component Analysis of the socioeconomic, demographic, and housing characteristics of the two cities. We found that: (1) stable-wealthy neighborhoods were more likely to have more, and more consistent, tree canopy cover than other neighborhood types; (2) decreases and increases in income were negatively associated with UTC in Washington, D.C. but not Baltimore, where income stability in both wealthy and impoverished neighborhoods was a significant predictor of UTC; and (3) the association of high socioeconomic status with UTC coverage varied between the two cities.     

Patch-level based vegetation change and environmental drivers in Tarim River drainage area of West China

Information on vegetation-related land cover change and the principle drivers is critical for environmental management and assessment of desertification processes in arid environments. In this study, we investigated patch-level based changes in vegetation and other major land cover types in lower Tarim River drainage area in Xinjiang, West China, and examined the impacts of environmental factors on those changes. Patterns of land cover change were analyzed for the time sequence of 1987–1999–2004 based on satellite-derived land classification maps, and their relationships with environmental factors were determined using Redundancy Analysis (RDA). Environmental variables used in the analysis included altitude, slope, aspect, patch shape index (fractal dimension), patch area, distance to water body, distance to settlements, and distance to main roads. We found that during the study period, 26% of the land experienced cover changes, much of which were the types from the natural riparian and upland vegetation to other land covers. The natural riparian and upland vegetation patches were transformed mostly to desert and some to farmlands, indicating expanding desertification processes of the region. A significant fraction of the natural riparian and upland vegetation experienced a phase of alkalinity before becoming desert, suggesting that drought is not the exclusive environmental driver of desertification in the study area. Overall, only a small proportion of the variance in vegetation-related land cover change is explainable by environmental variables included in this study, especially during 1987–1999, indicating that patch-level based vegetation change in this region is partly attributable to environmental perturbations. The apparent transformation from the natural riparian and upland vegetation to desert indicates an on-going process of desertification in the region.     

Effects of vegetation,urban density,building height,and atmospheric conditions on local temperatures and thermal comfort

This paper shows the effects of several variables, which co-cause the Urban Heat Island effect on temperature distribution and outdoor thermal comfort (by using the Predicted Mean Vote, PMV) on dense urban environments. The study was conducted by means of a three-dimensional microclimate model, ENVI-met 3.1, which forecasts the microclimatic changes within urban environments. The effects of building density (% of built area) and canyon effect (building height) on potential temperature, mean radiant temperature, and Predicted Mean Vote distribution are quantified. The influence of several types of green areas (vegetation on the ground and on roofs) on temperature mitigation and on comfort improvements is investigated for different atmospheric conditions and latitudes in a Mediterranean climate. The research quantifies the effects of the variables investigated on temperature distributions and in determining outdoor comfort conditions. Vegetation on the ground and on roofs mitigates summer temperatures, decreases the indoor cooling load demand, and improves outdoor comfort. The results of the study demonstrate that density and height of buildings in a city area influence potential temperature, mean radiant temperature, and Predicted Mean Vote distribution; for most of the cases examined higher density causes higher temperatures and with taller buildings vegetation has higher cooling effects. Considering the cooling effect of vegetation, a difference can be noticed depending on the amount of green areas and vegetation type. The results of this study show also that vegetation is more effective with higher temperatures and lower relative humidity values in mitigating potential temperatures, mean radiant temperatures, and PMV and in decreasing the cooling load demand.     

Using green infrastructure for urban climate-proofing: An evaluation of heat mitigation measures at the micro-scale

Urban green infrastructure (UGI) has been increasingly promoted as a key measure to mitigate heat stress in cities caused by the urban heat island effect and climate change impacts, including climate variability and extremes. However, comparable information concerning the performance of different UGI types to moderate such impacts is mostly lacking. This creates serious challenges for urban planners who need to decide on the most effective measures while considering spatial and administrative constraints. This study investigates how different types and quantities of UGI, i.e. trees, green roofs, and green facades, affect pedestrian thermal comfort. The study was applied to high-density residential areas under current and future climatic conditions. Climate change will on average increase afternoon Physiological Equivalent Temperature (PET) values by 2.4 K; however, this could be vastly reduced by different UGI scenarios. Planting trees had the strongest impact with an average PET reduction of 13% compared with existing vegetation. Trees shade open spaces and provide evapotranspirative cooling. Another valuable adaptation option is green facades, which have mitigating effects of 5%–10%. In contrast, the effects of green roofs were negligible. Our results indicate that increasing the share of green cover did not directly correspond to the magnitude of the PET reduction. Placing vegetation strategically in heat-exposed areas is more effective than just aiming at a high percentage of green cover. We conclude that our extensive comparative analysis provides empirical evidence to support UGI on the micro-scale and assists planners and decision-makers to effectively select and prioritise concrete measures to adapt to climate change.     

Development potential of sky gardens in the compact city of Hong Kong

Sky gardens have been actively studied and installed in different cities. Their development potential in compact developing cities has received little attention. Using remote sensing and GIS techniques, this study evaluates the vegetation configuration and development potential of sky gardens in urban Hong Kong, their underlying location, land use and building factors, and future planning and implementation concerns. Existing sky garden area is limited with sparse vegetation cover and low biomass. Existing podium gardens exceed roof gardens by about nine times. District development age has little effect on existing and potential sky gardens. Old towns have higher potential roof and podium gardens than new towns in most land uses. The effect of land use on potential sky gardens varies greatly by districts. Buildings with 10–20 floors have higher potential roof gardens in most districts. Building area is the main determinant of potential roof garden, and population density of potential podium gardens. Three scenarios of realization, namely minimum (20%), medium (50%) and maximum (80%), are adopted to project sky garden provision in individual districts. The projection extends to the contribution of new sky gardens to urban greening in terms of green cover and greening rate in districts. The challenges include susceptibility to typhoon damage in high-rise exposed sites, aggressive weed invasion, lack of roof-slab loading data in old buildings, and poor building maintenance. The opportunities include affordability of the new technology, enabling government policy, and establishment of scientific and research foundation in the local context. The development strategy could aim squarely at stringent technical standards and contractor skill requirement, and programme prioritization based on research findings. The study provides useful hints, approaches and recommendations for a systematic sky-garden action plan in Hong Kong and other similar compact cities.