Implementation of trees on buildings has substantially increased as an alternative solution to increase tree canopy coverage in dense cities, due to inadequate plantable space at ground level, and to achieve a range of environmental, social, and economic benefits. However, the requirements of this type of implementation have received little attention in the literature. This paper aims to identify and prioritise the opportunities and challenges of implementing trees on buildings by engaging with experts from various disciplines who are experienced in implementing trees on buildings using qualitative and quantitative approaches. This study demonstrates that although there may be challenges to implementing trees on buildings, opportunities significantly outweigh these challenges. Shading and moderating the microclimate was the greatest opportunity, whereas the quality of design, construction, management, and maintenance was considered the biggest challenge in implementing trees on buildings. This study also highlights gaps in the existing literature, including between different disciplines engaged with the tree implementation process and provides recommendations to reduce these gaps, enhance the implementation process, and increase the application of trees on buildings. In addition, the study discusses the implications of the results of this study for research, practice and policy, and compares the study findings with relevant green roof studies.