The role of bioretention as a site level infrastructure solution to economic and environmental issues associated with urbanization is well understood. Public agencies have developed extensive resources for their design and implementation that are well supported by research. However, the vegetative layer does not receive the same attention as the engineered components within guidance and research, leaving successful implementation up to the skill of individual practitioners. In order to realize the potential bioretention facilities have to integrate nature and city, comprehensive planting design guidance is required. This study begins the process of establishing such guidance, illustrating its potential through the creation of a design guide prototype called GreenKey. It suggests a framework for future planting design research and practice that integrates ecology, horticulture, stormwater engineering, soil science, microbiology and landscape architecture to ensure practitioners are equipped to navigate the complex task of designing resilient and engaging urban bioretention facilities.