Urban interstitial spaces are part of the urban green structure of the city, they are the unregulated interstitial spaces: vacant plots, former industrial area, or unused railway lines. In terms of their physical configuration and the apparent relaxation of rules, the interstitial sites differ from formal green space. They host different uses and users and so help to serve the diverse needs of urban inhabitants. Hence, a strong case can be made for the importance to consider the interstices as additional type of public green space. Unfortunately, interstitial spaces are not considered as public space. Management of interstitial spaces hinders the publicness of the interstices, and redevelopment often results in a formalization of green space, thereby losing the characteristics that helped to serve the diverse needs of urban users. This raises the question: what design interventions can increase publicness of urban interstitial spaces without formalizing green space? To answer this question, research was conducted in four distinct sites in Leipzig, Germany. One specific interstice serves as a case study for in depth-research and subsequent design interventions. The study draws from personal observations and experiences to develop a novel approach to a spatial design in urban interstices, aiming to increase publicness without formalizing the green space. The result is a design approach alternative to the so often applied tabula rasa approach. It provides flexibility in the level of informality between the different interventions. The interventions pay attention to the self-transforming nature of the interstice, they are a means to prompt a certain behavior without completely predefining the use of the site, aiming to incite curiosity for users to determine their own paths. In this way incorporating the interstitial space into the public green structure of the city without losing the distinctive qualities that interstitial space holds.