Green infrastructure can provide a wide range of urban ecosystem services, from recreation and health benefits (Tzoulas et al. 2007) to pollution reduction, biodiversity habitat and high temperature reduction (Norton et al. 2015). However, using exclusively formal greenspaces such as city parks and street trees poses two problems. First, implementing and maintaining green infrastructure in cities carries substantial costs (Naumann et al. 2010). Land acquisition may be prohibitive for rapidly growing cities with high land prices (e.g., Sydney, Singapore, Hong Kong), while implementation and maintenance costs may limit feasibility for shrinking cities (e.g., Detroit, Leipzig, Kyoto). Second, projects are often tied to expectations for economic returns on investment, which may not benefit local residents but can cause eco-gentrification (Wolch, Byrne, and Newell 2014). In this paper we draw upon recent research (Rupprecht and Byrne 2014; Rupprecht and Byrne 2015; Rupprecht, Byrne, Garden, et al. 2015; Rupprecht, Byrne, Ueda, et al. 2015) to argue that ‘informal urban greenspace’ (e.g. vacant lots, street and railway verges, brownfields and power lines etc.) could be used as green infrastructure, and that it indeed already performs this function to some degree. We discuss how informal greenspaces may complement traditional elements of green infrastructure, how both growing and shrinking cities may be able to integrate it into green planning strategies, and what challenges its use may pose. We conclude by presenting a multi-layered provisional roadmap of directions for future research on geographical, planning-related and ecological aspects of informal greenspaces relevant for its use as green infrastructure.