Project management literature have focused on either intra-organisational relationships or on vertical inter-organisational relationships. The purpose of this paper is to explore inter-project interdependencies and coordinating in multi-project contexts by using the notion of project ecologies. We adopt an organisational routines perspective to explore the coordinating practices managing those interdependencies. The empirical material underpinning our findings were collected and analysed through a case study of an urban development district, new to both the project ecology literature and the organisational routines literature. The findings highlight the existence and importance of horizontal interdependencies in project ecologies, as compared to the more commonly studied interdependencies in vertical relationships within and between projects. The need for horizontal coordinating is outside project managers’ regular focus on steering vertical relationships. Accordingly, the routines to manage the horizontal interdependencies in project ecologies are different to those in more engineered routines that are often described in project management guidelines.