This paper presents an agent-based model that explores the conditions for ongoing participation in community gardening projects. We test the effects of Ostrom’s well-known Design Principles for collective action and use an extensive database collected in 123 cases in Germany and two case studies in the Netherlands to validate it. The model uses the Institutional Analysis and Development (IAD) framework and integrates decision mechanisms derived from the Theory of Reasoned Action (TRA). This allows the analysis of volunteer participation in urban community gardens over time, based on the garden’s institutions (Design Principles) and the volunteer’s intention to join gardening. This intention is influenced by the volunteer’s expectations and past experiences in the garden (TRA). We find that not all Design Principles lead to higher levels of participation but rather, participation depends on specific combinations of the Design Principles. We highlight the need to update the assumption about sanctioning in such systems: sanctioning is not always beneficial, and may be counter-productive in certain contexts.