This paper contributes to the understanding of urban commons and how they might be (co)-designed. Insights from two cases are used to articulate how urban commons develop over time and to discuss how the approach of infrastructuring can enable urban commoning on a long-term basis. First, an overview of commons and urban commons is provided with a special focus on commoning, as in, the understanding of commons as an ongoing process rather than a stable arrangement. Thereafter, the paper gives an overview of the participatory design community's findings about co-designing commons, with infrastructuring proposed as a possible approach. By looking at the development of two urban commons over time, the paper tentatively presents an understanding of urban commoning. This emerges as a process that entails the exploration, reification, and reworking of collaborative arrangements over time. It is a process that requires transparency and accountability, and its transformative potential in relation to urban governance should be carefully considered. From these findings, the paper suggests that prolonged infrastructuring efforts for urban commons need to: (1) foster the understanding of the temporal and fallible nature of arrangements; (2) support accountability and transparency over time; (3) recognize and address the installed base; and (4) articulate democratic and governance aspects in commoning.