As climate change, biodiversity loss, material scarcity and rapid digitisation converge, many UK design projects are building new ways to care for shared resources. We call these ecological commons and study how design-mediated commoning turns ecological citizenship from an idea into everyday practice. Using an interpretive, multiple-case approach, we look at seven projects: farm-made biomaterials (Ag.Lab), an AI repair assistant for electronics (AI-Fixer), a civic land information tool (EcoLandS), participatory work on water justice (Flow.Walk.Drag), runner-led air quality sensing (Open Air), social housing retrofit (RCA), and a sensory, regenerative home retrofit (Wild House). We show three things. 1) Design does the practical work of care. It creates the guides, standards, toolkits and routines that let people share responsibility. 2) Projects last longer when openness is built in, for example through licences, data trusts and transparent model cards that reduce the risk of capture. 3) Values matter. Pride, pleasure and attachment keep people involved when money and momentum fade. We unite these lessons together as a simple guide for organisations: design for care, govern in a polycentric way with shared roles across communities, small firms, labs and public bodies, and measure success by the system’s ability to regenerate and share benefits. For business and government, this points to a shift from owning to stewarding, where legitimacy comes from holding assets in trust through open standards, participatory governance and resourced maintenance.