In the midst of the epoch of the Urban Anthropocene, citizen engagement is an important step on the path of creating local and global sustainability. However, the factors that motivate civic urban dwellers to become voluntary stewards of nature environments inside cities need research. This is an empirical study based on deep interviews and a grounded theory approach focused on the "inner world" of people in Warsaw, Poland, that engage in green area stewardship. Our approach reveals a commonly shared vision as the prime motivator powering agency in green area stewardship. This vision was articulated as creating a countryside within the city characterized by a stronger sense of community, a shared sense of place and an enhanced connection with nature. While other studies have found inner values or direct benefits as motivating factors for engaging in urban stewardship, we instead found a green vision for re-designing what the "urban" could be like as the prime motivator for transformation-a vision with potential global sustainability implications.