This essay reports on a ‘living lab’ approach to develop a new understanding of below- and above-ground ecological processes as the foundation for robust urban forest habitats. This experimental approach includes a series of design and implementation projects in the city of The Hague, the Netherlands. In contrast to mainstream greening projects led by local governments, these experiments enable urban trees to form more robust forest-like systems by creating a symbiosis between soil (organisms), trees, plant communities, and species. As implemented reference projects are limited, a learning-by-doing methodology was adopted. A transdisciplinary team, consisting of landscape architects/designers, arborists, botanists, municipal and private green space maintenance organizations, has initiated, implemented, and monitored a series of pilot projects. Analysis of ten natural reference locations in the surrounding countryside has helped to define natural and forest-like soil conditions and plant communities for the three living lab locations in the city. Local residents have been engaged in the design, implementation and maintenance process. Sharing insights so far contributes to the transition of reconnecting soil, nature, and people in cities.