The concept of “urban forest” (UF) is gaining momentum in urban planning in the context of climate adaptation. Principles from the field of urban forestry are mainstreamed into urban planning, but little is known about effective tools for the successful implementation of new UFs. This article presents explorative research comparing how three cities (Almere, Madrid, and Boston) are dealing with the planning of a UF project, and their alignment with distinct organisational and typological interpretations of a UF. We employed a mixed-methods approach to gain insights into the main goals of the project, their organisational structure, and the employed planning process through the analysis of project documents and expert interviews. Our results point to an effective mainstreaming of environmental questions among stakeholders, but also indicate a poor development of objective criteria for the success of a UF. We note that municipal planners circumvented current internal rigidities and barriers by relying on intermediaries and local academia as providers of external knowledge, or by facilitating experiments. Finally, our results show that there may not be just one UF type to achieve the desired environmental and social goals and overcome implementation barriers. Conversely, each of the governance and organisational models behind the implementation of each type present collaborative and mainstreaming challenges. Therefore, we see an opportunity in further research examining processes and institutions towards the collaborative building of UFs that could bridge gaps between top-down and bottom-up approaches and activate different types of agencies.