The mainstreaming of nature-based solutions (NBS) in urban areas is widely advocated for their cost-effectiveness and ability to integrate diverse natural features and processes into cities. Research highlights the environmental, social, and economic co-benefits of NBS in addressing urban sustainability challenges and building resilience. However, on-the-ground implementation often lags behind ambitions, limited to isolated projects without sufficient attention to long-term management. Institutional, organizational, and cultural barriers within city governments are significant hurdles. Mainstreaming NBS requires a focus on institutional structures and organizational routines, rather than simply integrating NBS into established practices. Existing city government structures often operate in departmental silos, follow rigid funding procedures, and lack broad public involvement. Even with novel approaches like co-creation, cities struggle with the necessary skills, time, and institutional support. This paper explores the institutional mainstreaming of NBS, aiming for a process-based understanding of how to develop the governance capacities needed for systemic, localized, and inclusive NBS planning. Institutional mainstreaming is viewed as an iterative process of creating and reforming the institutional order to influence how planning occurs, involving experimentation with novel governance practices and planning tools. The research investigates how city officials in ten European cities have experimented with innovative approaches to NBS planning, delivery, and stewardship, leading to changes in existing governance arrangements and strategies for mainstreaming NBS in urban governance capacities. The research was conducted within the Connecting Nature project, a five-year research and innovation project funded by the European Union’s Horizon 2020 programme, involving policy officers from ten cities, scientists, civil society organizations, and SMEs. The Connecting Nature Framework, a process-based tool, guided the application of innovative governance mechanisms across seven activity areas and three phases of NBS planning, delivery, and stewardship. The framework was applied iteratively in a co-creative and peer-learning process.

Existing literature extensively documents the benefits of Nature-Based Solutions (NBS) for urban sustainability, highlighting their environmental, social, and economic co-benefits. However, a significant implementation gap exists, with NBS often confined to isolated demonstration projects lacking long-term management. Studies consistently point to institutional, organizational, and cultural barriers within city governments as primary reasons for this gap. These barriers include fragmented departmental structures, rigid funding processes prioritizing economic cost-effectiveness over broader social-ecological benefits, and limited public participation. The literature emphasizes the need to address these institutional challenges to effectively mainstream NBS. Scholars advocate for focusing not just on integrating NBS into existing frameworks, but also on transforming the underlying institutional structures and organizational routines of urban planning. Successful mainstreaming requires fostering inter- and transdisciplinary collaborations, inclusive interventions, and addressing the mismatch between NBS’ systemic nature and ‘business-as-usual’ approaches within city governments. Previous research has highlighted the importance of institutional learning and the role of planners as institutional entrepreneurs in overcoming these barriers. The existing literature often identifies barriers and leverage points but lacks a comprehensive understanding of the specific strategies and mechanisms required to develop the necessary governance capacities for successful NBS implementation and scaling.

This research employed a qualitative comparative case study of ten European cities participating in the Connecting Nature project to scale NBS. The methodology combined knowledge co-production and reflexive monitoring to generate and integrate existing knowledge about governance barriers, mainstreaming strategies, and institutional changes. Three frontrunner cities (FRCs) – Genk (Belgium), Glasgow (United Kingdom), and Poznań (Poland) – with existing NBS experience and a commitment to scaling, were involved, along with seven fast-follower cities (FFCs) with less experience. All cities focused on urban green spaces, but their entry points varied: some started from a strategic level with city-wide strategies, others from scaling up single projects, and still others from replicating successful small-scale interventions. The Connecting Nature Framework, a process-based tool encompassing seven activity areas across three phases of NBS planning, delivery, and stewardship, guided the cities' work. A co-creative and iterative peer-learning process involved diverse stakeholders in co-developing the framework, formulating interventions, and learning from experiences. Monthly (later bi-monthly) reflexive monitoring sessions with each FRC facilitated discussion of daily experiences, challenges, and next steps. Knowledge transfer activities, including 1-to-1 learning sessions and knowledge hub sessions, promoted peer-to-peer support and best-practice sharing. The comparative analysis involved three iterative steps: clustering activities based on mainstreaming strategies, formulating stepping stones to overcome governance barriers, and analyzing the resulting changes in governance conditions (rules, relations, practices, discourses). The analysis was iterative, involving cities in refining the results and promoting knowledge exchange.

The study identified three main strategies for mainstreaming NBS in urban governance: 1. **Institutionalizing a systems approach:** This strategy addresses the need to connect NBS to broader policy goals and integrate them across departmental silos. Key interventions included creating narratives about NBS co-benefits, aligning NBS with existing policies and SDGs, making NBS strategies operational and legally binding, establishing cross-departmental working relations, and integrating NBS into funding and stewardship mechanisms. Examples include A Coruña and Nicosia aligning urban garden networks with sustainable transport and wellbeing policies, Poznań connecting open gardens with education agendas, and Glasgow using high-level initiatives to politically position NBS. Cities developed tools like the Business Model Canvas (BMC) to illustrate the wider value propositions of NBS and attract diverse funding sources. 2. **Institutionalizing inclusive collaboration:** This involves activating diverse partnerships and co-creative processes to involve various stakeholders. Cities initially faced challenges due to lack of experience with co-production. Key interventions involved creating institutional space for collaboration, developing skills and tools for co-production, defining specific goals and target audiences, using innovative methods like the BMC and EM|Path approach, and establishing lasting partnerships and co-management structures. Examples include Genk hiring a social innovation officer, Glasgow making community participation a key criterion, and A Coruña creating urban garden associations for self-management. Cities implemented capacity-building initiatives and continuous engagement activities to foster long-term collaboration. 3. **Institutionalizing reflexivity and continuous learning:** This strategy emphasizes reflexive monitoring and learning to link emergent knowledge about how NBS interact with their contexts. Key interventions included implementing reflexive monitoring to evaluate daily activities and adapt strategies, developing impact assessment approaches to collect evidence across multiple dimensions, establishing platforms for continuous learning, and embedding reflexive learning in decision-making. Examples include Ioannina using bi-weekly project meetings for reflexive monitoring, Poznań using it to link tangible and intangible results, and Glasgow employing citizen science approaches to collect data on open spaces. Cities established collaborations with academia to support impact monitoring and evaluation.

The findings demonstrate how policy officers in ten European cities, supported by the Connecting Nature Framework and a co-creative research process, have successfully interacted with their governance contexts to develop and scale NBS. The three identified mainstreaming strategies illustrate how NBS, as systemic innovations, can foster institutional learning and overcome ingrained barriers. The strategies, encompassing stepping stones and resulting changes in governance conditions, go beyond simply identifying barriers and leverage points, revealing how cities changed underlying governance arrangements to create conditions for a solutions-oriented agenda across multiple sectors. Changes ranged from new narratives and relations to institutionalised positions and collaborations. While some changes varied across cities based on entry points and contexts, frontrunner cities showed more success in extending changes to other programs. Despite progress, barriers persisted: inconsistent political support, short-term financing, insufficient staffing, and departmental silos. These highlight the need for top-down action and policy changes at national or EU levels. The study underscores the crucial role of institutional entrepreneurs – the city teams – in enacting the stepping stones. The co-creative research approach supported this entrepreneurship, with reflexive monitoring being highly valued. The peer-to-peer learning fostered knowledge exchange and reduced the gap between frontrunner and fast-follower cities. The study concludes by suggesting potential avenues for transferring the Connecting Nature Framework and furthering the mainstreaming of NBS.

This research provides valuable insights into mainstreaming nature-based solutions (NBS) in urban governance. The identification of three key strategies – institutionalizing a systems approach, inclusive collaboration, and reflexivity and learning – offers a practical framework for cities seeking to integrate NBS. The study highlights the importance of institutional entrepreneurship and the effectiveness of co-creative research approaches in supporting this process. Future research could focus on scaling up these strategies, addressing persistent barriers such as securing long-term funding and overcoming departmental silos. Further investigation into the transferability of the Connecting Nature Framework and the development of standardized methodologies for impact assessment and reflexive monitoring would also be beneficial.

The study's findings are based on a specific set of ten European cities participating in the Connecting Nature project, limiting the generalizability to other contexts. While the methodology incorporated reflexive monitoring and peer learning to enhance the rigor of the qualitative analysis, subjective interpretations might still have influenced the data analysis and interpretation of findings. The focus on the experiences of policy officers provides a specific perspective, potentially overlooking other relevant actors and dynamics within the complex urban governance landscape. Furthermore, the relatively short timeframe of the Connecting Nature project might not fully capture the long-term effects of the implemented mainstreaming strategies.