Enhancing blue-green infrastructure in German cities with the involvement of urban society: insights from Frankfurt/Main and Stuttgart Open Access

In recent years, increasing attention has been paid to blue-green infrastructure (BGI) in discussions about sustainable adaptation to climate change in cities (Ghofrani et al. 2017; Brears 2018; Thorne & O'Donnell 2020; Almaaitah et al. 2021; Casiano Flores et al. 2021). BGI contributes to climate adaptation by cities. The various interlinkages within BGI help provide a buffer for extreme weather events while offering additional ecological services such as cooling, the production of wetland crops, biodiversity and nature conservation, in addition to making cities more agreeable places in which to live and work (Ghofrani et al. 2017; Gunawardena et al. 2017; Brears 2018; Deely & Hynes 2020; Deely et al. 2020).

A key definition provided by Brears (2018) states that BGI is ‘a planned network of natural and semi-natural areas that utilise natural processes to improve water quality and manage water quantity by restoring the hydrological function of the urban landscape and managing stormwater’. BGI comprises natural elements, such as rivers, wetlands and forests, as well as man-made elements such as green roofs and retention ponds. The combined implementation of various BGI elements results in an interlinkage of green and blue infrastructures. This study broadens that definition by adding the natural balance alongside the water balance, as has also been done by, for example, Casiano Flores et al. (2021) and Deely et al. (2020). Additionally, in an urban context, BGI closely interacts with the technical water infrastructure, which is also included in discussions in this paper about BGI.

Despite the benefits of BGI over the conventional grey infrastructure, it has not yet become widespread as a concept (Ghofrani et al. 2017). BGI is rarely supported by or used for water management such as drainage or rainwater harvesting and irrigation, although interventions of this kind can significantly improve its impact (Trapp & Winker 2020). One reason for this could simply be that it is a relatively new approach, despite early publications promoting such systems dating back 15–20 years (Ghofrani et al. 2017). Another reason is that these systems are more complex, for example due to the connections necessary within blue and green infrastructure, but also owing to the institutions involved in its planning, implementation and maintenance (Deely et al. 2020; Almaaitah et al. 2021).

Initiated by the European Union's Biodiversity Strategy, Germany was an early adopter in Europe of BGI (Hu et al. 2020). Subsidies from the European Regional Development Fund can partly be used for investments in a ‘greener, CO₂-free Europe’, including adaptation to climate change, the preservation of biodiversity and green infrastructure. In Germany, government funding for the establishment of green infrastructure is inconsistent and heavily influenced by the regulations of individual federal states (Langendorf 2020). German planning law allows for the development of BGI (Deutsche Vereinigung für Wasserwirtschaft Abwasser und Abfall e.v. 2021). The municipal stormwater fee provides discounts or exemptions for surfaces based on their contribution to runoff, thereby incentivising green roofs if these comply with existing technical rules (Nickel et al. 2014). In the 1970s, Germany was the first European country to support the construction of green roofs (Brudermann & Sangkakool 2017). Direct subsidies and legal regulations provide the best incentives, with Germany offering a wide variety of them (Burszta-Adamiak & Fialkiewicz 2019).

Unlike many other countries, Germany has a decades-long tradition of research and lecturing at universities on greening buildings. For this reason, but also owing to special municipal promotion opportunities, the implementation of green roofs and facade greening is also higher than in many other countries. For example, stipulations contained in land use plans together with stormwater fees have just as much of an effect in Germany as financial incentives; many cities with more than 50,000 inhabitants operate municipal subsidy programmes for green roofs and facades (Burszta-Adamiak & Fialkiewicz 2019; Bundesverband Gebäudegrün 2020).

Deely et al. (2020) provide a useful overview of the barriers faced, classifying them into the following categories: institutional and governance, sociocultural, knowledge, technical and biophysical, and funding and market. There has been limited research on BGI and urban society (Haeffner et al. 2017; Leonhard et al. 2019; Williams et al. 2019), with slightly more research undertaken on barriers in general (Deely et al. 2020; Almaaitah et al. 2021; Casiano Flores et al. 2021; O'Donnell et al. 2021). In the present study, urban society is understood as a subgroup of the population that feels a sense of responsibility for the city (Hoffmann-Axthelm 1993). In terms of BGI, urban society was narrowed down to three groups that are able to influence it: city residents, actors in the housing and real estate sector, and municipal employees. This study analysed ways in which to enhance BGI in German cities by ascertaining the attitudes and requirements of urban groups in relation to BGI, the barriers to achieving it and the measures needed to overcome these, as perceived by the various groups. Overall, recommendations were identified to develop BGI and improve climate adaptation with a focus on the perspective of urban society. In terms of extreme weather events, this research concentrated on the effects of periods of hot, dry weather since most research to date has focused on flooding (Williams et al. 2019; Deely et al. 2020; Almaaitah et al. 2021; Casiano Flores et al. 2021).

The overall research question was: How can processes around BGI be enhanced to improve adaptation to climate change from the perspective of urban society, specifically from the perspective of a city's residents, real estate actors and municipal administrations? This was elaborated using the following guiding questions: What are the similarities and differences in the perceptions, expectations and requirements of the various urban society groups investigated here? What barriers does urban society identify in relation to communications, planning and implementation around BGI? What can potentially be done to overcome these barriers in their opinion?

To achieve the above-mentioned objectives of this study, a number of methodical steps were combined. Using a broad actor analysis, relevant actors and actor groups in cities were identified and selected groups addressed (Rohrbach et al. (accepted)): (1) experts in the field of real estate management were asked about their perceptions and about the opportunities and obstacles presented by BGI in qualitative social science interviews, and (2) the perspectives of residents of two German cities – Frankfurt/Main and Stuttgart – were obtained using the participatory and transdisciplinary method of so-called future workshops.

The aim of qualitative research is to grasp and understand a research field in its complexity, and claims to describe ‘realities of life “from the inside” from the perspective of the acting people’ (Flick et al. 2005). Thus, qualitative research can provide in-depth insights into subjective perspectives on a certain topic. Qualitative expert interviews (Meuser & Nagel 2009) – as conducted in this study – are used to explore existing expert knowledge as well as understand the perspectives of experts, in this case in relation to the implementation of BGI.

As a first step and in preparation for the expert interviews, an actor analysis was carried out (Rohrbach et al. (accepted); Zimmermann 2010). The aim of this study was to identify key actors who determine the current state of BGI, their influence and scope for action in relation to BGI, as well as how they influence each other.

The actor analysis was designed to be an iterative process and started with desk research, primarily using information available online. The acquired information was added to a visualisation of the actor network using the following categories: green elements, water resources, technical elements, actors, institutions and guidelines, informal cooperation between actors and decision-making levels. Feedback from the transdisciplinary research team and information gained during the expert interviews were then used to expand the network and elaborate on the differences between the two cities. The visualisations of the networks were validated during the expert workshops and finalised accordingly.

Based on this actor analysis, 18 experts from the housing and real estate sector and 15 experts from the municipal administrations were selected for qualitative social science, semi-structured expert interviews.

The experts in the real estate sector came from housing associations, public buildings, real estate investment trusts, (landscape) architecture, urban planning, umbrella organisations in the housing industry and associations active in the field of sustainable construction and greening of buildings. The municipal actors were mainly municipal planners from different sectors: urban and green planning, urban water management, transport planning and building supervision.

The expert interviews were analysed using qualitative content analysis (Mayring 2015). The main categories of the analysis were based on the above-mentioned research objectives of the research.

Participatory and transdisciplinary methods were also applied in the form of so-called future workshops and expert workshops. The ‘future workshops’ were conducted with around 15 residents from each of the two study cities of Frankfurt/Main and Stuttgart. The objectives were to understand the perception, role and meaning of BGI from the viewpoint of residents.

‘Future workshops’ (‘Zukunftswerkstatt’) are a method for developing perspectives on future solutions and give the initiatives introduced by city residents equal consideration in terms of their influence on (urban) development processes (Jungk & Müllert 1988). The method is currently used to ‘develop visions for the future […]’ (Nanz & Fritsche 2012). Siebert describes the goal of future workshops as ‘developing imagination for sustainable, socially and environmentally compatible solutions to social problems’ (Siebert 2010).

The residents were randomly selected from the respective city's municipal registers and they attended a one-day creative workshop. Following an introduction to the topic, there was a diagnosis phase: small groups analysed the situation with green spaces in the sample districts and the need for action. At the start of the next stage, known as the ‘utopia’ phase, one of the research partners framed the challenges presented by climate change and urbanisation for BGI, and highlighted possible actions from a scientific perspective. In the implementation phase, the participants evaluated examples of solutions and measures, as well implementation opportunities and barriers. The results were analysed using qualitative content analysis (Mayring 2015).

Table 1 shows the categories of the category-based analysis of the actor analysis, the expert interviews as well as the future workshops.

Finally, the results of the three investigated groups and preliminary recommended actions were presented and discussed in two expert workshops. Invitations were extended to staff from the municipal administrations working in BGI in Frankfurt/Main and Stuttgart. Some of them had already been interviewed as municipal experts. The workshops were held online for half a day.

The workshops included results from the actor analysis, major findings from the ‘future workshops’, and results of the expert interviews, along with recommended actions. The aims of the workshops were to verify the preliminary results and discuss and finalise recommended actions. The results of the workshops were then integrated into the overall research findings.

Based on all the above methodical steps, an analysis of the barriers and measures for implementing BGI was performed using the framework of Deely et al. (2020). The barrier dimensions were: institutional and governance (I&G), sociocultural (SC), knowledge (K), technical and biophysical (T&B), and funding and market (F&M). Each dimension comprised various barrier types. The analysis in this study was performed in two stages: (1) the results were analysed within the three investigated groups using the above barrier dimensions, and (2) The threat level for each barrier type and the overall threat level for each barrier dimension were determined. Overall, it should be noted that not all the barrier dimensions were investigated since the interviews were already underway when this framework was published and they could only be taken into consideration when the results were being analysed.

Measures to enhance BGI were identified in a three-step process. First, the measures and recommendations highlighted in the interviews were collected and classified. Second, they were analysed in accordance with the overall barriers identified. Third, weak measures were elaborated further, additional measures developed and inappropriate measures eliminated. The overall set of measures was then discussed in the two above-mentioned expert workshops to complement and finalise the selection.

The analysis of the two workshops revealed that these participants had a high level of understanding and knowledge about plant species and the impact of urban greening on the microclimate. The majority of participants in both cities stressed the need for an expansion and further development of urban greening in urban areas. They did not consider reinforcement of the status quo to be satisfactory in light of expected changes. At the start of the workshop, they introduced aspects into the debate such as the relevance and impact of urban greening on residents’ quality of life and its importance for local recreation and air quality. They mentioned the need for it to be properly managed, gave examples of best practices and their own experiences, and lastly highlighted aspects they felt were missing and conflicts of interest, for example between urban greening and traffic. Overall, the conclusion was that urban greening is extremely important to residents, especially in terms of recreation, quality of life and biodiversity. They discussed options and potential new or improved forms of interactions between municipalities and residents, such as irrigation of public green areas by citizens and management of water supply in order to enhance urban greening, even offering to become involved themselves.

Water was mentioned in discussions about urban greening irrigation. They were clear that rainwater or greywater should primarily be used for this rather than drinking water. Residents of Frankfurt/Main also mentioned the risk of torrential rain and its impact on the drainage system, which was a less important consideration for participants from Stuttgart. When evaluating the blue-green connections in existing examples of practical solutions, the focus was interestingly not on the ‘if’, but rather on the ‘how’ of using other water resources. Questions about the practicability of irrigation were emphasised, such as how to modify buildings to provide greywater sources effectively and how to solve the problem of storage and transport of rainwater or (treated) greywater, for example, to where it is needed in the public space.

However, the discussion around water revealed the influence of existing conditions: high building density in the selected neighbourhoods, the actual situation following an ‘above average’ hot summer, and the workshop focus on hot, dry periods of weather rather than on torrential rain and flooding. Although the citizens were randomly selected from neighbourhoods that are typical for the two cities, a self-selective effect of the participants should be noted – only people who are interested in this kind of topic would attend this kind of workshop. Nevertheless, the participants represented residents of all ages as well as people of different genders with different cultural, economic and educational backgrounds. Therefore, the results can still be deemed to be typical and highly relevant.

Overall, city residents were keen to get involved themselves, with some willing to take over looking after a piece of public green space in terms of mowing and watering, for example. However, most of them were unaware of public initiatives promoted by the municipal departments’ websites with regard to funding, private sponsorships, etc. Here, there was clearly a requirement for more and different forms of communication, e.g. via social media.

Two subgroups were identified in the analysis of interviews with representatives from the housing and real estate sector. The first can be characterised as actors who seek to provide (affordable) accommodation and who do not give much thought to improving the urban climate. They mainly see the challenges and barriers associated with enhancing BGI, including innovative forms of urban greening and alternative water resources. They are either deterred by the challenges or conclude that these approaches have been around for some time, but have not yet become established, such as greywater. As these are not yet up and running, they are not interested in them. Most of the representatives from housing companies, associations and actors in commercial property clearly belong to this subgroup.

The second subgroup mainly comprised representatives from organisations and architects that focus on sustainability in the building sector or work for housing initiatives. They mentioned the same challenges and barriers as the first group, but had solutions based on their own experiences or from a transfer of knowledge.

Quantitatively, the first subgroup was larger and had the greatest impact on the market. Overall, these interviewees described their industry as fairly conservative and risk averse, focusing on safe, common systems that are well known because the construction and management of facilities puts them under considerable financial pressure. It remained unclear whether the risk aversion they mentioned was just an argument they were using as an excuse or an actual fact. They classified BGI, and in particular newer forms such as green facades, as too risky and still at an experimental stage. Only extensive green roofs were judged to be state of the art and could be implemented without risk. They also reported that the shareholders of companies willing to go in this direction rejected interventions of this kind. To date, only a few of them have tried to introduce forms of greening that would be viewed as successful if they cut the costs of facilities management in other areas. Moreover, in the case of office buildings, art or water fountains were preferred over green spaces in terms of providing attractive elements. Moreover, according to actors from the real estate industry, funding programmes for BGI set up by many German municipalities are not attractive due to the bureaucracy and high workload involved with little probability of receiving funding. Altogether, it is unlikely that this sector will voluntarily take an active role in implementing BGI.

Both municipalities were mindful that their cities have to adapt to climate change. BGI was seen as a key tool by most of the municipal employees who are aware of its potential. Nevertheless, differences emerged between the various actors. Two key actors are the department of parks and green spaces, which is responsible for green areas and parks, including cemeteries, which are usually park-like areas with a high amount of urban greening in Germany, and the department responsible for urban drainage. The ‘inner circle’ around those two key actors is rounded off by the urban planning department and the environment department, both of which play an active role in the two cities, while the transport department, also considered a potential key player due to its considerable influence on the design and shaping of urban areas, is on the sidelines.

While managers of urban greening focus on boosting and enhancing urban greening and occasionally discuss irrigation, the departments managing urban water streams concentrate more on torrential rain and flooding. Irrigation and alternative water sources, which are necessary to meet the requirements of climate adaptation, are not among their core activities due to their existing roles and duties; instead the funding system arranged in line with the tariff structure hinders re-use of such water resources, e.g. drained water for irrigation in most cases. This division of responsibilities hampers the development of BGI because the department of parks and green spaces is left to find sustainable solutions on its own even though its employees do not have the relevant knowledge to deal with alternative water resources such as rainwater or greywater. At the same time, by opting to use alternative water resources, they take on the risk of water management, e.g. the requirement to deal with large amounts of water after heavy rainfall. According to planning actors in both the planning and environment departments, legal requirements for the implementation of blue and green elements need to be improved.

Although these different points of view do exist, there is growing interest in BGI at different levels, such as pilot projects, new development plans, subsidy programmes for private building owners, and revisions of statutes. Coordination groups, round tables and smaller project-related working groups have been set up to provide information, exchange ideas and work on responsibilities to improve adaptation to climate change and prepare the ground for the implementation of relevant measures. Whether the focus is on climate adaption or mitigation also varies, depending on the involvement of departmental heads or officers in charge and the content and output of the groups. This leads to a difference in focus in relation to BGI.

During the analysis of the results, it became clear that these could be grouped under the classifications ‘green infrastructure’, ‘blue infrastructure’, ‘relevance of transformation’ and ‘willingness to take action’. While green and blue infrastructures address the three groups’ relationships with BGI and their appreciation of it, ‘relevance of transformation’ and ‘willingness to take action’ can be seen as the foundation for enhancing BGI.

All the urban actors interviewed highlighted the importance of urban greening and their appreciation of it, while all three groups emphasised the relevance of urban greening in relation to climate change and urban adaptation requirements as well. Positions diverged when it came to the inadequacy of maintaining of the status quo and the need to upgrade and expand existing urban greening. While the majority agreed with it, the first subgroup in the housing and real estate sector and individual municipal employees disagreed since the cost-benefit trade-offs were considered to be negative. Moreover, for actors in the housing and real estate sector, (accessible) green areas are accompanied by other problems such as complaints by tenants, noise nuisance and litter, e.g. when the areas become meeting places for young people. They also rejected the idea of maintenance being undertaken by residents as they may lack the skills and be unreliable and it may even not be legally possible as insurance policies would not cover them for accidents. It is clear that community-based initiatives are harder to manage, but solutions do exist, hence this reaction may be a response in order to avoid greening.

This division in positions also accounted for the need for urban greening to change. Here, various alterations were mentioned, such as green areas having to become more heat resistant, necessitating a change in species composition. When it came to the management of green areas, both municipal actors and representatives of the real estate industry were persuaded of the need for heat-resistant trees for urban adaption, but the reduction in evaporative cooling as a consequence of heat-resistant urban greenery was not perceived to be a problem. Additionally, there is a need to improve the biodiversity of existing green areas, e.g. lawns, and adapt management practices such as mowing.

A key issue was the extent and way in which urban greening contributes to urban climate adaptation and makes a difference. Here, the judgements among the different groups and interviewees varied. Residents did not differentiate between private and public urban greening. Residents and engaged property owners also assumed that every piece of green helps, regardless of its size and design, but the majority of actors in the housing and real estate sector and some of the administrative staff were doubtful that small contributions at individual property level could make any difference at all. Depending on the assessment of this issue, they came to very different conclusions about the forms and types of urban greening required, their personal tasks and duties, as well as the responsibilities of other stakeholders. Some representatives from the real estate sector were convinced of the positive effects of urban greening, but clearly saw the responsibility for this lying with the municipality. Others also pointed out that their responsibility is only to comply with the legal requirements set by municipalities during the planning of new buildings.

The topic of water was divided into two extremes: heavy rainfall and flooding and periods of dry, hot weather. The discussion was less about the blue infrastructure and its water bodies, streams, etc., than about water resource management in a more general way. While all three groups mentioned heavy rainfall and flooding, the focus was on hot and dry periods and the role water and blue infrastructure play here. Residents immediately started discussing requirements for water storage and irrigation, with the latter necessary regardless of whether it is a private property or a public space. In their view, one way of fulfilling requirements would be to use water sources other than drinking water. Actors in the housing and real estate sector clearly followed a strategy of avoiding as much as possible dealing with water management, such as rainwater harvesting and irrigation, in order to reduce costs, although certain costs would be passed on to tenants as part of their service charges. No irrigation would be the best solution for them. In the area of housing, passing on the costs to tenants should be possible from a financial perspective, particularly as both cities are experiencing a housing shortage. For commercial real estate, there is currently no incentive in the case of Frankfurt/Main due to an oversupply of office space, with offers ranked and evaluated according to their incidental renting costs. At the municipality level, however, attention is very slowly being paid to the requirements and impact of irrigation. There is consensus that more green space is needed, but as yet no awareness of the continuous need to irrigate green spaces. Cooperation and a different allocation of tasks between the different specialist municipal authorities to manage water resources more effectively during hot, dry periods are not on their agenda at all.

Water resources mentioned in the interviews barely went beyond discussions about drinking water and rainwater. Residents were the only participants to mention service water produced from domestic wastewater streams, such as greywater. Service water, groundwater retention and runoff from wastewater treatment plants were barely raised at all except by a few interviewees who are very engaged, well-trained experts in the housing and real estate sector and municipal employees. Finally, in the case of Stuttgart, it was interesting to see the limited knowledge among that city's participants since it has very detailed information on protecting and managing its mineral springs and also runs pilot projects, such as for using runoff from sewage treatment plants for irrigation purposes.

No one disputed climate change or the need for cities to adapt to it. Residents discussed adaptation and transformation in an integrated way, already automatically linking together urban greening, water resources, mobility issues and quality of life, e.g. in discussions about aspects of land use. Representatives from housing associations saw their tasks and duties very much in the area of climate protection, for example by promoting energy-efficient buildings, with BGI not featuring at all for them or at best being seen as a marginal topic since it competes with climate protection measures for funding, land, etc. Only innovative interviewees with a focus on sustainability in the building sector were able to describe integrated solutions reflecting both climate adaptation and mitigation. Furthermore, most actors in the housing and real estate sector were unconvinced by existing BGI solutions since they felt they were too complex and not sufficiently familiar. The only exception to this was extensive green roofs as a measure to comply with drainage requirements. This is the start of a vicious circle: as they are not sufficiently familiar with BGI, they assume that they are not at the right level of readiness; therefore, it is not an attractive option to consider and so is ignored while it remains unfamiliar.

Municipal employees painted a very heterogeneous picture when it came to the relevance of transformation. Although adaptation to climate change was not disputed, the topic itself and its consequences were not integrated consistently into the various departments and work routines. A lack of knowledge and competencies was identified and clearly highlighted by participants in the final workshops. Even the workshop participants themselves showed certain knowledge gaps in the debate, e.g. concerning the security of their own water supply regime, with an absence of prioritisation as well as too many goals preventing the implementation of BGI. Furthermore, negative experiences with BGI, which they had either had themselves some time ago or had heard about, still featured prominently in their considerations.

Willingness to take action varied considerably between the groups. Residents signalled that they were willing to get involved, indicating significant potential for the improvement of green areas. Residents also stated clear expectations about the roles and duties of municipal employees to take action. At the same time, they felt that there was a lack of information from the municipality regarding options for engagement and communication about ongoing activities, despite the municipalities themselves considering their information and communication management to be comprehensive and appropriate. Many of the representatives from the housing and real estate sector described themselves as a group of actors who go as far as the guidelines require them to go. They would only become more involved and play a larger role and make a greater contribution if the requirements to do so were stricter. In comparison, the analysis of the interviews revealed that municipal employees were unaware that they can require housing and real estate actors to take action, and probably considered their scope and power to be quite limited.

Municipal employees said that they would be willing to act, but there was no political support for it. It is likely that this statement contains bias. Naturally, clear political backing (e.g. by the mayor) would help, but some activities can be undertaken without it, such as the implementation of pilot projects to gain experience. Moreover, the existing lack of clarity about roles also indicated that the action taken so far has been too superficial. If there had been more action, there would be solutions with regard to the allocation of tasks or at least any conflicts between departments concerning the allocation of tasks would be more evident. In addition to the absence of political backing, funding is also a crucial issue: the municipal interviewees mentioned a clear lack of allocation of financial (and human) resources, actors from the housing and real estate sector emphasised the large amount of bureaucracy and uncertainties around funding, while residents were often unaware of the funding options available in their city.

When looking into the legal aspects, municipal actors in particular wanted legal safeguards for their planning and implementation processes to allow action to be taken and reduce risks for the municipality. This is in line with professional associations in the German water and wastewater sector that point out clear gaps in the law with regard to the coupling of blue and green infrastructure and the need for legislation (DWA 2021).

One area of interest was to examine differences between the cities in terms of the importance of BGI and how it is dealt with and managed. The results showed differences in relation to urban greening. For residents in Stuttgart, an important topic was air pollution and its reduction, and how urban greening might contribute to this. In contrast, the residents of Frankfurt/Main referred to the sustainable handling of water resources and conflicts of interest between irrigation of urban greening and water shortages during lengthy hot and dry periods. These differences can be explained by extensive coverage of these issues in the media and society at large in recent years. Stuttgart, which is located in a basin almost entirely surrounded by steep hills, has had and continues to have a major problem with particulate matter, with one of its measuring stations known to be a major German hotspot for particulates (Landesanstalt für Umwelt Baden-Württemberg (LUBW)), whereas there is an almost historical water conflict between Frankfurt/Main and one particular supply area, the Vogelsberg, which is a regular subject of debate (Schilling 1993; Früh-Müller et al. 2016).

In the housing and real estate sector, no differences were identified between the interviewees, but the interviews with municipal actors highlighted considerable variations. First, the differences identified among residents were also observed in this group. For example, Stuttgart employees have a strong focus on urban greening combined with environmental aspects and biodiversity, but water is not a particular issue. In Frankfurt/Main, meanwhile, there is greater knowledge about the dependencies and interactions between blue and green infrastructures. The differences when exploring the topic of BGI were even more interesting. Work on BGI by municipal actors in Stuttgart appears to be less coordinated, with them trying out BGI in various pilot projects and hoping that it will develop from there. Areas of opportunity are exploited to provide experience on the ground, to be followed later by arrangements for necessary cooperation, management rules, etc. The opposite appears to be true in Frankfurt/Main in terms of handling BGI. Here, there is considerable activity around adopting and changing municipal regulations and exploring future forms of cooperation and management practices, however the number of pilot projects appears rather small. It would be very interesting to monitor these developments and investigate them further.

Based on the structural conditions mentioned by the interviewees, the following main barriers to BGI were identified: knowledge and awareness deficits, low real estate sector engagement, governance and policy issues, and insufficient public resources (for details, see Table 2). This confirms, at least in part, the findings of other studies (Deely et al. 2020; Casiano Flores et al. 2021; O'Donnell et al. 2021; Dorst et al. 2022). In the interviews, the conditions of collaborative governance were barely mentioned at all as barriers, which is interesting because the key literature focuses on this as a crucial obstacle, as outlined in the review of Almaaitah et al. (2021).

For a more in-depth investigation of barriers, this study built on the framework of Deely et al. (2020) and produced surprising results. Before this detailed analysis along different dimensions and themes, the greatest barrier to BGI was expected to be the institutional and governance dimension (Almaaitah et al. 2021) rather than the technical and biophysical dimension. Moreover, a greater impact was anticipated for funding and market as well as sociocultural barriers. Although this outcome was surprising, when looking at the average values for each dimension, the dimensions I&G and F&M came next in the ranking of relevance. The high expectations regarding sociocultural aspects might be explained by the focus of the various stakeholder groups in the investigations, showing a typical effect of when matters are viewed from a limited perspective.

Nevertheless, the high ranking on the T&B dimension across all investigated groups was very surprising. Many very reasonable barrier types, ranging from plant selection to irrigation difficulties, and several conflicts of interest were mentioned. A potential explanation for this high ranking could be that there was a basic commitment to adapt urban areas to climate change, with the important role of BGI confirmed by nearly all the interviewees. Hence, it is possible that the interviewees felt uncomfortable expressing their own existing concerns after having provided this confirmation. Therefore, they could mention barriers relating to material and ecological aspects under the cover of objective and logical arguments, which would mean that many of the stated barriers should be regarded as pretexts. As the validation of the expert opinions was not part of the expert interviews, e.g. ranking of the stated barriers, this potential explanation remains a hypothesis.

Another interesting observation came when evaluating the results in relation to the different stakeholder groups. A clear result was that residents had the greatest interest in adapting to climate change, were keen on BGI, appreciated its great relevance for transformation, and were willing to take action (see also Section 3.2.4). Meanwhile, municipal officers seemed less confident about it and actors in the housing and real estate sector were the most opposed to this approach, at least this was the case for the subgroup active in the commercial sector. This result was confirmed by the barrier identification framework. Residents came up with the fewest barriers, followed by municipal officers, and then actors in the housing and real estate sector. It could of course be argued that residents are the stakeholder group furthest away from planning, implementing and running BGI. This might be true, but cannot be the sole reason since private houses and apartments and thus property owners were taken into consideration in the future workshops, and hence were counted as residents, and had already implemented BGI.

This finding is in contrast to other European countries where limited citizen engagement is reported to hinder BGI (Dorst et al. 2022) or only occurs when citizens are informed about the impact of BGI (Derkzen et al. 2017). Comparable German research within the water-sensitive design approach (Leonhard et al. 2019) and the Sponge City approach (Chen & Chen 2020), however, show that urban society in Germany is willing to support BGI. There is usually a mix of intrinsic motives for and interests in improving living conditions for the community (Wang et al. 2021). Nevertheless, Ambrey et al. (2017) demonstrate that Australian citizens do not accept adaptation to climate change as a motivation for BGI; instead, BGI is only accepted and people's perceptions changed by the argument that it improves socio-economic living conditions.

A wide range of measures were identified from the results on urban society's perceptions, expectations and requirements. The measures were assigned to four categories at the city level: information and communication, institutional changes, financing and costs, and education and training (for details, see Table 3). The measures presented were either identified during the interview analysis or developed during the classification and elaboration undertaken by the authors during the evaluation stage (for details, see Methods section). The aim was to focus on the barriers identified by urban society.

All the listed measures were rated as appropriate or at least worthy of consideration when presented and discussed with municipal employees in the final expert workshops in Stuttgart and Frankfurt/Main. Of course, due to existing settings, projects, conditions and measures, preferences varied. By way of example, one measure per category is outlined below to provide more details and insight.

Competitions and prizes can help shine a spotlight on BGI as a way of improving climate adaptation, as well as raise awareness and provide information about well-designed and suitable implementations. Furthermore, this measure rewards proactive individuals and initiatives, and encourages activity. New competitions and prizes for BGI can be set up or BGI can be included as a criterion or category in existing competitions. It might also be interesting to hold competitions for young people ranging in age from kindergarten level to university students to raise awareness of BGI.

Many measures can be summarised under the umbrella of collaborative governance. Coordination of BGI at different levels can be grouped into different activities, such as interdepartmental cooperation in overarching working/steering groups, supportive politicians and expression of political will, the creation of an overarching coordinating body, the adaptation of planning processes, and vertical networking between politics and administrations. The measure highlights that the enhancement of BGI within existing areas of emphasis, types of work and methods of cooperation is insufficient and needs to be adapted. New cooperation, communication and reconciliation schemes are required.

Cross-cutting topics require exchange and decisions between stakeholders at particular moments in the planning processes that have not yet been necessary. Therefore, a phase of experimenting with alternative planning options is needed before a new process is identified to replace the existing one. An overarching working or steering group would also help moves in this direction where new processes/tasks that are not easily assigned to a particular person or department can be initiated and worked on jointly, or a joint team can be established to monitor them.

New and possibly more intensive forms of networking and cooperation are also required between politicians and administration. This not only includes the requirement for a supportive politician to drive the topic forwards and offer political support for difficult processes, but also the interaction between municipal officers and local politicians who have in-depth knowledge of their own areas and local people.

Furthermore, it became clear from the results that the housing and real estate sector also needs to be explicitly addressed. The analyses clearly show that they do not see it as their role to contribute to climate adaptation; they take decisions and act in a risk-averse manner and also doubt the impact they can have by managing green areas and water resources differently at the property level. Here, municipal employees supported by political decision-makers should establish direct contact with these actors to emphasise the relevance of their contributions and establish how to overcome the sector's risk aversion. Depending on the results of their exchanges, specific measures could range from developing coping strategies and overcoming risks all the way to imposing instructions and guidelines.

A major barrier is a lack of knowledge about the cost of BGI, especially in relation to its planning, implementation, maintenance and operation. Estimates range from very cheap and easy to manage to extremely expensive and requiring skilled employees. Compilations of costs for typical green, blue and blue-green infrastructures would help overcome this barrier and increase certainty about a subject that is currently fairly unknown and of major concern in decision-making. A specific focus should be on operation and management because there is very limited knowledge and information available specifically in this area. Moreover, this information must be communicated thoroughly to increase cost transparency for all stakeholders active in the sector.

Such information is also important for architects and planners to design BGI in an appropriate way in line with client preferences and operational and management budgets. This information is often lacking or the knowledge available is inadequate. It should also be integrated into training courses and continuing education.

It is important to ensure that stakeholders in key professions within BGI are qualified. This concerns municipal officers in the area of blue and green infrastructure and in urban and transport planning. Some of them only have limited knowledge about the potential of BGI to enhance climate adaptation in urban contexts. Furthermore, architects, landscape architects and planners, and tradespeople and professionals in specialist companies need to acquire knowledge about the options and potential benefits of BGI, particularly with regard to its design, for combating the effects of climate extremes such as long dry periods and heavy rainfall, but also to integrate important operational and maintenance aspects into their design and planning.

There are various options for continuing education. First, BGI could be added to existing training courses and continuing education, such as in sustainable building, climate protection and energy efficiency to make it appealing to architects and planners. For example, energy consultants could be trained on heat protection with blue-green elements, as well as the combination of green and energy infrastructures. Second, courses offering training on BGI, from planning to operation and management, would be of benefit. It would be helpful to have courses that are designed for single professions, as well as mixed courses to initiate exchanges. Third, to ease the way for such courses, teaching material and guidelines on how to set up such courses would contribute to them becoming more widespread. A national coordinator would be of help here. Last but not least, job shadowing is an option. This means working alongside someone who is already knowledgeable and intensively involved in BGI. Learning from another person who ideally works in the same profession and/or department can be very effective.

Overall, it would be advisable to select different measures at the same time to increase their impact. This is a strategy that has been adopted successfully in Germany in the case of green roofs (Burszta-Adamiak & Fialkiewicz 2019). Alongside specific measures, recommendations relating to urban society were identified to boost the impact of the selected measures. First, political backing is needed, especially for municipal employees, to implement BGI fully and explore the maximum contribution it can make to climate adaptation. It needs political decision makers who will take up this issue and make it their vision. Politicians encouraging the implementation of BGI are also important to boost its profile among residents. Without this, the implementation and contributions of BGI will be limited. However, if such backing is lacking, it does not mean that nothing can be done, but measures should be selected that take this into account. A strategic decision might be the selection of measures that can help win the support of decision-makers.

Second, it became clear that alliances are also needed within and between municipal departments, as well as within urban society in general, in order to enhance BGI. Most likely to be responsible for this work is the municipal department leading the processes around BGI, but they are not able to do this on their own. Therefore, the identification of potential allies and the benefits to them of pushing ahead with BGI is important in order to create a stable network of support and collaborators. Round tables and working groups have been established in Stuttgart and Frankfurt/Main to work on climate adaptation and mitigation. These groups help forge alliances, encourage action from other departments/municipal employees, and identify and push ahead with key measures. It is recommended that they focus more on BGI as a measure to counter climate change, and establish respective subgroups if the various working groups and round tables are not designed for such level of detail.

Third, actors in the housing and real estate sector also need to be addressed. The actors are currently unaware of the task, its importance and the relevance of this sector's contribution. Here, a combined set of measures is needed with direct instructions to municipal housing associations so that they become involved, from joint pilot projects to an overall campaign containing exchanges, information packs and funding for first movers all the way to enforcement in law. This can also be initiated by citywide municipal steering groups.

Fourth, it is important in parallel to push the enhancement of BGI throughout the city on a strategic level. Of course, certain measures and activities should reach out to the whole of urban society, but it is advisable to select certain focus areas where measures and forces are shared. The criteria for selecting such areas could be the severe impact of climate change, a spatial structure that allows and/or forces action, potential benefits/resources in civil society from NGOs, clubs and associations in the area, supportive local politicians, and upcoming activities in the area of blue and green infrastructure.

Finally, it is recommended that work be bottom-up and top-down at the same time to make best use of opposing movements and include the whole of urban society from the outset. It is advisable to start with small, local projects to try things out, gain experience and build on this for the purposes of upscaling (bottom-up approach). At the same time, measures on a strategic level should be initiated, such as the amendment and harmonisation of statutes (top-down approach). In doing so, experience acquired from small projects can help when drafting appropriate statutes, while changing statutes is a longer process, which means that they can help disseminate the tested and adopted measures immediately after approval. This creates a dynamic situation that encourages primarily hesitant stakeholders to participate. In particular, when innovative peers in the respective groups start to take action and the innovation is judged to be ‘fashionable’, it goes from being niche to more mainstream.

This study focused on urban society, specifically on the residents, housing and real estate sector and municipal administration of the cities of Frankfurt and Stuttgart, and investigated ways to enhance BGI, with a particular focus on periods of hot, dry weather. As a methodological approach, an empirical set-up of different workshops and interviews was used plus an analysis of barriers via the barrier identification framework of Deely et al. (2020).

The analysis regarding the perceptions, expectations and requirements of the various groups in urban society investigated here showed that they agree that the BGI needs to be enhanced to allow cities to adapt to climate change. Where there are considerable differences, however, is in their willingness to get involved. Within urban society, the residents are most in favour of BGI and are willing to support it as a general concept, as well as newer forms of BGI more specifically, followed by municipal employees. One interpretation is, that residents experience and suffer directly negative climate change effects in the cities and also combine benefits of more urban greening for BGI with other needs, like enhanced amenity to stay, noise reduction or traffic calming, etc. BGI was seen as a key tool by most of the municipal employees who are aware of its potential. Two key actors are the department of parks and green spaces and the department responsible for urban drainage. The ‘inner circle’ around those two key actors is rounded off by the urban planning and environment departments, while the transport department, also considered a potential key player, is on the sidelines. Actors in the housing and real estate sector were found to be the most reluctant or averse ones. Overall, the interviewees from this sector described their industry as being fairly conservative and risk averse and classified BGI, and in particular newer forms such as green facades, as too risky and still at an experimental stage.

At the same time, the research focus on adaptation to periods of hot and dry weather emphasised interesting aspects for urban society within BGI, such as the selection of plant species, irrigation requirements and careful water management using alternative water resources. While the selection of plant species was clearly a topic raised by the municipal experts, residents also discussed the requirements involved in managing urban green areas, such as mowing and watering. A major finding was that residents and engaged property owners assumed that every piece of green helps, regardless of its size and design. In contrast, the majority of actors in the housing and real estate sector and some of the administrative staff were doubtful that small contributions at the individual property level could make any difference at all. This led to very different conclusions about the forms and types of urban greening required, their professional tasks and duties, as well as the responsibilities of other stakeholders. Irrigation requirements were considered a minor issue by everyone except residents. Water resources mentioned in the interviews barely went beyond discussions about drinking water and rainwater. Residents were the only ones to mention service water produced from domestic wastewater streams, such as greywater. Service water, groundwater retention and runoff from wastewater treatment plants were not mentioned at all except by a few interviewees.

In terms of the relevance of transformation, this was of importance to residents, but the majority of actors in the housing and real estate sector ignored it. There were considerable differences between municipal employees. A lack of knowledge and competencies was identified and clearly highlighted by the municipal employees themselves. A similar picture can be drawn for the willingness to take action demonstrated by the groups. While residents signalled their willingness to contribute, actors in the housing and real estate sector stated that they would only become more involved if the requirements were stricter. Municipal employees said that they would be willing to act, but there is a lack of political backing, financial (and human) resources as well as legal safeguards to allow action to be taken and risks for the municipality to be reduced.

The analysis of barriers regarding communication, planning and implementation around BGI showed that action is required to overcome a wide variety of barriers that were mainly highlighted in the area of technical and biophysical barriers, followed by institutional and governance and funding and market. These results were surprising. Before this detailed analysis along different dimensions and themes, the institutional and governance dimension was expected to emerge as the greatest barrier to BGI. A potential explanation for this high ranking could be the overall basic commitment to adapt urban areas to climate change and the important role of BGI. Hence, it could be that the interviewees felt uncomfortable expressing their own existing concerns and hesitated to outline, e.g. their financial priorities, and therefore mentioned barriers relating to material and ecological aspects under the cover of objective and logical arguments. Unfortunately, the present research was unable to validate this hypothesis.

A wide range of measures was identified in light of the results on the perceptions, expectations and requirements of urban society. The measures were assigned to four categories at city level: information and communication, institutional changes, financing and costs, as well as education and training. In addition to the measures directly addressing the planning and implementation process from the groups’ feedback, it became very clear that strategic and transformation-oriented information and communication are also crucial aspects. All the investigated groups highlighted a distinct lack of information or difficulties in finding adequate information. This caused at least insecurities and in the worst case the complete abandonment of BGI. Moreover, negative examples remained in people's memories for a very long time. Overall, political backing, alliances between stakeholders, activation of the housing and real estate sector, and a focus on selected urban areas in which to initiate action are recommended for successful enhancement of BGI, while a combination of bottom-up and top-down activities should be commenced to achieve maximum impact.

The authors would like to thank the German Federal Ministry of Education and Research for funding the research project ‘INTERESS-I – Integrated strategies to strengthen urban blue-green infrastructures’ (grant number: 01LR1705C1). The authors also wish to thank all the interviewees, workshop participants and Gesa Matthes for their time, interest and valuable discussions. These results would not have been achieved without them. Last but not least, the authors of this article would like to thank the whole INTERESS-I consortium for the joint research and for constructive and fruitful discussions.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.