The growth of urbanization in developed and developing countries is one of the most important social and economic phenomena in the overall development of different aspects of human life [1]. However, urban ecosystems are where interactions between anthropogenic activities and the natural environment are at their most intense ^[2][3][2,3]. Over the years, the expansion of cities has led not only to a reduction in green areas but also to greater environmental damage from the production of heat, waste, water, and air pollution and has generally had a negative impact on biodiversity ^{[4][5][6][7][8]}[4,5,6,7,8]. There is an urgent need for integrative planning of green cities if we are to meet the growing environmental, social, and economic challenges posed by the negative effects of urban development [9]. Cities of the future should strive to be greener by designing and managing green infrastructure that can improve urban resilience and livability ^[10][11][10,11]. Even though urban forests and green open spaces in cities (tree-lined streets, gardens, parks, wetlands) are essential, there is often limited availability of ground-level spaces suitable for nature-based solutions.

Green roofs, typically consisting of vegetation planted above a series of layers that protect the roof substrate and improve the system’s performance, can, therefore, represent an ideal supplementary space ^[12][13][14][12,13,14]. In particular, extensive green roofs (EGRs) require a shallower growing media and little maintenance compared to intensive green roofs (IGRs), which involve a thicker growing media and more intensive gardening [6].

Vegetated building roofing dates back to at least the Neolithic Era (8000–4000 BC), as they provided protection for buildings in harsh and climatically extreme environments [15]. For example, in the Arctic and the semi-arid continental lands of Central Asia, the scarcity of trees gave rise to a vernacular architecture where roofs were insulated with soil and living grasses from natural meadows (Figure 1a,b), known as “sod-roofs” [16]. The use of vegetation on buildings has also been recorded in the milder climates of medieval and modern Europe, particularly the beautiful, intensive green roofs built by aristocratic families and religious organizations [13]. These architectural structures gained complexity over the years with the invention of reinforced concrete, which allowed the construction of multi-story buildings with wide flat roofs and, as a result, provided many more opportunities for creating ornamental gardens above ground level [16].

Studies carried out in Germany in the mid-twentieth century stimulated the birth of modern EGRs; thereafter, spontaneous vegetation growth was observed on flat roofs, demonstrating nature’s ability to create green roofs on buildings [13]. Le Corbusier [17] was already talking about green roofs in 1927 and listed them among his ’Five Points of Modern Architecture’, citing their functional benefits in protecting reinforced concrete from temperature changes, as well as their recreational value ^[18][19][18,19], while in 1936 the roof gardens created in South America by Roberto Burle Marx became famous [20].

Extensive green roofs have been the subject of several review papers and research papers, although these tended to focus on individual aspects ^[21][22][23][21,22,23]. Such studies have shown how EGRs help mitigate the urban heat island (UHI) effect, reduce temperature through evapotranspiration ^[12][14][24][12,14,24], and stormwater run-off by retaining water ^{[12][14][25][26][27]}[12,14,25,26,27]. They absorb air pollution ^[14][28][14,28] and act as a sound barrier due to the thickness of the substrate and vegetation layers ^[29][30][29,30]. Green roofs also provide ecological benefits by supporting urban flora and fauna biodiversity and functioning as ecological corridors ^{[6][31][32][33][34]}[6,31,32,33,34]. Finally, some revision work highlights how green roofs can affect different types of ecosystem services ^[35][36][35,36].

Although the historical and geographical background seems quite clear, interest in this topic has lacked consistency and homogeneity both in chronological and geographical terms. The same can be said for the themes chosen for study; although the spectrum of topics is very broad, interest in addressing them is not homogeneous. There is still a need for a more detailed analysis of ‘where and when’ interest in modern green roofs spread in the context of scientific research and ‘why’, i.e., with what goal such studies were made. Additionally, green roofs may be of interest to a wide range of disciplines, particularly engineering, architecture, agronomy, and botany. Another interesting question in need of clarification is ‘who’, i.e., the professional background of the study authors.

The botanical aspect of green roofs and their design, on the other hand, tends to be neglected, with little attention paid to plant selection. Although plant species play an important aesthetic role, which is a crucial architectural priority and important for psychological well-being ^[8][37][8,37], the plants selected are also fundamental to the functionality of green roofs. For this reason, care should be taken to select the most appropriate plant species ^[6][38][6,38], and attention must be paid to the influence that soil depth, local climates, water availability, and planting density have on roof-based plants ^{[19][39][40][41]}[19,39,40,41].

2. Origin and Evolution

Although the principles of EGRs can be found in early twentieth-century modern architecture, one can say with certainty that their introduction and related research began some decades later. This probably happened, like with many other innovations, as a result of the complex socioeconomic and political conditions created by the two world wars. There is now a truly global interest in research into this subject driven by curiosity, fashion, and necessity. With regard to the geographical origin of EGRs, the vast database confirmed the role of Germany and Switzerland in early implementation and scientific research, which then expanded worldwide ^[42][43][48,49]. There is a margin of error in the precise quantification of scientific papers in the field since the indexing of papers began later and was implemented differently in different countries. Recent and rapidly spreading interest in places like Southern Europe, China, and the United States may be associated with an increased awareness of environmental issues and economic benefits ^[44][50]. Indeed, over the past three decades, environmental degradation has become a source of collective concern, leading many national and international organizations to launch specific initiatives based on sustainability in order to counter the progress of climate change and environmental degradation ^[45][46][51,52]. However, despite the need to address these issues collectively, countermeasures are not taken in the same way and at the same time by all nations, and cultural, political, and economic factors can influence how nations approach these strategies. This divergence is also clear in the present study. The challenges related to political decisions and the market are evident, even when considering countries that emerge as the most active in this context, such as the United States, China, and Europe, where green roofs have become a central theme. Despite the establishment of organizations like GRHC (Green Roofs for Healthy Cities) or EFB (European Federation Green Roof and Walls) with the aim of promoting and encouraging the adoption of green roofs and walls in their respective countries, it is observed that regulatory frameworks and political incentives remain insufficient. Additionally, there is a lack of in-depth analysis and research on the management of the green roof market. For instance, although China is recognized as a global leader in constructing new areas every year, it still lacks adequate regulations on green roofs to stimulate their development ^[47][53].

3. Aims and Approaches

From the earliest published papers, the researchers found that despite being a multidisciplinary field, the study of green roofs is too often treated in a monodisciplinary manner ^[43][48][49,54]. The only negative finding common to all the papers was related to costs. This confirms the conclusions of Chen et al. ^[49][55] and Dong et al. ^[44][50], who identified maintenance, design, and construction costs, poor provisions for EGR adoption, and lack of subsidies as the main disincentives to the implementation of EGRs. A general evaluation of the various approaches suggests little consideration was given to ecological concerns. Usually, when engineer/architect teams specify the selection method for plants, their decisions are made mostly on a technical or practical basis by considering morphology, canopy capacity, aesthetics, easy availability, and low maintenance. The terminology used is another element that highlights the need for more emphasis on plant selection. Authors often refer to vegetation with generic and minimalist terms such as ‘grass’, ‘grass-like’, and ‘Sedum-like’. This denotes how, in some cases, vegetation is seen superficially as purely aesthetic or as an accessory. Ecological and environmental factors have recently been receiving more interest due to the role of biodiversity in environmental management as a hot topic, even in urban contexts ^{[43][50][51][52][53]}[49,56,57,58,59]. This can also be seen in national and international policy trends. One such policy is the recently adopted Biodiversity Strategy 2030 ^[54][60], which aims to “bring nature back into our lives”, in line with the goals of the Green Deal ^[55][61].

4. Botanical View

In most countries, from the United Kingdom and Germany to America and China, Sedum species (Crassulaceae) are the plants most used. Their selection is justified by the belief that they are the most suitable species for this environment, and they are often recommended in guidelines. It is undeniable that species selection is a challenge, and selecting drought-resistant species such as Sedum sp. on extensive green roofs is easy for any professional figure to justify ^[44][50][56][50,56,62]. There is increasing awareness of the need to employ a wider range of Sedum species by considering biogeographical and bioclimatic factors, using native species ^[57][58][63,64], or considering physiological and ecological factors like prioritizing species able to tolerate water stress ^[59][60][65,66], possess specific photosynthetic qualities (e.g., CAM, C4) ^[60][61][66,67] or can adapt to pioneer conditions ^[62][63][68,69]. However, the progress of research is slow. For example, only about a hundred species appear in papers published in North America despite its extreme environmental complexity and wide range of climatic conditions ^[42][48]. From an ecological point of view, we should be aware that Sedum species are not native to some parts of the world, and research should also focus on considering the suitability of other plant species for green roofs ^[48][54]. Sedum could be replaced with other species suited to the green roof construction environment by using ecological species selection methods [6]. In this way, by considering the variety of functions performed by different plant forms, green roofs can be designed so as to promote conditions beneficial to plants and maximize benefits ^[54][64][60,70]. Species diversity in green roofs has often been seen as simply an aesthetic issue ^[65][71]. Time and again, plant diversity has been shown to play an important role in the functionality of these infrastructures. For example, it can improve substrate cooling ^[66][72], prevent invasive weeds ^[67][73], and conserve water ^[48][68][54,74]. Plants are also crucial to social, psychological, and sometimes even to ethnobotanical considerations ^{[69][70][71][72]}[75,76,77,78]. In some cases, for example, green roofs are used to grow food and treat water ^{[73][74][75][76]}[79,80,81,82].