This study first uses ArcGIS 10.4 to establish a geographic information database of the productive rooftop landscape of commercial complexes in Chinese mainland, exploring the spatial distribution aspects and influencing factors by the overlay analysis method. Then the overall development and operation characteristics are summarized based on the attribute data.

In the geographical distribution analysis, the data are obtained via two steps. 1) Preliminary information collection. Because the productive rooftop garden of a commercial complex typically has a natural education or commercial use, this necessitates the use of external publicity to have a better economic benefit. The more common method is to attract customers through the network propaganda platform. Therefore, the data in CNKI literature database, the Baidu search engine, and Dianping APP, WeChat, etc., were retrieved using critical words such as “sky farm”, “sky castle”, “urban farm”, “productive rooftop landscape”, “commercial garden” on a large-scale, then we called the owners to confirm the information. 2) On-site survey. This research validated the information by field investigation to ensure the truthfulness and accuracy of the data. Although in this paper, the real locations and all the information of all the productive roof landscapes are obtained whenever possible, there may be some cases in which the information cannot be obtained due to lack of public availability. Therefore, there may be omissions in the collection of cases, and the distribution of the cases is limited to some extent. In addition, some of the roof productive landscapes, such as MODO in Nanjing Aqua City and Haha Farm on the roof of Wuhan New Mass building, have ceased operation and are not included in the statistics. The incomplete statistics up to January 2020 were sorted and the name, location, attribution, property rights, business model, and other attribute information were obtained for the 38 cases still in operation in Chinese mainland.

The research on the factors impacting the spatial distribution includes various natural and social factors. 1) Natural factors: This type of productive garden is hardly affected by terrain, soil, and other conditions because it is located on the roof. Therefore, this study primarily explores the impacts of temperature and precipitation on the spatial distribution of the cases. 2) Social factors: The production of the productive rooftop landscape is one of the important strategies to alleviate urban problems in the context of rapid urbanization, but is subject to many social factors, such as the degree of regional urbanization, the development of shopping mall complexes, residents' demands, rules and policies. Because the degree of regional urbanization represents the development of shopping mall complexes and the residents' demand for urban green space and public space to a certain extent, this study mainly probes the level of urbanization, and relevant rules and policies of various provinces. The annual average temperature and annual precipitation of all provinces (regions, cities) in 2015 of the Data Cloud of Chinese Academy of Sciences (CAS) were selected to represent the distributions of temperature and precipitation, and this was overlaid with the case spatial distribution map to obtain the relationship between the productive rooftop landscapes of commercial complexes in Chinese mainland and temperature/precipitation. In addition, the data of urbanization ratio in 2019 were extracted from the China Statistical Yearbook of the State Statistical Bureau, and the provinces with relevant roof green support policies were separated. Relevant system policies involve legislation of urban greening regulations, etc., technical specifications, guidance policies, financial subsidies and other incentive policies (Dong et al., 2020). The relationships between spatial distribution and natural/social factors can be analyzed by superimposing the distribution map of the productive landscape with urbanization degree and policy information.

In the operation characteristic analysis, this study analyses the attribute data in the database from the aspects of time, scale, theme positioning, operation mode, and location.

This study selects the practices of Chengdu and Shenzhen as two key case areas. On the one hand, the productive rooftop gardens in these two cities are in good operating condition and at a stage of mature development. On the other hand, the selection of two different coastal and inland regions for investigation helps eliminate the impact of regional differences on the results. The research objects were seven specific cases, including Mengtian urban farm Yintai store, Longhu store, Wanda store, Fangtian skyfarm Kaide store, and Pengruili store in Chengdu; Skyfarm, and Lezhi Village in Shenzhen.

The design layout feature analysis is accomplished via field research, which unifies the Unmanned Aerial Vehicle data collection and on-the-spot mapping, draws its plane layout and the landscape design diagram, and finally summarises and analyses its common layout characteristics.

In addition, 40 questionnaires were sent to the public on-site, and among them, only 36 were valid. There were seven semi-structured interviews for personnel directly related to productive rooftop landscapes, and the interview results were compiled into questionnaires. The research objects include two roof production landscape management operators and five staff members responsible for reception and introduction. During the interviews, some managers did not disclose personal information, and the team did not make in-depth inquiries due to privacy considerations. Based on the results, the cognition degree and participation willingness of ordinary citizens for the productive rooftop garden were analysed, as well as the obstacles and perceived benefits in operators' practices and operation processes. Furthermore, the characteristics of the development status of this type of case were analysed.

3.1.1 Spatial distribution characteristics

The spatial distribution of the rooftop gardens is unbalanced, and most of them are located on the southeast coast and in Sichuan province, accounting for 84.21% of the total. In other inland provinces and cities, they are less widely distributed. Precisely, the productive rooftop gardens in Chinese mainland are concentrated in 20 cities in 10 provinces (municipalities) in the south. Among them, Sichuan (7), Zhejiang (7), and Shanghai (7) have relatively high numbers of productive rooftop landscapes, accounting for 55.26% of the total; Guangdong (4), Jiangsu (4), and Anhui (3) are the next; and Hunan (2), Fujian (2), Hubei (1), Hainan (1) also emerged with one or two each (Fig. 1).

3.1.2 Factors influencing the spatial distribution

(1) Natural factors: 1) In terms of temperature, all the cases are situated in areas above the national annual average temperature. The higher temperature conditions in southern cities help the growth of productive crops, extend the growth cycle of rooftop crops, and ensure the roof's perennial landscape effect (Fig. 2a). 2) In terms of precipitation, the cases are all concentrated in the southeast of the 800 mm isoprecipitation line. Except for the Sichuan area, the annual precipitation where the other roofs occur can attain more than 1000 mm. Abundant water can meet crop growth needs, reduce irrigation input and consumption, and aid project construction and promotion (Fig. 2b).

(2) Social factors: 1) More than 75% of the cases occurred in provinces where the urbanization rate exceeded the national average. A higher level of urbanization can not only meet the diverse needs of residents for urban green space or public space life, but also promote the introduction of relevant policies to a certain extent, so as to promote the production of this type of productive landscape (Fig. 2c). 2) More than 95% of cases are located in the provinces and cities (Fig. 2d) that have promulgated urban roof greening policies. Chengdu launched a three-dimensional urban space greening work plan in the early 21^st century. Shenzhen's “Code for the Design of Roof Greening” set requirements for roof greening planting areas as early as 2009. In 2011, Shanghai included roof greening in the 12th Five-Year Plan for green development, indicating that relevant policies can effectively promote the construction of this type of green roof. In contrast, even though the Beijing-Tianjin area has a higher level of urbanization and has also introduced relevant policies, no such cases have appeared. This can be attributed to climatic conditions, leading to the need to build a greenhouse on the northern roofs, although this approach would also need to meet the relevant requirements of buildings, structures, and even floor area ratio, which hampers their development.

3.1.3 Development and operation characteristics

(1) The basic state of construction: 1) As far as the construction time is concerned, since the first commercial complex rooftop agricultural garden was built in Shanghai in 2013, the development trend of this kind of rooftop garden was good in the subsequent seven years. The number of completed rooftop landscapes continued to increase and the practical quantity attained a small climax in 2016 and 2017 (Fig. 3a). 2) In terms of the scale, 77.4% of the roofs have areas between 1000 and 5000 m², while some of the more extreme cases involved roofs over 10000 m² and some less than 1000 m² (Fig. 3b).

(2) Operation theme orientation: Most of the cases are built in the form of children's paradise, skyfarm, happy farm, theme park, etc. More than 80% of the cases were dominated by the parent-child experience, with the other seven focusing on production, tea-making, catering, sports, group building, and others (Fig. 3c). Based on the analysis of operating conditions, the business model of combining nature education with parent-child recreation activities for young children, primarily aged 4-12 years, is the most mature and stable.

(3) Operation mode: All cases are designed, constructed and operated by professional teams. The building property rights owners rent out the site or cooperate to take part in the project. Generally, the profit model adopts the business model of “admission + catering + picking + curriculum + leasing + agricultural leisure experience”. The design team specialises in urban eco-agriculture technology, including rooftop agriculture, campus agriculture, and balcony agriculture business projects.

(4) Project location: Comparing the locations of projects within the various cities, the first-tier cities, such as Shanghai, Shenzhen, and Hangzhou, primarily focus on the outer ring line, while the rooftop gardens in the second- and third-tier cities, such as Ningbo, Mianyang, and Fuzhou are located at the centre of the city (Ling, 2019). On the one hand, this difference may be because they are associated with the structural condition of building roof itself and the construction of new commercial complexes. The building structure of the old city center may need to be more radically transformed to meet the construction needs of roof landscaping. For most of the newly-built large-scale commercial complexes, the roof load and other construction conditions are considered early in the planning, which is more conducive to the production and development of this type of productive rooftop landscape. In addition, iiMedia research data show that the complex projects in the second- and third-tier cities in China have expanded rapidly. Additionally, the new commercial complexes in the first-tier cities are expanding in a decentralized manner. Based on a report released by the Shanghai Business Development Research Center, taking Shanghai as an example, the spatial distribution of new urban commercial complexes is taking the traditional downtown as the core pattern, and developing it into the multi-core pattern of various regions, so as to effectively avoid the competition of multi-functional duplication. On the other hand, this difference may be associated with the higher rent in the centre of the first-tier cities than that in the surrounding areas, and the second- and third-tier cities can promote economic development via new types of industries.

3.2.1 Layout features of landscape design

(1) Graphic design: 1) For the sake of safety, the selection of productive rooftop landscape sites usually has certain requirements for the height of the wall, or a certain distance between the roof and the edge of the roof for the purpose of ensuring public safety. In the overall layout, buildings or temporary structures are arranged in the periphery for staff use or indoor classrooms, while the outdoor distribution area is set in the center of the site for tourists to experience the plants, so that people can hardly tell the difference between the roof and the ground. 2) The roof site is planned according to the form of the roof to utilize the use of the site with greatest flexibility. An example is Mengtian urban farm Yintai store (Fig. 4), where the design of landscape and function are along the path during layout, making full use of that building's irregular roof space. 3) The circulation generally has the following two features. Visitors can reach the roof via elevator; and designers organize the circulation by extending the path to increase the experience of feeling the productive landscape. 4) Functional partitioning of the internal experience is designed based on the relationship between the functions and functional characteristics. As shown in Fig. 5, according to functional use, the roof plane from west to east can be roughly divided into an experience area, a productive planting area, and an animal breeding area, with the harvest square as the core and the walkway can be used for different functions.

(2) Landscape design: 1) In terms of space separation, the structures, height differences, fences, climbing plants, etc. are often used to subdivide the space to form subspaces of different scales, different functions, and different atmospheres. 2) Species selection mainly centres around the productive plants, including vegetables, fruits and other colourful productive seasonal landscape. 3) As for functional facilities, the rooftop landscape is often used for a series of projects related to children's education, leisure, and play, among other uses. Corridor walkways can be used for landscape kiosks and other landscape products to meet people's different social needs and make profits. Some of the projects can include setting up of facilities for animal breeding, fish farming, or petting pigs, hamsters and other small animals. Additionally, relevant games can be set up according to the characteristics of the animals.

3.2.2 Citizens' participation willingness

The results of the questionnaire survey show that although 75% of citizens have no planting experience, they are willing to participate in the projects. About 88.89% of citizens are willing to venture into rooftop agriculture in public buildings, and 42.31% of citizens hope to venture into rooftop agriculture in rented land for self-planting and domestic consumption. The reasons for the unwillingness to engage in rooftop agriculture include the lack of time, planting knowledge, and suitable places for such projects. All the same, the high percentage of citizens who are willing to venture into rooftop agriculture confirms the huge potential of rooftops as productive landscapes and for the future market.

3.2.3 Operator interview results

Interviews with personnel at several of the facilities gave us some insights on the main issues from the perspective of the operators.

(1) Advantages and disadvantages of commercial complex building roofs: 1) The roof rent is cheap, and it is otherwise difficult to have such a large floor area in the city; 2) The additional commercial attributes bring passenger flow to the development of productive landscapes, and the rooftop garden can also attract customers by combining it with surrounding shops to form a community of interests.

(2) Obstacles in actual operation: 1) Restrictions on roof planning laws and regulation; 2) Limited funding, and high economic cost compared with other roof functions; 3) Lack of agricultural planting knowledge; 4) Unsuitability of roof structure for transformation, and safety concerns; 5) Poor relations and disputes between shopping malls and residents in the surrounding area.

(3) Benefit perception: What is the most important from the operator's perspective is whether the project can bring economic benefits and social transformation, which are of higher value than ecological and environmental benefits.

The development of the productive rooftop garden of commercial complexes in China is on the rise, with huge market potential. According to statistics for the year 2020, there is active development of new productive rooftop gardens of commercial complexes in many cities, such as Hefei, Nanchang, and Zhengzhou and others. However, regarding the general course of development of this type of rooftop landscape, the development process is not smooth. Some projects that were launched with great expectations have closed down or are in bad shape due to poor management. Examples of such projects include the Magic bean sky in Nanjing, Haha Farm in Wuhan, and Skyfarm No.8 Warehouse store in Shenzhen. Overall, the existing problems fit into five main categories.

(1) There are flaws in relevant policies and regulations, and the legality of the productive rooftop garden is not yet to be ascertained.

At present, there are no comprehensive laws or regulations for the rooftop productive garden. The development of a productive green roof involves the planning bureau, the construction bureau, the city management, and the garden bureau. When several government departments are involved, each department has its own set of standards, some of which are incompatible with the standards of the others. Although some policies on roof afforestation and subsidies have been introduced successively in developed cities, the lack of a sound policy system and the funds to support and guarantee the construction of productive rooftop garden still remain. This is especially true for large-scale commercial complexes involving multi-faceted linkage management, which has led to difficulties in the implementation of such projects. For example, Skyfarm No.8 warehouse store had perfect facilities and a beautiful design when it opened, but due to illegal construction and other issues that have led to the dismantling of part of the structure, the beauty and function of the landscape have been weakened. At the beginning of its operation, the pet area had several animal species including sheep, pigs, horses and other large animals, which were dearly loved by both children and adults. However, owing to poor sanitary conditions and other reasons, the farm was the subject of complaints and large animals were prohibited. Now visitors can only see hamsters and other small animals on weekends. This is also one of the reasons why the development of the farm is becoming increasingly off-track.

(2) The functional orientation is often not accurate, and a contradiction exists between the simplification of customer groups and the diversification of demands.

Based on the theme of parent-child education, most of the existing productive rooftop landscapes of commercial complexes are designed mainly for children under 12 years old and parent-child families, although the operation mode is relatively stable, the customer group is relatively singular. Under the background of the steady development of social economy, consumer groups are significantly differentiated, and it is difficult for any single productive rooftop landscape to meet the experience needs of consumers of different ages. For example, Shenzhen Natural Science Experience Center was initially positioned for plant science popularization, sightseeing and appreciation. However, it could not attract younger children, therefore, the Center gradually began to provide a series of programs such as teaching and amusement facilities to stabilize the passenger flow

(3) The planning design of the productive rooftop garden is unreasonable.

China's rooftop agriculture started late, and so the construction of productive rooftop gardens of commercial complexes is in the stage of practical exploration, and a standard design system has not been formed. For this reason, there are problems related to plane layout and landscape layout design, among other aspects. Skyfarm, for example, once opened a branch on the ninth floor of Maoye Department Store in Nanshan District, with a two-story scale, but later closed it because the location did not take into account the inconvenience of vertical transportation. The roof of the building was too high, going up and down took a long time and only the goods elevator was available for transport purposes, which seriously affected passenger flow, leading to its closure.

(4) The supporting technologies are incomplete.

Rooftop agriculture is involved with such issues as floor load, waterproofing, etc., and has very high requirements for the relevant supporting technologies. At present, the construction technologies, cultivation technologies and anti-seepage technologies of rooftop agriculture in China are relatively mature, but the popularity is relatively low. Most of the workers engaging in rooftop productive landscape construction are ground construction teams, so there will inevitably be technical problems in the construction process. For example, in Bao'an District of Shenzhen, the roof waterproof treatment of Lezhi Village was not done well, and this caused disputes with shops on the floors below because of seepage problems.

(5) The maintenance and final-period management are ignored.

The investigation found that many commercial complex roofs venture into “rooftop agriculture” blindly. The initial construction is quite grand, the assumption is perfect, but they fail to take into account the fact that late maintenance and management require a lot of human labour and material costs. The maintenance and operation are often ignored for the purpose of reducing the cost, which not only leads to low ecological benefits, and also little economic benefit. Take Skyfarm No. 8 warehouse store as an example, a special team was assigned to carry out maintenance of the productive plants and landscape matching, to ensure the roof landscape always remained alive and well throughout the year. Such professional teams were laid off to reduce costs, and now the productive landscape areas are often green and barren. This is also the main reason why the garden is now operating in a state of decline.

4.2.1 Establishing and improving laws and policies

At present, there is still a lack of laws and regulations specially for rooftop agriculture. In order to drive the sustainable development of the productive rooftop landscape, it is necessary to establish or improve policies, regulations, rules and systems conducive to the development of rooftop agriculture. For instance, legislation should be promoted that allows rooftop agriculture to be treated as the same as the ground agriculture, with capital subsidies, etc.; Standard regulations should be established to make the designs of productive landscapes on the roofs of commercial complexes have a standard design mode and measurement standard; Environmental health policies and urban management systems should be improved, the system should be adjusted according to the characteristics of roof agriculture and the commercial complex; Authoritative management mechanisms should be established to avoid confusion and ambiguity in management; And incentive mechanisms should be established to stimulate people's enthusiasm and improve the public participation.

4.2.2 Clarifying the functional orientation

Subject to location, site, customer group, commercial value and other conditions, overall planning should be carried out in the early stage of construction, so as to judge the potential target group and functional orientation. For example, the theme of R-farm roof farm in Wuhan Yintai Creative City has focused on the parent-child interaction from its establishment in 2015 to 2019. In 2019, the theme was changed to a place for team-building and party activities for urban youth. So it expanded the target customer group, with young people and new families as the main customer group, which is in line with the business philosophy of the commercial complex where it is located, i.e., meeting the needs of urban people. Furthermore, the ecological landscape can be combined with multi-demand functions. Using a reasonably streamlined layout for orderly integration of multiple functions can make the roof landscape meet the needs of customer groups to the greatest extent, and also enrich the functional attributes of the commercial complex. In addition, supporting facilities can be added to improve the functions according to different needs. For example, the rooftop landscape with a parent-child theme can be supported by colorful children's entertainment facilities to meet the children's play needs while increasing the experience sense of the atmosphere of the farm. Alternatively, the rooftop landscape with a sports and fitness theme can be equipped with a variety of outdoor fitness equipment or fitness projects, such as horseback riding, etc.

4.2.3 Rationalizing planning and design

(1) Architecture space design: The main body of this kind of landscape is the production space. In the process of construction and design, the spatial streamline should be considered synthetically, and the planting space, traffic space and leisure space should be divided reasonably. Moreover, the landscape elements should highlight the different spatial characteristics and enrich the spatial levels. The transportation system should combine the inside and outside environments of the site, combine the entrance and exit lines of the commercial building, and enhance the functions of guidance and evacuation. At the same time, the production space should allow for convenient fertilization, removal, and picking of insects, and so on.

(2) Landscape construction and design: As the most important landscape element, plants in the productive space on the roof of the commercial complex play ecological, economic and ornamental roles. Rich and varied seasonal beauty can be produced through the planning and combination of plants, adjusting measures to local conditions, and highlighting the local characteristics, so as to form a rooftop plant landscape which integrates production and viewing. Usually, the mixed planting mode is adopted, and the landscape types are mainly productive plants with garden plants. Additionally, based on meeting the requirement of rooftop waterproofing and the urban environment, water bodies can be properly arranged, in combination with the functions of landscaping, production and ecology. Additionally, it can be designed with rainwater utilization which is a kind of sustainable circulation development mode.

4.2.4 Perfecting related technologies

Compared with the ground, productive rooftop landscapes require advanced technology, including infrastructure, plant cultivation, irrigation and seepage control, etc. In landscape design, consideration should be given to the load on the building for reasonable layout, materials and construction techniques, as well as the use of lightweight materials. Rather than traditional soil culture, the technology of soilless cultivation can be applied according to its specific conditions. Because rooftop plants are not directly planted in the ground, lack of moisture and supply of groundwater, irrigation and anti-seepage measures are extremely important. Therefore, roof anti-seepage technology should meet the relevant standards, and irrigation systems should maximize the balance of crop water storage and water-saving. In addition, productive rooftop landscapes can augment other ecological facilities, such as solar energy and rainwater collection systems, to achieve energy conservation and environmental protection (Luan et al., 2017).

4.2.5 Paying attention to final-period maintenance and management

In addition to the planning and design of the productive rooftop landscape in the early stage, the final-period planting maintenance and operation management are also very important, being among the most important factors for the sustainable development of this type of productive landscape. At present, the final-period maintenance and management need to be supported by a lot of manpower and material resources. Generally, it is expensive to employ professional teams for operation. But this part of the investment is essential, otherwise it may cause greater losses overall due to poor management. In the future, the government and the rooftop agriculture department can organize regular productive rooftop landscape training courses to cultivate professionals in order to reduce the costs and promote the development of this type of landscape.