Objective In recent years, urban greening has attracted more and more attention from governments at all levels. The open space covered by vegetation can be called green space. The green space in the urban built environment includes both the “formal green space” featuring clear land ownership and active management and maintenance, and the “informal green space” featuring vague ownership and loose management. Under the background of urban stock development, urban formal green space plays a limited role in meeting the recreation, livelihood and welfare needs of residents, while facing the challenges of incremental land use constraints and decentralized construction land use rights. Therefore, the function and potential of informal green space (IGS) have attracted the attention of scholars. IGS is a more ecological, economic and resilient green space form, and transforming IGS into a public open space that is convenient for residents to use and has ecological service functions is one of the ways to improve the living environment of high-density urban areas under land constraints. This research aims to, by clarifying the connotation of the IGS concept and analyzing the research hotspots and research progress on IGS in foreign countries, provide experience for the planning, management and protection of urban IGS in China.

Methods Based on the core database of Web of Science, this research selects the keywords of “informal (urban) green space(s)” or “informal (urban) greenspace”, the literature type of “article” and the language of “English” for retrieval, with a total of 47 English articles being retrieved as the basis of analysis. The number of citations is an important standard to measure the influence of papers, and the research hotspots of IGS can be obtained based on the analysis of highly cited papers. By analyzing the top 10 most frequently cited papers, it is concluded that relevant research of foreign scholars mainly focuses on four directions: Identification technology, recreation value, ecological value and renewal model. After that, the research progress on informal green space in foreign countries is systematically analyzed from these four directions.

Results As for the practical geographic information analysis technology of IGS, scholars usually combine remote sensing technology and field survey to identify and count IGS. This survey method has high accuracy, but the labor and time costs are high and the survey scope is limited. Recently, some scholars have combined Google Street View with machine learning to identify and classify green areas in labeled streetscape photos. In addition, participatory geographic information systems are used to obtain IGS geographic information from the public. IGS has three unique recreational values: First, IGS can trigger diverse recreational perception of residents, and age stage and cultural background will affect residents’ perception of IGS ; second, IGS can meet the flexible and diverse needs of residents, providing residents with more flexible, natural and uncontrolled outdoor activities; third, IGS occupies a high proportion in urban space, which can greatly enhance the connectivity of urban green space network and improve the fairness of access to green space. The ecological value of IGS is mainly reflected in two aspects: Enriching urban biodiversity and improving urban ecosystem regulation services. First, IGS can reduce the negative impact of urbanization on biodiversity. Second, IGS has the same or even better ecological regulation functions as formal green space, such as alleviation of urban heat island effect, rainwater retention, biological carbon sequestration, and air purification. At the same time, foreign practices show that low-cost renewal methods can be used to enhance the support of IGS to social economy and urban culture. The renewal model of IGS can be summarized into three categories: Overall renewal of public policy formulation, top-down support for transformation by government decision-making, and bottom-up promotion of transformation by social forces.

Conclusion Combined with the national conditions of China and the lessons drawn from foreign theories and practices, it is suggested to optimize the construction of IGS in domestic cities by using the three strategies of actively responding to the needs of residents, creating geographic information database and innovating planning management paradigm, so as to promote the construction of urban greening in a fair, ecological and economical way. First of all, the renewal and management of IGS should actively respond to residents’ renewal intention and co-construction intention, consider the preference of vulnerable groups for the use of community green space, and improve procedural fairness and participation fairness in urban renewal. Secondly, at present, the base map of IGS is still difficult to obtain, and it is urgent to establish IGS geographic information database incorporating natural environment and human society. Finally, in order to retain the unique recreational and ecological value of IGS, it is necessary to formulate targeted safeguard policies, pay attention to regional overall planning and flexible planning, establish coordination mechanisms and improve organizational mechanisms.