Informal green spaces in urban environments: a systematic review of definitions, values and assessment methods

As urban land resources continue to diminish, conflicts among competing land-use demands have become increasingly prominent. Unlike traditional green spaces, informal green spaces (IGS) provide unique advantages, including adaptive spatial configurations, low maintenance requirements, and multifunctionality. However, concepts, potential contributions, functions, and possible risks of IGS have not been comprehensively explored so far. By reviewing 159 articles published between 2002 and 2025, the following key findings were identified: 1) Although no consensus has yet been reached regarding the definition of IGS, existing studies generally agree on several core characteristics, including informal planning, spontaneous vegetation, and spatial fragmentation. 2) Both Chinese and international studies recognize the ecological and economic values of IGS. Chinese research focuses on social functions related to urban renewal, whereas international studies emphasize ecological values. In both contexts, socio-economic value studies are limited by small sample sizes and narrow scopes. 3) Various assessment methods have been used to evaluate IGS values. However, most studies focus on single-dimensional values and lack a comprehensive assessment framework. 4) IGS are also associated with potential risks, such as safety hazards and management difficulties. Nevertheless, existing research mainly remains confined to problem identification, with limited attention given to systematic risk assessments. Based on these findings, future directions are proposed to address the existing gaps. Future research should explore the various subtypes of IGS, with a particular emphasis on how different methods apply to different IGS. Additionally, establishing an integrated assessment system and a comprehensive risk management framework is essential. Furthermore, efforts should be directed toward promoting the multifunctional, integrated use of IGS.