Greens in the Gaps: Diversity and the Ecological Potential of Urban Spontaneous Vegetation in Sidewalk Ecosystems

Green spaces are essential for urban environments, but urban expansion often results in fragmented patches and narrow pavements unsuitable for tree growth. Consequently, there is a pressing need for alternative vegetation in urban landscapes where tree planting is impractical. Urban spontaneous vegetation (USV)-plants that establish naturally without cultivation-shows promise for urban landscaping, and yet has been underexplored in urban ecology. This study was the first systematic survey to examine the composition of USV in Chiang Mai, Thailand, across seven urban locations. The survey was conducted along 13 sidewalk routes (totaling 33.24 km), documenting all non-tree vascular plant species. A total of 63 USV species from 24 families were recorded, predominantly colonizing pavement gaps, cracks, and curbside cracks. The most diverse family was Poaceae, with 15 species. Among the 61 identified species, 32 species (52%) were non-native. Seven species were found in all surveyed locations, highlighting their adaptability to challenging urban conditions. Fifty USV species are medicinal plants. Many species exhibit characteristics that are ideal for sustainable landscaping, such as drought tolerance, low maintenance requirements, and ornamental value. This study highlights USV as a key component of green infrastructure and provides new insights for urban sustainable landscaping.