The rural-urban fringe (RUF) is the indistinct boundary zone outside the urban area. It is also a highly vulnerable, sensitive, and ecologically valuable space, owing to the complex demographic composition, unbalanced economic development, and dramatic changes in land use and ecosystems. Different plants spontaneously emerge across RUF areas, playing an important ecological role in regulating climate and providing varied habitats for plants and animals. Few studies on urban sprawl have examined the plant diversity of RUF areas. Thus, more information is needed about the composition, diversity, and variability of spontaneous RUF species. Accordingly, this research implemented a field study across 248 survey plots in the RUF of Chengdu City, China. The composition and diversity of spontaneous species in five RUF habitat types during different seasons were analyzed. The effects of habitat types and seasons on α-diversity and species composition dissimilarities were evaluated using two-way analysis of variance, Principal coordinates analysis, and pairwise permutational multivariate analysis of variance, respectively. Across the seasons, 313 species of spontaneous vascular plant were recorded in RUF plots. Escaped medical and edible species accounted for 53.04% of species, consistent with the RUF inhabitants' needs. Among exotic plants, 41 of 74 were invasive species posing a high risk of species invasion. Trees were the dominant vegetation type only in autumn
herbs were the dominant vegetation in other seasons. Both season and habitat types significantly affected spontaneous species composition and α-diversity
moreover, habitat type had a greater effect than season. Farmland, with the highest α- and β-diversity levels, had the optimal resilience under environmental change. Its species composition was most similar to that of building land. Furthermore, both farmland and road land exhibited a significantly elevated proportion of exotic species in comparison to other habitats. Exotic species easily spread to cities and villages through the connectivity of road lands and farmland. In contrast, woodland had the lowest adaptive capacity under environmental change, with the lowest α- and β-diversity levels. With the escalation of anthropogenic disturbance, the prominence of the dominant species within the community markedly increased. However, as the foundation of occasional species conservation, it is the most important habitat type to protect. This study contributes to the understanding of changes in ecological processes in the RUF of megacities caused by urbanization and provides an important reference for RUF habitat conservation.