Seagrasses are integral components of coastal ecosystems, providing a wide range of essential services such as habitat for marine organisms, stabilization of sediments, nutrient cycling, and carbon sequestration. These ecosystem functions are crucial not only for the ecological health of coastal zones but also for the protection of shorelines from erosion, making seagrasses vital for both marine biodiversity and coastal resilience. Among the numerous species of seagrasses, Zostera caespitosa, a key species in the temperate coastal regions of China, plays a significant role in maintaining the structure and function of seagrass meadows. Despite its importance, there is a lack of detailed studies on the reproductive ecology and seed germination of Z. caespitosa, particularly under varying environmental conditions such as temperature and salinity. This knowledge gap limits our understanding of how this species might respond to changing environmental factors, which is crucial for successful restoration and conservation efforts. The study aimed to investigate the reproductive biology of Z. caespitosa, including its reproductive phenology, seed morphology, and the effects of environmental factors (temperature and salinity) on seed germination. Additionally, this study aimed to contribute to the development of effective restoration strategies for seagrass meadows in temperate regions by providing data on the optimal conditions for seedling establishment. The research was conducted in Sanggou Bay, Shandong Province, China, which serves as a representative site for temperate coastal ecosystems. Field observations were carried out from April to July 2025 to monitor the reproductive phenology of Z. caespitosa. The reproductive cycle was characterized by several key phases: flower clusters first appeared on April 2, followed by the onset of flowering on April 28, fruit formation on May 25, and seed release from June 17 to July 15. Throughout these phases, the water temperature ranged from 7.2°C to 21.2°C, indicating the species' adaptation to the moderate thermal conditions typical of temperate coastal regions. Reproductive shoot height increased steadily from 33.2 ± 7.5 cm in early April to 73.1 ± 12.5 cm in mid-July. Seed production peaked in mid-June, with an average of 32.7 ± 9.1 seeds per shoot, and gradually decreased as seed dispersal began in late June. Seed morphology analysis revealed that the seeds of Z. caespitosa were elliptical in shape, with an average length of 3.46 ± 0.32 mm and width of 2.12 ± 0.14 mm. The seed coat exhibited distinct longitudinal ribs and secondary thickening, which suggest mechanical strength that helps the seeds resist the hydrodynamic forces typically present in coastal environments. The seeds had a high starch content in the endosperm, occupying approximately 85% of the seed volume, providing a reserve of energy necessary for seedling development. The embryo had a U-shaped structure, which included the radicle, shoot, and cotyledons, all of which are essential for the initial stages of seed germination and seedling growth. To assess the effects of environmental factors on seed germination, laboratory experiments were conducted using a 3×6 factorial design. The experiment included three temperature levels (5°C, 15°C, and 25°C) and six salinity levels (5, 10, 15, 20, 25, and 30). The results indicated that temperature was the primary factor influencing seed germination (Two-way ANOVA, P = 0.0056). The highest germination rate (77.78%) was observed under conditions of 15°C and salinity 10. Seeds germinated most successfully at 15°C, while the main effect of salinity and the interaction between temperature and salinity were not statistically significant. Although a trend of decreased germination was observed at high salinity (≥20), these results suggest that Z. caespitosa exhibits a broad tolerance to salinity, which is an important consideration for seagrass restoration efforts in areas with variable salinity. The statistical analysis confirmed that temperature had a significant effect on seed germination, and moderate temperatures (around 15°C) were found to favor seedling establishment, as evidenced by the significantly higher germination rates across salinity treatments at this temperature compared to 5°C and 25°C. This finding aligns with previous studies on other seagrass species, including Z. marina, which has been shown to exhibit temperature-dependent patterns in seedling recruitment and survival. The results of this study emphasize the critical role of temperature as a key driver in shaping seed germination and seedling success, offering valuable insights for restoration planning. This study contributes to the broader understanding of seagrass reproductive ecology, providing detailed account of the reproductive phenology and seed germination of Z. caespitosa in China. The findings are significant for the restoration and conservation of seagrass meadows in temperate coastal regions, particularly in China, where Z. caespitosa is an important species. By identifying the optimal conditions for seed germination, this research provides a practical framework for future restoration projects, enabling more effective management of seagrass ecosystems in the face of environmental changes. In conclusion, the results of this study enhance our understanding of the reproductive processes of Z. caespitosa and provide important guidelines for the restoration of seagrass meadows. The findings underscore the importance of temperature as a determining factor for seed germination and highlight the need for further research into the ecological requirements of seagrass species in temperate regions.