Phenology – the timing of seasonal biological events – is a sensitive indicator of climate change and ecosystem dynamics. Long-term, broad-scale phenological data are crucial for understanding and predicting plant responses to environmental change. However, until the mid-2000s, European phenological observations were scattered across national networks, limiting large-scale analyses. In response, the Pan European Phenology (PEP725) database was established 15 years ago as an open-access, reference-grade infrastructure for plant phenology data. PEP725 unifies observations from over 30 countries, compiled from 1868 through the present, with all records standardized to a common protocol. The database now contains more than 13 million phenological records for c. 265 plant species across 46 phenophases, making it the world's largest repository of ground-based plant phenology data. We highlight key scientific insights and cross-sector applications enabled by the dataset, and share technical lessons learned. Looking ahead, we outline a roadmap for PEP725's evolution – including new data contributions, technological upgrades, global integration, and community engagement – to ensure it remains a vibrant, open community resource driving phenology science forward. We invite the plant science community to utilize, contribute to, and further cocreate this phenological data platform.