Citizen science data reveals winter warming delays cherry bloom in the Pacific Northwest, USA

Societal Impact Statement Climate change is altering ecological systems, including the phenology of flowering plants. Shifts in the bloom date of cherry trees are a global concern considering their cultural, agricultural, and horticultural importance. Ornamental cherry is a prominent component of the University of Washington campus (Seattle, USA), providing an opportunity to engage volunteers in citizen science that quantifies shifts in bloom in the Pacific Northwest. These phenological shifts affect horticultural systems, cherry festival planning, and synchrony with pollinators, with direct implications on local economies, tourism, and landscape resilience for future generations. Summary We monitored cherry bloom date on the University of Washington campus, Seattle, USA, in three flowering cherry species and cultivars to develop a predictive model for estimating bloom, and to quantify changes in bloom date in Somei‐yoshino cherry ( Prunus x yedoensis ) between 1966 and 2024. We worked with citizen scientists to record bloom date, and for Prunus x yedoensis , bloom phases, using ArcGIS Field Maps between 2012 and 2024. We also examined newspaper archives to reconstruct the observed bloom date for Prunus x yedoensis prior to 2012. We used a published modeling framework to develop species‐ and cultivar‐specific models to predict bloom. We observed different thresholds for chill and heat requirements across flowering cherry species and cultivars, and general congruence between observed and model‐predicted bloom dates for each. Using the longer time series in Prunus x yedoensis , we observed that warmer winters slowed the accumulation of required chill units, while warmer springs led to required heat units accruing more rapidly. The net effect of warming winters and springs resulted in a delay in the bloom date of Prunus x yedoensis by ~2 days per decade between 1966 and 2024. Shifts in the bloom date of flowering plants could result in phenological asynchrony with pollinators, with cascading effects across ecosystems. Further research is needed to understand the complex responses of flowering plants to shifting climatic conditions.