Allium ursinum, a dominant herb layer species in nutrient rich, deciduous forests of Central Eu-rope, has considerably expanded in the past decades. As this species mainly relies on regeneration by seeds, we wanted to analyse if and how the timing of phenological phases, climatic factors and resource availability correlated with flowering intensity as a potential factor for its success. We rec-orded annual inflorescence numbers in a population of the Göttinger Wald from 1981 until 2017 and conducted phenological observations across the same time span. In addition, inflorescences were counted in a fertilization experiment contrasting a control with a nitrogen, a phosphorus and a nitro-gen + phosphorus fertilization variant from 2010 to 2017. To investigate the effect of disturbance, inflorescence numbers were counted two to four years after a local summer storm in the southwest-ern foothills of the Harz Mountains in different disturbance categories after windthrow. Our results demonstrate that the length of the growing period for A. ursinum was a strong predic-tor of the number of inflorescences produced in the following year from 1981 to 2017. Climatic factors that had a significant effect on inflorescence numbers were all linked to growing period length. The effect of an increasing trend in growing period length on inflorescence numbers of A. ursinum, was, however, not directly obvious through an increasing flowering intensity with time. Instead, an exceptionally long growing period in 1991 synchronized the flowering behaviour of the population leading to years with a very high flowering intensity. These recurrent high flowering years may have facilitated the expansion of A. ursinum e.g., by increasing the probability of successful seedling establishment or of an accidental dispersal by ungulates. Results of the fertilization experi-ment indicate that atmospheric nitrogen deposition alone does not lead to an increase in inflorescence numbers. Highest numbers were recorded with a combined application of nitrogen and phosphorus. Inflorescence numbers also benefited from an increase in light availability after windthrow but de-creased in the third year after disturbance. Nevertheless, the high reproductive effect after disturb-ance might enable the persistence of this closed forest specialist against other competitors. Overall our results indicate that recurrent high flowering years induced by long growing periods and a high resource availability including light, phosphorus and nitrogen may have influenced the successful expansion of A. ursinum in the past decades and can explain new occurrences and the efficient gap filling in already A. ursinum-rich stands.