Fagus sylvatica L. (Fagaceae), European beech, is one of the most important deciduous tree species in Central Europe and the most common broadleaved tree species in Germany. We investigated the leaves of six individual Fagus sylvatica trees growing in a beech forest in Kiel, Schleswig-Holstein, Germany, for seasonal variations in the content of phenolic natural products over three consecutive growing seasons. The investigated compound classes comprised hydroxycinnamic acid and flavonoid derivatives. The content of phenolic compounds showed clear trends in all years. A sharp decline in the total content of phenolic substances was observed from mid-April to the end of May. During the summer months, the content of phenolic compounds remained stable with only slight fluctuations until fall. The values for individual trees deviated more pronouncedly from one another in spring, but converged during the course of the growing period. These trends, despite differences in absolute values, were identical in three consecutive growing seasons (2016–2018). Our results contribute to a better understanding of the dynamics of plant natural products of deciduous trees in temperate climates caused by seasonal variations. Display omitted •Phenolic natural products were monitored over three consecutive growing seasons.•Hydroxycinnamic acid and flavonoid derivatives were assessed separately.•After a sharp decline in spring, contents of phenolics compounds remained stable until fall.•Sharp decreases of phenolics in spring were observed in all three years.•The observed fluctuations are discussed in comparison to literature data.