In this work we deal with the importance and use of phenology. With the help of short-term observations we are able to determine the influence of the weather on the development of vegetative and generative plant organs. We focused on monitoring the onset and duration of selected vegetative (leaf bud spikes, leafing, leaf yellowing, leaf decay) and generative (blooming) phenological phases in the woods of Corylus avellana L., Prusnus spinosa L., Cerasus avium L. and Quercus robur in Detvianska kotlina during the years 2016 - 2018. We used meteorological data from the nearest weather station Vígľaš-Pstruša to evaluate the phenological manifestations of forest trees in relation to weather conditions with a focus on the occurrence of temperature extremes. For the area of the Detvianska basin, we have created a phenological calendar that shows 10%, 50% and 100% onset of phenological phases of leaf bud release, leafing, flowering and yellowing of leaves.