Objective – to reveal the direction of the shift in the timing of the beginning of flowering of tree species in the conditions of the dendropark under the influence of changes in climatic conditions in the period 2008–2017 vs 1960–1969 as well as to conduct a comparative analysis of different methods of selecting of temperature indexes during recalculating of positive temperatures into effective ones and to determine the effect of individual winterspring months on the phenodata of the onset of the flowering of the species. Material and methods. The subject of the researches – three groups of species of woody plants that differ in terms of flowering: with the beginning of flowering in March–April, May and June. To characterize the thermal regime of the periods studied were used the air temperature parameters of Prilukskoy meteorological station. Data processing was carried out using the computer program “Excel”. Observation of the phenological phases of development of woody plants in the period 2008–2017 were conducted as per method of L.S. Plotnikova (1972). The statistical data processing of these phenological observations was carried out according to the method of G.N. Zaitsev (1981) in the modification of V.I. Ivlev (2014). Results. The conducted researches established that over the past 10 years in Chernigov region the annual air temperature has changed towards warming: the average annual air temperature increased by 1.8 °C, in winter – by 2.7 °C, in spring – by 2.4 °C, in summer – by 1, 6 °C, in autumn – by 0.7 °C. The greatest warming was observed in March – by 3.9 °C. The increasing in air temperature leads to the movement of the beginning of flowering in the direction of acceleration, which in the dendropark conditions varies within 2–15 days. The optimal option for recalculation of positive temperatures into effective ones is the formation of the sum of effective temperatures by a smooth mathematical function. A close inverse correlation was found between the sums of the effective temperatures in some months and their combinations and the phenodata of the beginning of the flowering of tree species. Conclusions. An analysis of the results shows that the formation of the sum of the effective temperatures necessary for the blooming of the species occurs from the beginning of the year. The greatest influence is manifested in the month preceding the month of flowering of the species.