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On the basis of original phenological observations (1955-2009) and meteorological data on Saint Petersburg meteostation the authors study the influence of bioclimatic cyclicity on plants of introduced and native arboreal flora in Saint Petersburg. Time periods are divided into 3 groups: early-warm, normal and late-cold. The bioclimatic data on early-warm and late-cold cycles are calculated. The early-warm and late-cold cycles alternate, with domination of the early-warm cycles under the tendency of climate warming at the beginning of the 21st century. The results of arboriculture should be estimated taking in mind the adaptation of plants in the late-cold cycles, but the seed propagation may give better results in the early-warm cycles. The last ones have been always considered as favourable for plants. But nowadays more and more species produce negative reaction to the earlywarm cycles. In modern conditions the significance of phenological investigations and especially of long uninterrupted observations on arboreal pheno-indicacators becomes more important.