In this paper spring and autumn temperature thresholds of 5°C and 10°C, active air temperature sums and lengths of growing seasons with temperature threshold of 5°C and 10°C were analyzed. The analysis were made for 10 meteorological stations for the period 1961-2011. The dates of occurrence of phenological phases of selected plants (oxeye daisy, beech, apple, winter wheat and maize) has been investigated. It was observed a tendency of earlier appearance of spring phenological phases and later appearance of autumn phenological phases. As a result extension of vegetation periods in the Republic of Slovenia is present. Dates with occurrence of phenological phases of selected plants from 8 phenological stations were connected with the results of the analysis of meteorological parameters from 8 meteorological stations. The highest correlation coefficients were obtained for the temperature parameters (0.8), while the average precipitation quantities, number of days with frost, potential evapotranspiration and water balance showed a low correlation coefficient values with the appearance of phenological phases.