We have studied in alder the pollen differentiation as a marker for the dormancy to obtain some useful indications which could be useful for forecasting the beginning of the pollen season. In central Italy, the meiotic division occurred at the end of September and the first haploid mitosis divisions were monitored at the beginning of November. From this time the pollen volume decreased on average by 12% reaching the minimum value at the beginning of December, before increasing again in the second half of the month, and then decreasing when anthesis occurred in January. During this period the lengths of the equatorial and polar axes changed in different ways. The beginning of pollination happened when the chilling requirement to break dormancy was fulfilled with 740 chill-units, and 42 growing degree days were accumulated. Pollen volume and P/E ratio could mark the various periods of temperature requirements for flowering to occur.