It is commonly accepted that the vegetative period is the part of the year during which the mean daily air temperature attains at least 5°C. The report here summarized refers to the study based upon the monthly averages of air temperature from the period 1931-1990, measured at nine weather stations, representing various geographical conditions in Poland. The average dates of the beginning and end of the vegetative period confirm the regularities in the spatial distribution of these characteristics over the area of Poland, which are known from the literature of the subject. The vegetative period starts, generally, at the turn of April, and ends at the turn of November (Tables 1 and 2, Figs. 1 and 2). Length of the vegetative period ranges from 190-200 days in the north-eastern Poland to more than 230 days in the western part of the country. It is the shortest in the mountains, lasting approximately 185 days (Table 3, Figs. 1 and 3). During the 60-year period under study the earliest start of the vegetative period in the lowland part of Poland occurred on January 30th (in 1990), while the latest - on April 25th (1941, 1955). The respective extreme dates for the mountainous areas were March 25th (1934) and May 5th (1980). Thus, within the area of Poland it may happen that vegetation starts to grow already in the last week of January, but it may also start to grow as late as in the first week of May. The dates of the end of the vegetative period for the lowland Poland (Table 2, Figs. 1 and 2) range from October 9th (1946, 1976) to December 3rd ( 1951 ). In mountains the vegetative period ended the earliest on the last day of September ( 1931) and the latest - on November 18th (1963). Thus, the end of the vegetative period may occur in Poland between the last week of September and the first week of December. The length of the vegetative period (Table 3, Figs. 1 and 3) ranges from 170 days (1941) to 291 days - the latter being the absolute maximum of the period under study- in 1990. Hence, the difference between the extreme lengths of the vegetative period amounts to 121 days. In the mountains, though, the shortest vegetative period lasted 159 days (1931), while the longest - 221 days ( 1934). The analysis of the coefficients of correlation (Table 4) of the dates of the beginning and end of the vegetative period with its length showed that the length of this period is more strongly linked with the date of beginning than with the date of end. This is most probably connected with the fact that the starting date of the vegetative period in consecutive years displays greater variability than the ending date. Equations of linear regression (Table 5) indicate that the length of the vegetative period increases at the rate of 1 to 3 days per decade. This is is most probably connected with the acceleration of the start of vegetation by approximately 0.5 to 1.5 days per decade, coupled with the delay of its termination by approximately 0.5 to 1.5 days per decade. The analysis of the periodical changes (Fig. 4) showed that the 7-year cycle of the starting dates of vegetation is worth special attention, along with the 3- and 4-year cycles of the termination dates. The true length of the 7-year cycle for the start of the vegetative period gets bigger as we go from the west (7.6 years) to the east (7.9 years) of the country. The spectra of oscillations of the growing season's end are, on the other hand, characterized by high uniformity over the area of Poland.
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