The length of the annual growing season is the period between the day when the average daily air temperature in spring exceeds the temperature threshold of 5 °C and the day when it drops below this value in autumn.

Air temperature at 5 °C is generally recognised as the lower temperature threshold for plant vegetation, while the period with average air temperatures exceeding the temperature threshold denotes the length of annual cycle of vegetation. The temperature threshold so determined is used as one of the conditions for the classification of agri-ecological zones.

The rise in average air temperatures and the increased frequency of days with temperatures exceeding the vegetation threshold influence the prolongation in the length of annual cycle of vegetation. The latter has a non-homogeneous character, with appreciable deviations within the period in question (i.e. 1961-2007). The 5-year running average indicates a slight cyclic oscillation. The last cycle of 5-year running average values indicates the prolongation in the length of annual cycle of vegetation since the 1990’s, which is also made strikingly evident by the comparison of average values for the periods 1961-1990 and 1991-2007.

The adjustments to the changed length of cycle of vegetation are expressed as gradual changes in agricultural technology, which, owing to the nature of agricultural production, is a lengthy process. In introducing new technologies into agricultural practice, special account is to be taken of environmental factors, the laws of environmental protection and supporting information systems (agri-meteorological data). In accordance with the National Environmental Action Programme, reduction of the burden caused by the changing length of cycle of vegetation (introduction of new varieties) will require upgrading of existing legal and economic instruments for environmental protection (biodiversity, sustainable development), acting in line with the adopted international conventions (Framework Convention of the United Nations on Climate Change, United Nations Convention to Combat Desertification, Convention on Bilogical Diversity), strengthening supervision over sources of environmental hazards, heightening environmental awareness, enhancing the knowledge on environmental issues, and encouraging sustainable forms of agricultural production.