The growing season of Poland in the changing climate based on phenological observations

The growing season is the period when weather conditions (e.g., precipitation, temperature, wind, etc.) in a given area support plant growth and development. This study examines how climate change has influenced the growing season duration in Poland over the past nearly 80 years, based solely on phenological observations. The research was conducted for two 15-year periods: the pre-warming period (1946-1960), before significant global climate warming became evident, and the warming period (2007-2021), characterized by the additional influence of the "greenhouse component" on climate trends. The former dataset was sourced from the Yearbooks of Phenological Observations, which had not been previously available to a wider audience and were digitized by the authors for this study. The latter dataset was obtained from the database of the Institute of Meteorology and Water Management-National Research Institute. The results indicate an increasingly earlier onset of the vegetation period and a slight delay in its end over time, leading to an extended growing season. Today, as a result of climate change, its duration has increased to over 240 days in the west, while in the central lowlands and the Lublin Upland, it has reached 220-230 days. However, it remains almost unchanged along the coast and in northeastern Poland. Due to its location in a transitional temperate climate zone, Poland experiences high weather variability, which is also reflected in fluctuations in the start dates and duration of growing seasons.