Field cultivation of vegetables in Poland is strictly related to weather conditions. Low temperatures occurring from November to March limit vegetable cultivation in the field. However protecting plants with row covers can prevent losses from untimely frosts and freezes and provide a means to modify the environment around the plant, favorably resulting in more rapid growth, earlier maturity, and possibly increased yields. Experiments were carried out in 2008–2010 at the experimental field of the Agricultural University of Kraków (N 50º4’, W 19º50’) in order to determine the impact of flat covers made from biodegradable nonwovens produced by the Institute of Biopolymers and Chemical Fibres and CENARO in Łódź. Seeds of frost-resistant ‘Glacier’ cultivar of onion were sown in the end of August, and after a significant drop in temperature occurred in the second half of November plants was covered with biodegradable nonwovens such as: Bionolle 59 g m^-2 and 100 g m^-2 and IBWCH 50 g m^-2 and 75 g m^-2, as well as standard nonwoven polypropylene PP 50 g m^-2. A plot that remained uncovered throughout the winter served as the control. The covers were kept until spring and were removed one (2009) and two months (2010) before harvest. During harvest, measurements of plant height and marketable yield were taken. In the laboratory harvested onions were estimated for ascorbic acid, dry matter, soluble sugar and nitrates content. The investigations showed that covering onion with biodegradable nonwoven material had a positive impact on yield in the first year of the experiment. There were no significant differences in the yield quantity in the second year. Contents of the analysed organic compounds showed no regular patterns in both years of the experiment. In 2009, the vitamin C, dry matter and nitrate contents were higher than in 2010. In the second year of the study, the control onions had the most soluble sugars.

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