The aim of our thesis was to evaluate, in how many days the lettuce plants reached the marketable mass of rosette of 300 g. In recent years, many merhants decided to sale lettuce per piece not per weight. For that reason, it is important to know, how much time cultivars of lettuce plants need to reach desired rosette mass. Three lettuce cultivars of rosette lettuce type (Lactuca sativa L. var. crispa) were included in our research. Lettuce plants were planted in 2 periods (spring-summer and sumer), in both period on the open field. Plants were started to harvest, five weeks after the plants were transplanted on the field. Plants were collected every two day, every time aproximatelly 15 plants per cultivar. Each plants were weighted 3 times: the whole plants, above ground part of plants and marketable part of plants. Cultivar 'Susybel' grew faster at the beginning of the growing period, but stoped with growth when temperature increased up to 30 °C. The yield of cultivar 'Susybel' was 27 t/ha in the first growing period and 28 t/ha in the second growing period. Cultivar 'Aquarel' showed the best results regarding the growth during the period with higher temperatures. The cultivar 'Aquarel' is suitable for spring and summer growing periods. The yield of cv. 'Aquarel' was 28 t/ha in the first and 29 t/ha in the second growing period. Cultivar 'Myrtel' seems similar to cultivar 'Aquarel' in terms of the increasing plant mass during the growing period. It has grown well in both growing periods, but the marketable mass of rosettes were lower in comparison with both other cultivars. The yield of cultivar 'Myrtel' was 26 t/ha in the first and 27 t/ha in the second growing period. The result of our experiment showed that lettuce plants need 6 weeks in the spring growing period and 7 weeks in summer growint period to produce rosette mass of 300 g.
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