The cultivation of sugar beet as a winter crop harvested in autumn of the next year is expected to contribute to a marked yield increase. Sown in autumn the plants have to survive frost during the winter. The study thus aimed to characterize the optimal growth stage, in which maximum winter hardiness is reached, and to determine the lowest temperature sugar beet plants can survive in this optimal growth stage. Furthermore, the importance of weather conditions (temperature, snow) in relation to the growth stage of the plants was assessed with a PCA (principle component analysis). From 2009 to 2012 field trials with 5 genotypes at 3 locations in Germany were conducted, which were accompanied by greenhouse experiments with controlled frost experiments. The survival rate after winter was mainly affected by the environment (year × location, 93%), while the genotype effect (1%) was rather low. An optimal growth stage for maximum survival was determined at a thermal time after sowing of 600–900°Cd (base temperature 3°C). The greenhouse experiments revealed that in this optimal growth stage the plants survived a minimum temperature of −7°C (−6°C to −8°C). In the field trials, the impact of the growth stage reached before frost (46%) on the survival rate after winter was considerably higher than the actual weather conditions during winter (17%). In particular too much advanced growth(dry matter yield of root and leaves, root diameter) resulted in a high susceptibility for frost damage. Regarding the weather conditions, the number of frost days with snow and the minimum temperature during winter without snow had the highest importance for survival. The knowledge of the required thermal time to reach maximum winter hardiness can be used to optimize the sowing date of autumn sown beets in different environments. However, a conflict may occur between the aim to obtain optimal winter hardiness and to reach maximum yield in the next year.