The cultivation of Miscanthus has attracted growing interest despite its yield instability. Therefore, understanding what causes such instability is of primary interest for breeding. Our objectives were to estimate the genetic parameters-genetic variance and genetic heritability-and genetic correlations for flowering time-related traits in a biparental Miscanthus sinensis diploid population, and divide the year effect into age and growing season effects using a staggered-start design. The population was established with single plants organized with this design and consisted of two genotype groups established twice in a same field, in 2014 and 2015, with a total of 159 genotypes and 82 common genotypes between the groups. Soil conditions being identical between both stands, the growing season conditions corresponded to climatic conditions. All plants were extensively phenotyped for different panicle and anther emergence traits in 2018 and 2019. All traits were delayed by 3 weeks in 2019 compared to 2018, which was explained by climatic conditions that occurred before the floral transition, mainly a 3 degrees C decrease in temperatures. When dividing the year effect, the genotype x growing season interaction was much higher than the genotype x age interaction. This increased the genotype x growing season interaction variance compared to the genotype x age interaction variance: the growing season effect decreased the genetic parameters for all flowering time-related traits, up to 20% for broad-sense heritability. Interestingly, most traits responded similarly to this effect. Therefore, M. sinensis breeding for flowering time must be conducted under contrasted climatic conditions to select more stable genotypes.