The paper covers the issues of the formation of the generative organs of switch-grass plants (Panicum Virgatum L.) in the correlation with the varietal features and the weather conditions in an inter-phase period of “flowering-seed formation”. Switch-grass is one of the most promising perennial cereal plants to produce bio-fuel. Widespread introduction into production is held back by low seed emergence, which is due to its large dormancy state. Therefore, to study the causes of this phenomenon and the ways to reduce it is of a real relevance. In the vegetation years, the size of pollen grains ranged from 22.9 to 23.6 mkm depending on the varietal features and the weather conditions in a phase of flowering and pollen formation. A significant difference in a pollen size in a correlation with the varietal features was not recorded; the tendency towards the increase or decrease of its size was seen though. In the years under study, the sizes of pollen grains changed under the effect of weather conditions, it occurred even within one cultivar. The pollen of all the cultivar samples had the smallest size in 2018 and 2019; and in the vegetation years of 2020-2021 the average pollen sizes were much larger for all cultivars than in 2018 and 2019. Over the years under study, on the average, no significant difference in the sizes of embryo and seed, depending on the varietal features, was recorded. The growing weather conditions had a significant effect on the sizes of embryo and seeds, their length and width, and it amounted to 98-100%. The most favorable weather conditions for the processes of flowering and seed formation were created in 2019, as compared with those in 2020 and 2021, which ensured the formation of larger sizes of both embryos and seed in all the cultivars. The correlation-regression analysis of the data showed a strong linear correlation between the length and the width of embryo and the weather conditions in a phase “flowering-formation” of seed with a determination coefficient R2=0.9163 and a correlation coefficient R=0.96 for the length of embryo and R2=0.9613 and a correlation coefficient R=0.98 for the width of embryo.