The effects of increasingly pronounced climate changes on pedunculate oak forests are manifested in decreased vitality, dieback, onset of pests and diseases and the absence of acorn yield. For the past several years acorn yield has either been poor or partial, making it insufficient for management needs. For this reason, in 2023 the company Croatian Forests Ltd announced a tender for the transfer of forest reproductive material of pedunculate oak, including seeds and seedlings, from the European Union countries to the Republic of Croatia. Scientific research in the field of forest seed production, as well as the use of quality seeds, are increasingly gaining importance because the preservation of the natural regeneration cycle, as well as the conservation of the gene pool and biological diversity, set challenges to forest experts, both in science and in practice. In the clonal seed orchard “Plešćice I”, the time of entry of clones into the phenophase of bud elongation was monitored from 2014 to 2018. According to the obtained data, all clones were classified into three clusters or groups by cluster analysis: early-flushing, intermediary-flushing and late-flushing. The clones were divided into two groups according to the position of parent plus trees in the stands. Acorn samples of all clones were collected from the 2015 harvest and seedlings were grown under controlled conditions. The acorn from the group of early-flushing clones had the highest average values of the morphological traits that indicate size: width, volume and mass of one acorn, whereas the average values of acorn length were equal to the values of the intermediary clone group. The average values of these traits in the late-flushing clone group were the lowest, while the difference with the other two groups were statistically significant for acorn length, volume and mass. In the case of acorns of late-flushing clones, which had the smallest size, germination was slower compared to the other two groups. Although no significant differencewas found between the groups in terms of average values of the root collar diameter, root system mass in dry condition, and Dicken's quality index, which indicate seedling quality, the group of early-flushing clones attained higher average values of these variables in comparison with the other two groups. A correlation between the width, volume, and mass of one acorn and mass of the root system in dry condition for all clones and for the majority of the groups, was positive and weak, but statistically significant. By separately sowing pedunculate oak acorns of different sizes and acorns originating from early-flushing and intermediate or late-flushing clones from the clonal seed orchard “Plešćice I”, it is possible to grow seedlings of the desired age adapted to certain environmental conditions, provided the correct sowing density is applied. The introduction of high-quality forest reproductivematerial in thequalified categoryto appropriate sites is one of the important prerequisites for successful regeneration and quality of the future stand.