In the world production of hazels, there is no a generally accepted hazel cultivar as a universal pollinator, but cultivar composition is rather determined based on cultivar characteristics, which are manifested in given growing conditions. One of the most important parameters that determines the cultivar composition of hazel is the knowledge of
reproductive biology of cultivars, which have numerous specificities such as allogenic
fertilization and incompatibility. Besides the above, the process of pollination
and fertilization of hazel is further complicated by the different dynamics of cultivar flowering
and discrepancies in terms of the period of flowering. All these factors influence the cultivars in
orchard to have a low level of fruit set and yields are below expectations. As a contribution to
the knowledge of phenology of hazel flowering the flowering of several hazel cultivars was observed
in the area of Banja Luka region. The observation was carried out over the period of two
years at the site of Jošavka (Čelinac) on ‘Romai’, and ‘Tonda Gentile Romana’ cultivars and on
two clone cultivars of ‘Istrian Long’ with different phenological characteristics. All cultivars
were grafted onto the Corylus colurna L. rootstocks. The results of research show that temperature
fluctuations during the phenophase of flowering, significantly affect the initiation and length of
hazel flowering, as well as the inter-pollination of cultivars in the orchard.