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Over last 130 years the American assh was often used for afforestation of Croatian lowland areas where the native narrow leafed ash (Fraxinus angustifolia Vahl) did not grow well. Due to unequivocal species identification of the American ash F. americana L. or F. pennsylvanica Marshall, pollen samples of concerned specimens were collected. Based on species pollen samples of F. americana and F. pennsylvanica, collected from trees growing at the Royal Botanical Gardens Hamilton in Canada, and on their distinctive morphological features, pollen samples of American ash trees from a plantation Črnovščak in Dugo Selo near Zagreb and from arboricultures in Zagreb were determined as Fraxinus pennsylvanica.

Hermaphroditic flowers blooming unseasonably on an oak tree (»the green oak«) of uncertain hybrid origin prompted a detailed investigation into the biology of the flowering, and the morphology of various traits of the tree, which is growing in an urban area near Zadar, northern Dalmatia, Croatia. Hermaphroditic flowers were also detected on one holm oak tree from Weihenstephan, Germany. Since the first oak tree was described as a hybrid between Q. cerris f. austriaca Q. ilex, scanning electron microscope investigations of pollen and hairs on the leaves of three species were made, on: Q. ilex L., Q. cerris L. and the »green oak«. Oak pollen is of elliptical shape and tricolpate. The ornamentation of the pollen grains of Q. cerris and the »green oak« were similar concerning the ornamentation of the exine – warty and rough and with small rounded protrusions – as well as the size of the pollen grain. Q. ilex pollen was much smaller and the surface lacked the small irregularly shaped protrusions typical of the other oaks.Within samples of holm oak pollen, significant difference in the surface structures was observed. Concerning the different types and number of hairs on the leaf surfaces, Q. ilex revealed a much greater number of stellate hairs on the upper leaf side as compared to Q. cerris and the »green oak«. Simple uniseriate hairs and simple unicellular hairs were missing on the leaves of Q. ilex, but were present on both the other oaks. To sum up the results, the »green oak« and Q. cerris reveal some similarities whereas Q. ilex does not.