ABSTRACT
Juglans are mainly restricted to the northern hemisphere; however, there is an incursion southward via South America into the southern hemisphere, where species such as J. australis, J. boliviana, J. neotropica and J. venezuelensis are found. Taxonomic studies of the genus have identified four sections: Cardiocarya, Juglans, Rhysocaryon and Trachycaryon. Walnuts are notoriously difficult to root from cuttings; consequently, most of the nut producing varieties, and some species for which seed is rarely obtainable, are propagated by grafting. Apart from budding in the field during the summer, which is successfully carried out in areas with high summer temperatures, grafting is restricted to the dormant season. Walnuts are characterised by wood which can contain a large volume of pith, within which are chambers or voids. Walnut grafts using rootstocks other than Juglans regia failed due to the development of a black line at the point of the union and subsequent death of the scion.
