Walnut trees are any species of tree in the plant genus Juglans, the type genus of the family Juglandaceae, the seeds of which are referred to as walnuts. All species are deciduous trees, 10–40 metres (33–131 ft) tall, with pinnate leaves 200–900 millimetres (7.9–35.4 in), with 5–25 leaflets; the shoots have chambered pith, a character shared with the wingnuts (Pterocarya), but not the hickories (Carya) in the same family.
The common name walnut derives from Old English wealhhnutu, literally ‘foreign nut’ (from wealh ‘foreign’ + hnutu ‘nut’), because it was introduced from Gaul and Italy.
The two most commercially important species are J. regia for timber and nuts, and J. nigra for timber. Both species have similar cultivation requirements and are widely grown in temperate zones.
Walnuts are light-demanding species that benefit from protection from wind. Walnuts are also very hardy against drought.
When grown for nuts, care must be taken to select cultivars that are compatible for pollination purposes; although some cultivars are marketed as “self fertile”, they will generally fruit better with a different pollination partner. Many different cultivars are available for growers, and offer different growth habits, flowering and leafing, kernel flavours and shell thicknesses. A key trait for more northerly latitudes of North America and Europe is phenology, with ‘late flushing’ being particularly important to avoid frost damage in spring. Some cultivars have been developed for novel ‘hedge’ production systems developed in Europe and would not suit more traditional orchard systems.
Walnuts are very attractive trees in parks and large gardens. Walnut trees are easily propagated from the nuts. Seedlings grow rapidly on good soils. The Japanese walnut in particular is known for its huge leaves, which have a tropical appearance.
Edward Step the horticulturalist had this to say about the The Walnut (Juglans regia).
In the Golden Age, when man lived happily on a handful of acorns, the gods fed upon walnuts, and so their name was Jovis glans—the nuts of Jupiter—since contracted into Juglans. Those who delight in obvious interpretations by appealing to the modern meanings of words similar in construction may be pardoned for supposing that Walnut-trees were formerly trained against walls; but, like many other obvious interpretations, this is wide of the mark. Some have gone back to the Anglo-Saxons for help, and though the result arrived at is in all probability the correct one, it is almost certain that the Anglo-Saxons knew nothing of the matter, and would scarcely trouble to give a name to something they had never seen. The Walnut is a native of the Himalayas, the Hindu Kuh, Persia, Lebanon, and Asia Minor to Greece. The learned Roman, Varro, who was born B.C. 116, and died B.C. 28, mentions it as existing in Italy in his day; and Pliny tells us it was brought thence from Persia. The date of its introduction to Britain is usually set down as about the middle of the sixteenth century, but it was probably at least a century earlier, for Gerarde, writing at the close of the sixteenth century, describes it as a tree commonly to be seen in orchards, and in fields near the highways, where a very new importation was not likely to be found. But to return to the name: there can be little doubt that it is a contraction of Wälsh-nut (in modern spelling, Welsh-nut), meaning foreign. This is German, and while the modern sons of the Vaterland write it Wallnuss (occasionally Wälshenuss), the Dutch form is Wallnoot. That this is the true derivation is made pretty certain by Gerarde, who calls it “Walnut, and of some Walsh-nut.”
That the new importation was fully appreciated in Europe for its fruit may be judged by the extent to which its cultivation had spread in Evelyn’s day, for he tells us the trees abounded in Burgundy, where they stood in the midst of goodly wheat-lands. He says: “In several places betwixt Hanau and Frankfort in Germany no young farmer is permitted to marry a wife till he bring proof that he hath planted and is a father of such a stated number of [Walnut] trees, and the law is inviolably observed to this day, for the extraordinary benefit the tree affords the inhabitants.”
The Walnut is a handsome tree, growing to a height of forty to sixty feet, with a bole twenty feet or more in circumference, and a huge spreading head. The bark is of a cool grey colour, smooth when young, but as the tree matures deep longitudinal furrows form, and it becomes very rugged. The twisted branches take a direction more upward than horizontal, but in early summer they are almost completely hidden by the masses of large and handsome leaves of warm green colour and spicy aroma. I once rejoiced in the occupation of a garden that held two Walnut-trees, and though they had not attained to the fruiting age, their possession was a delight to me; but then I am one of those who enjoy their fragrance, which is unbearable to some persons. The large leaves are formed after the fashion of the Ash-leaf—broken up into a variable number of lance-shaped leaflets with scarcely perceptible teeth.