Persian walnut, English walnut, common walnut, California walnut (Eng), nogal, noguera (Spa), noguer, noguera (Cat), intxaurrondoa (Baq), nogueira (Glg), nogueira (Por).
“On Saint Giles's day, shake the walnut tree”; “On Saint Urban's day harvest your walnuts”; “On Saint Matthew's day winnow the walnuts”.
(Spanish popular sayings)
As its specific name indicates, the walnut is a regal, majestic plant, that can be up to 25 or 30 m tall, and which has a broad crown that casts a deep shade under the canopy. The bark is silvery-grey and longitudinally fissured. The leaves are deciduous, alternate and compound with 5 to 9 leaflets, always in an uneven number (odd-pinnate). They are oval and have an entire margin. The male and female flowers are separated on the tree; the female blooms are inconspicuous, but the male blossom is showier and arranged on long hanging stems known as catkins, which favour the wind-dispersion of pollen. The fruits are walnuts. They are surrounded by a fleshy green cover that dries out when it is ripe, becoming brown or purpley in colour.
The walnut prefers valley bottoms with deep rich soils, as long as they are not too acidic or flooded, so that it can better develop its powerful root system. It withstands the cold well, but does not like late frosts, excessive drought or severe pruning. In places where walnut trees grow, it is rare to find other plants beneath its shade: there is an allelopathic effect, i.e., the remains of fallen leaves contain tannins and other compounds that inhibit the germination and growth of other species.
There is much controversy over the origin of the walnut. The pollen fossil record indicates the presence of this species after the ice age, suggesting that it took refuge in some areas and later recolonised the Iberian Peninsula, aided by humans and even hibridising with cultivated varieties, but disappearing in its wild form. The other possibility is that it completely disappeared and was later reintroduced. What is sure is that the Romans expanded its cultivation, principally for its fruit. According to various authors, this tree is considered to be a native of southeast Asia or China, and some people believe that there are still representatives of these ancient varieties on the Iberian Peninsula. Today it is cultivated throughout the majority of the territory.