Juglans regia (the Common walnut, Persian walnut, or English walnut), is the original walnut tree of the Old World. It is native in a region stretching from the Balkans eastward to the Himalayas and southwest China. The largest forests are in Kyrgyzstan, where trees occur in extensive, nearly pure walnut forests at 1,000–2,000 m altitude (Hemery 1998)—notably at Arslanbob in Jalal-Abad Province.
Juglans regia is a large deciduous tree attaining heights of 25–35 m, and a trunk up to 2 m diameter, commonly with a short trunk and broad crown, though taller and narrower in dense forest competition. It is a light-demanding species, requiring full sun to grow well.
The bark is smooth, olive-brown when young and silvery-grey on older branches, with scattered broad fissures with a rougher texture. Like all walnuts, the pith of the twigs contains air spaces, the chambered pith brownish in colour. The leaves are alternately arranged, 25-40 cm long, odd-pinnate with 5–9 leaflets, paired alternately with one terminal leaflet. The largest leaflets the three at the apex, 10–18 cm long and 6–8 cm broad; the basal pair of leaflets much smaller, 5–8 cm long, the margins of the leaflets entire. The male flowers are in drooping catkins 5–10 cm long, the female flowers terminal, in clusters of two to five, ripening in the autumn into a fruit with a green, semi-fleshy husk and a brown corrugated nut. The whole fruit, including the husk, falls in autumn; the seed is large, with a relatively thin shell, and edible, with a rich flavour.
The wood is of very high quality, and is used to make furniture and rifle stocks.
Walnut ink, made by boiling the whole fruit or letting it oxidize, then releasing onto the exterior, is dark brown in color and darkens as it oxidizes. It can also be used to stain wood.
Other names include Walnut (which does not distinguish the tree from other species of Juglans), Common Walnut and English Walnut, the latter name possibly because English sailors were prominent in Juglans regia nut distribution at one time. In the Chinese language, the edible, cultivated walnut is called 胡桃 (hú táo in Mandarin), which means literally "Hu peach," suggesting that the ancient Chinese associated the introduction of the tree into East Asia with the Hu barbarians of the regions north and northwest of China.
In Flanders, the saying (in Dutch) goes "Boompje groot, plantertje dood", meaning "By the time the tree is big, the planter sure will be dead". The saying refers to the relatively slow growth rate of the tree.