Juglans regia

Juglans regia, known by various common names including the common walnut, English walnut, or Persian walnut amongst other names, is a species of walnut. It is native to Eurasia in at least southwest and central Asia and southeast Europe, but its exact natural area is obscure due to its long history of cultivation.

The species has numerous cultivars which produce the edible walnut consumed around the world and produced predominately in China. It is widely cultivated across temperate regions throughout the world including those of Eurasia, Australia, and the Americas.

The genus name, Juglans, is derived from two Latin words, jovis, which means Jupiter, the chief god of the ancient Roman religion; and glans meaning an acorn or nut. The specific epithet, regia, is defined as regal or royal in references the quality of the fruit and edible nuts.

Juglans regia is a large deciduous tree, attaining heights of 10–25 metres (35–80 feet), and a trunk up to 2 m (6+1⁄2 ft) in diameter, commonly with a short trunk and broad crown. The tallest accurately measured specimen is in the Lagodekhi Protected Areas of the country of Georgia, 29 m (95 ft) tall, and the stoutest in Spain, 9 m (30 ft) in girth. Heights of 34 m in the wild in Kyrgyzstan and 32 m in cultivation in Britain have been reported, but not verified.

The bark is smooth, olive-brown when young and silvery-grey on older branches, and features scattered broad fissures with a rougher texture. Like all walnuts, the pith of the twigs contains air spaces; this chambered pith is creamy-white at first, becoming brown in older twigs. The leaves are alternately arranged, 20–45 cm (8–17+1⁄2 in) long, odd-pinnate with (3–)5–9 (most often 7) leaflets, arranged in opposite pairs with one terminal leaflet. The largest leaflets are the three at the apex, 10–20 cm (4–8 in) long and 6–10 cm (2+1⁄2–4 in) broad; the basal pair of leaflets are much smaller, 5–8 cm (2–3 in) long. The margins of the leaflets are entire, but with toothed margins on seedlings, and in the cultivar 'Laciniata'. The leaves open fairly late in the spring (typically mid May in Britain), and are red-brown at first, becoming their mature dark yellow-toned green by mid June.

The male flowers are in drooping catkins 5–10 cm (2–4 in) long, and the female flowers are 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 with the husk breaking open; the seed is a large and edible nut with a rich flavour; the shell is thick in wild populations, while most cultivated plants have been selected for a thin shell.

One of the centres of origin and diversity of J. regia is Iran, and another further northeast in Central Asia. In Central Asia, genetic diversity is very patchy, with the highest diversity found around Sariosiyo in Uzbekistan, and conspicuously lower in the Arslanbob forests in Kyrgyzstan (these now thought to be of cultivated origin). Another native glacial refugium population with high genetic diversity is found on the southern fringes of the Alps in northeast Italy. However, as with other old and widespread cultivated plants, it is not easy to reconstruct the original distribution and determine the borders of the past natural ranges. There are many reports concerning the earliest fossil pollen and nuts of J. regia, and the conclusions that various authors draw are somewhat contradictory. Taken together these finds suggest that J. regia possibly survived the last glaciations in several refugia, as the compilation of the data shows most likely southern Europe, the Near East, China, and the Himalaya.

The largest forests are in Kyrgyzstan, where trees occur in extensive forests at 1,000–2,000 m (3,300–6,600 ft) altitude, notably at Arslanbob in Jalal-Abad Province.