Juglans regia

• 1. Introduction

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  Juglans regia goes by many names: common walnut, English walnut, Persian walnut. None of them quite captures what this tree actually is. Its Latin name translates as "Jupiter's royal acorn," a title bestowed by the Romans who spread it across their empire. That name is a clue to something worth examining. This is a tree whose fame so outran its origins that even specialists struggle to say where it comes from. Its natural range has been blurred almost beyond recognition by thousands of years of human cultivation. What drove people across continents to carry its seeds? Why does a Greek island hold a folk warning that planting one will kill you? And why did a group of Italian witches choose a walnut tree, specifically, as the site of their most sacred rites? Those questions are worth following.

• 2. A Name Fit for a God

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  Jovis and glans: Jupiter's nut. The Romans who coined the genus name Juglans were not being modest. They reserved the word regia, meaning regal or royal, for the fruit itself, a judgment on its quality rather than its lineage. The Latin common name was nux Gallica, meaning Gallic nut, after the region of Galatia in Anatolia, which sits at the western edge of the tree's presumed native territory.

  In English, the name took a stranger path. The Old English term was walhhnutu, a compound of wealh (foreign) and hnutu (nut). According to the Oxford English Dictionary, it meant the nut of the Roman lands, of Gaul and Italy, as distinguished from the native hazel. So English speakers named it after what it was not: a foreign thing, something that arrived from elsewhere.

  The name "English walnut," used mainly in North America, turns that logic inside out. Alan Mitchell put it plainly: "The Americans call it 'English Walnut', which is plainly an error by the early settlers." Walter Fox Allen's 1912 treatise on the subject confirmed that in America the tree had been called English walnut simply to distinguish it from native black walnut and butternut. Meanwhile, in China it is known as 胡桃, or hu tao, meaning Hu peach, because ancient Chinese associated its arrival with the Hu peoples from the northwest. In Mexico it is called nogal de Castilla, linking its introduction to Spaniards from Castile.

• 3. Origins Obscured by Time

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  Iran is one of the recognized centres of origin and diversity for this species. Another lies further northeast, in Central Asia. But pinning down the tree's true original range is a task that has humbled botanists for generations. Its cultivation history stretches back possibly as long as two thousand years in parts of west Asia and southern Europe, and that longevity has overwritten the natural record.

  Genetic diversity within the species is patchy and revealing. The highest diversity has been found around Sariosiyo in Uzbekistan. The forests of Arslanbob in Kyrgyzstan, now thought to be of cultivated origin, show conspicuously lower diversity despite their size. A native glacial refugium population with high genetic diversity survives on the southern fringes of the Alps in northeast Italy. The fossil pollen and nut record adds more questions than answers. Taken together, the evidence suggests the tree survived the last glaciations in several refugia, most likely in southern Europe, the Near East, China, and the Himalaya.

  The earliest evidence of walnuts in the Levant comes from wood remains dated to the Middle Bronze Age, discovered in northern Israel. The tree appears once in the Hebrew Bible. By the fourth century BC, Alexander the Great was carrying what the Greek botanist Theophrastus called the karuon he Persike, the Persian nut, from Iran and Central Asia into Macedonian territory. Those early introductions hybridized with local terminal-bearing forms to produce trees with lateral fruiting and larger fruit. It was this superior type that the Romans then spread through southern Europe and northern Africa.

• 4. The Anatomy of the Tree

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  The tallest accurately measured specimen of Juglans regia stands in the Lagodekhi Protected Areas of Georgia, at 29 metres. The stoutest, measured in Spain, reaches 9 metres in girth. Unverified reports from Kyrgyzstan put wild specimens at 34 metres; a cultivated tree in Britain reportedly reached 32 metres. In cultivation, mature trees may reach 50 feet in both height and width and live more than two hundred years, developing massive trunks exceeding 8 feet in thickness.

  The bark shifts character as the tree ages: olive-brown and smooth when young, then silvery-grey with broad fissures on older branches. Cut a twig and look at its cross-section and you will find chambered pith, creamy-white at first, turning brown with age. This structure of air-filled chambers within the twig is a trait shared by all species of Juglans. The leaves are large and odd-pinnate, usually seven leaflets arranged in opposing pairs with one terminal leaflet; the three at the apex are the largest. They open late in spring, typically mid-May in Britain, first appearing red-brown before settling into their mature dark yellow-toned green by mid-June.

  Male flowers appear as drooping catkins; female flowers come in small clusters of two to five and ripen into what most people think of as a walnut: a green, semi-fleshy husk enclosing a brown, corrugated nut. In wild populations the shell is thick. Centuries of selection by cultivators have produced thinner-shelled varieties. The whole fruit, husk included, falls together in autumn.

• 5. The Ecology of Exclusion

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  Other plants often will not grow beneath walnut trees. The mechanism is juglone, a chemical found in the fallen leaves and husks. It acts as a natural herbicide, clearing the ground around the tree and reducing competition. Horses that eat walnut leaves may develop laminitis, a serious hoof ailment.

  The tree is also a host to Rhagoletis juglandis, the walnut husk fly, which lays its eggs inside the husks of the fruit. Not all cultivars suffer equally. The varieties 'Eureka', 'Klondike', 'Payne', 'Franquette', and 'Ehrhardt' are among the most susceptible, linked to differences in husk softness or thickness.

  As a species, Juglans regia demands light. It requires full sun to grow well and tends to grow taller and narrower when competing for canopy space in dense forest. Its closest relative is Juglans sigillata from western China and the far northeast of India, which differs mainly in having nine to eleven leaflets rather than five to nine, and a much thicker, harder shell.

• 6. Witches, Warnings, and Folk Memory

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  On Skopelos, a Greek island in the Aegean Sea, a local legend holds that whoever plants a walnut tree will die as soon as the tree can see the sea. Most planting on the island is done not by people but by field rats of the subfamily Murinae. In Flanders, the same idea takes a blunter form. A folk saying translates as: "By the time the tree is big, the planter surely will be dead." Both sayings reflect a plain biological fact: the walnut is a slow grower and a late fruiter. A person who plants one for its nuts is planting for their grandchildren.

  Benevento in southern Italy holds a darker tradition. The city is the historical home of an ancient practice of stregoneria, Italian witchcraft. Witches from across Italy were reputed to gather at a sacred walnut tree in Benevento for the Witches' Sabbath. In 1526, a judge named Paolo Grillandi wrote of witches in Benevento who worshipped a goddess at the site of an old walnut tree. That written record is nearly five hundred years old, and it captures a tradition that was already ancient by the time Grillandi set it down.

  The legend did not stay local. In 1812, it inspired the ballet Il Noce di Benevento, meaning the walnut tree of Benevento, composed by Salvatore Viganò and Franz Xaver Süssmayr. Niccolò Paganini later adapted a theme from that ballet into a violin piece called Le Streghe. The Beneventan liqueur Strega still depicts the famous walnut tree with witches dancing beneath it on its label.

• 7. Wood, Dye, and Nutrition

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  Freshly cut live walnut heartwood can appear Dijon-mustard in colour, darkening to brown within a few days. Dried lumber settles into rich chocolate-brown to black, with cream to tan sapwood. The grain can produce unusual decorative patterns, known among woodworkers as curly, bee's wing, bird's eye, and rat tail. It is prized for its durability, lustre, and chatoyance, that shimmering depth of figure, and is used in high-end flooring, furniture, veneers, guitars, gunstocks, and handles.

  The green husk surrounding the nut is rich in a yellow-brown to dark-brown dye. Anyone who has shelled walnuts knows this firsthand: the fingers stain strongly brown and stay that way. An extract from the husks can dye cloth, and the dye carries notable antibacterial properties.

  The kernel itself is nutritionally dense. In a 100-gram reference amount, walnut kernels provide 654 calories. They are 65% fat and 15% protein. Manganese, phosphorus, magnesium, zinc, and copper are all present in amounts exceeding twenty percent of the daily value. The fat profile leans heavily polyunsaturated, at 49%, with monounsaturated fats at 10% and saturated fats at 6%.

• 8. A Global Crop

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  In 2022, world production of shelled walnuts reached 3.9 million tonnes. China accounted for 36% of that total, making it by far the dominant producer. The United States, Iran, and Turkey were the next largest contributors. California's central valley, with its rich, deep soil and long summers, is the heart of American production. Other major growing regions include France, Serbia, Greece, Bulgaria, Romania, and Hungary in Europe; Baja California and Coahuila in Mexico; and Chile in Latin America.

  The tree is cultivated extensively between 30 and 50 degrees of latitude in the Northern Hemisphere and between 30 and 40 degrees in the Southern Hemisphere. In recent years cultivation has expanded into New Zealand and southeastern Australia. J. regia germplasm reached China roughly two thousand years ago from Central Asia; European colonists brought it to the Americas in the seventeenth century. The cultivar group known as Carpathian walnuts refers not to the species as a whole but to selected cold-tolerant strains with high-quality nuts. Juglans hindsii and its hybrid with J. regia are commonly used as grafting stock to support cultivated trees, a practice that reflects how thoroughly the walnut has been shaped by human hands since Alexander the Great first carried it west from Persia.