Walnut trees span a surprisingly wide swath of the planet, from the mountain valleys of Central Asia to the hardwood forests of the eastern United States and the cloud forests of the Andes. The genus Juglans contains roughly two dozen species, and their collective range covers parts of every continent except Antarctica and Australia (though Persian walnuts are commercially grown in Australia and New Zealand today). Understanding where walnuts grow means looking at two overlapping maps: the wild, native ranges where different species evolved, and the much broader cultivated footprint that human trade and agriculture have created over millennia.
An Ancient Lineage With Deep Roots
Walnuts are old. Members of the genus Juglans are classified as Tertiary relict tree species, meaning they trace back to forests that existed tens of millions of years ago, before ice ages reshaped the planet’s vegetation zones.1Journal of Systematics and Evolution. Reconstructing evolutionary history of Chinese walnuts (Juglans) Fossil evidence shows walnuts once covered a much larger range across the Northern Hemisphere, and the species alive today are essentially survivors that held on in scattered refugia while glaciers advanced and retreated. That deep history helps explain why wild walnut species pop up in such geographically disconnected places: each modern species descends from populations that were stranded in favorable pockets and then evolved in relative isolation.
The Persian Walnut’s Central Asian Homeland
The walnut most people eat, the Persian or English walnut (Juglans regia), is native to the mountainous interior of Central Asia. Its original range stretches across a broad arc from the western Himalayas through Afghanistan, Iran, and into the Tien Shan and Pamir-Alay mountain systems of Kyrgyzstan, Tajikistan, and western China. Genetic studies point to the western Himalaya as the likely core region of Persian walnut genetic diversity.2PubMed Central. Population genetic insights into the conservation of common walnut (Juglans regia) in Central Asia Iran and Afghanistan are widely considered among the main centers of origin and domestication.3Nature / Scientific Reports. Genome-wide patterns of population structure and association mapping of nut-related traits in Persian walnut populations from Iran using the Axiom J. regia 700K SNP array
Within this homeland, wild walnut populations are not spread evenly. Population genetics work has identified distinct groupings in the Ili Valley of Xinjiang (China), the Dushanbe region of Tajikistan, and several sites in Kyrgyzstan, including the famous Sary-Chelek and Arslanbob forests.4PubMed Central. Historical spread routes of wild walnuts in Central Asia shaped by man-made and nature Some of these populations appear to be truly wild, while others, like certain populations in Xinjiang, may have been introduced by humans centuries ago.2PubMed Central. Population genetic insights into the conservation of common walnut (Juglans regia) in Central Asia Sorting out which walnut groves are genuinely wild and which were planted by ancient peoples turns out to be one of the trickier puzzles in walnut biogeography.
How Trade Routes Rewrote the Map
The Persian walnut’s global distribution today owes as much to human history as to climate. Genetic analysis combined with historical and ethno-linguistic records shows that ancient trade routes, including the Persian Royal Road and the Silk Road, enabled long-distance dispersal of J. regia from Iran and the Trans-Caucasus into Central Asia, and from western to eastern China.5PubMed Central. Ancient Humans Influenced the Current Spatial Genetic Structure of Common Walnut Populations in Asia Traders carried walnut seeds along caravan corridors, and favored trees were planted in new territories for food, oil, and timber. The Romans brought walnuts westward across the Mediterranean and into what is now France, Spain, and Italy. Later, Spanish and English colonists carried them to the Americas.
This human-assisted dispersal means the current spatial genetic structure of Persian walnut populations across Asia still carries the imprint of ancient trade networks. Populations that sit along known Silk Road corridors tend to share more genetic material with distant Iranian or Caucasian populations than with their geographic neighbors, a pattern that makes no sense without human movement as the explanation.
Where Persian Walnuts Are Grown Commercially Today
Persian walnuts are now cultivated commercially across a remarkably wide geographic range that includes west-central Asia, southern Europe, North and South America, Australia, and New Zealand.3Nature / Scientific Reports. Genome-wide patterns of population structure and association mapping of nut-related traits in Persian walnut populations from Iran using the Axiom J. regia 700K SNP array China is the world’s largest producer by volume, followed by the United States (almost entirely in California’s Central Valley), Iran, Turkey, and Chile. Major European producers include France, Romania, and Ukraine.
What unites these far-flung growing regions is climate. Persian walnuts need a temperate zone with warm, dry summers for nut development and enough winter cold to satisfy their chill requirements. Without a certain number of cool hours during dormancy, the trees leaf out erratically and set fewer nuts. Researchers in Iran have been working to identify genotypes with lower chilling requirements, because climate change is expected to reduce winter chill in many traditional growing regions over the coming decades.6Europe PMC. Exploring low-chill genotypes of Persian walnut (Juglans regia L.) in west of Iran Finding walnut varieties that can thrive with less winter cold is becoming a priority for breeding programs worldwide.
This chill requirement also explains why Persian walnuts do not grow well in the tropics, even at elevation. They need a genuine winter. The equatorial limit of commercial production roughly follows the subtropical belts, with Chile and parts of southern China at the southern edge and California, Turkey, and northern Iran at the northern core.
Eastern Black Walnut in North America
The eastern black walnut (Juglans nigra) is the best-known native walnut of North America and occupies one of the broadest natural ranges of any walnut species. It grows as scattered individual trees or small groups throughout the central and eastern United States. Its range stretches from western Vermont and Massachusetts westward through New York to southern Ontario, across central Michigan and southern Minnesota, and into eastern South Dakota and northeastern Nebraska. The southern boundary runs through western Oklahoma and central Texas, then curves east to northwestern Florida and Georgia, though it skips much of the Mississippi River Delta.7PubMed Central. Black Walnut
Black walnut grows best on deep, well-drained soils in coves and river bottoms, particularly across the Appalachians and the Midwest. It is valued as much for its dark, richly figured timber as for its thick-shelled nuts, and prime black walnut logs can fetch thousands of dollars. The tree is also famous for producing juglone, a chemical compound released from its roots, leaves, and hulls that suppresses the growth of many nearby plants. This allelopathic trait shapes what can grow in the vicinity of a black walnut and has made the tree both admired and cursed by gardeners and farmers alike.
Other North American Walnut Species
Black walnut gets most of the attention, but North America is home to several other Juglans species, each with a more limited range.
Butternut (Juglans cinerea), sometimes called white walnut, historically ranged across much of the northeastern United States and southeastern Canada, overlapping with the northern portion of black walnut’s territory but extending into cooler, more northerly terrain. Butternut has been in steep decline. Across its range, butternut populations dropped roughly 58 percent between two major forest inventory periods, falling from about 22 million trees to around 10 million.8PubMed Central. Regional patterns of declining butternut Juglans cinerea L. suggest site characteristics for restoration The primary culprit is butternut canker disease, a fungal infection that has devastated populations throughout the species’ range.9Canadian Journal of Forest Research. Susceptibility to butternut canker disease in field trials is influenced by disease level and hybridization Butternut is now considered a species of conservation concern in both the United States and Canada, and researchers are exploring whether hybridization with Japanese walnut (a related Asian species sometimes planted as an ornamental) might offer some disease resistance, though hybrids complicate the goal of preserving genetically pure butternut populations.
Arizona walnut (Juglans major) occupies a very different niche. It is native to the American Southwest and Mexico, where it grows primarily in riparian areas, near springs, and in other spots with shallow groundwater. You find it threading along canyon bottoms in mixed deciduous and pine-oak woodland rather than forming the dense stands that black walnut sometimes achieves in rich Midwestern bottomlands.10Fire Effects Information System. Juglans major, Arizona walnut Its range extends from central Arizona and New Mexico south into the Sierra Madre of Mexico. The nuts are small and hard-shelled but still important food for wildlife.
California black walnut (Juglans californica) and Hinds’ walnut (Juglans hindsii) fill out the remaining North American picture. J. californica grows in the coastal ranges and inland valleys of southern California, while J. hindsii is restricted to scattered sites in northern California. Both face habitat pressure from urban development. J. hindsii is widely used as rootstock for commercial Persian walnut orchards in the state.
Walnuts in East Asia
East Asia has its own cluster of walnut species, the most widespread being the Manchurian walnut (Juglans mandshurica). This cold-hardy tree grows across northern and northeastern China, the Korean Peninsula, and parts of Japan. Phylogeographic studies using both nuclear and chloroplast DNA have revealed that J. mandshurica survived the last ice age in at least two separate refugia in northern China, one in the northeast and one further west, rather than retreating to the subtropical south as many temperate trees did.11PubMed. Nuclear and chloroplast DNA phylogeography reveal two refuge areas with asymmetrical gene flow in a temperate walnut tree from East Asia This finding was somewhat surprising and suggests that pockets of suitable habitat persisted at relatively high latitudes even during peak glaciation.
China is also home to several other walnut species and subspecies, some of which are themselves considered Tertiary relics.1Journal of Systematics and Evolution. Reconstructing evolutionary history of Chinese walnuts (Juglans) These include Juglans cathayensis and regional variants found in subtropical mountain forests. The evolutionary history of Chinese walnuts is complex, with ancient hybridization events and long periods of geographic isolation creating a mosaic of related forms that taxonomists still debate.
Walnuts in South America
The Southern Hemisphere’s native walnut representation is concentrated in the tropical Andes. The Andean walnut (Juglans neotropica) is a long-lived, deciduous broadleaf tree that grows in tropical montane cloud forests along the Andes Cordillera, from Venezuela through Colombia, Ecuador, and into Peru and Bolivia.12IAWA Journal. Exploring monsoon precipitation signal in the Peruvian tropical montane cloud forest: Andean Walnut wood anatomy plasticity Unlike its temperate-zone relatives, J. neotropica grows in warm, moisture-rich environments at middle elevations, typically between about 1,000 and 3,000 meters above sea level. Its timber, known as “nogal” in Spanish, is prized for furniture and carving. The species is considered threatened across much of its range due to logging and habitat conversion.
This Andean presence is biogeographically interesting because it represents a deep branch of the walnut family tree. The genus Juglans is thought to have dispersed into South America millions of years ago via the Central American land bridge, and J. neotropica has been evolving in relative isolation from its Northern Hemisphere cousins ever since. It is not a tree that was introduced by Europeans; it was already well established before human contact.
Climate Change and Future Range Shifts
The map of where walnuts can grow is not static. Climate modeling for Persian walnuts in Europe predicts a northward shift of suitable habitat, with simultaneous losses at the southern edge of the current range driven by increasing temperature seasonality. The magnitude of the projected change varies considerably depending on the climate model and emissions scenario used, with predicted range contraction ranging from about 17 percent to as much as 85 percent of the current European distribution area.13Regional Environmental Change. Possible changes in spatial distribution of walnut (Juglans regia L.) in Europe under warming climate The wide spread in those projections reflects genuine uncertainty, but the direction of change is consistent: walnut-friendly conditions are creeping north while southern Mediterranean areas are becoming too hot and dry.
For growers, the practical question is whether new northern territories will warm up fast enough to compensate for losses in the south. Scandinavia and the British Isles have already seen experimental Persian walnut plantings that would have seemed impractical a generation ago. At the same time, traditional walnut regions in southern France, Spain, and parts of Iran may see yields decline as winters become too mild to satisfy chill requirements. The breeding work on low-chill genotypes mentioned earlier is a direct response to this trend.6Europe PMC. Exploring low-chill genotypes of Persian walnut (Juglans regia L.) in west of Iran
Diseases That Are Redrawing Walnut Boundaries
Disease is an active force in shaping where walnut trees persist. In North America, Thousand Cankers Disease (TCD) has emerged as a serious threat to black walnut, particularly in the western United States where the species was planted well outside its native range. TCD involves a partnership between a tiny bark beetle and a fungus; the beetle bores into branches and carries the fungus with it, and the fungus produces numerous small cankers that eventually girdle and kill the tree. The disease was first formally described in Colorado, where it eliminated black walnut along the state’s Front Range.14Plant Health Progress. Black Walnut Mortality in Colorado Caused by the Walnut Twig Beetle and Thousand Cankers Disease
TCD has since been detected in the eastern United States, including Tennessee, where tree mortality and decline have been observed in multiple counties.15Forests. Initial Assessment of Thousand Cankers Disease on Black Walnut, Juglans nigra, in Eastern Tennessee The concern is whether the disease will spread more widely into black walnut’s native heartland in the Midwest and Appalachians, where the species has enormous ecological and economic value. Research into why western populations have suffered more severe mortality suggests the explanation is not simply a more aggressive fungal strain in the west. Instead, environmental conditions like prolonged drought, higher beetle population densities, and possibly multiple introductions of the fungus from different source locations all contribute to worse outcomes in western states.16Frontiers in Forests and Global Change. Differential Virulence Among Geosmithia morbida Isolates Collected Across the United States Occurrence Range of Thousand Cankers Disease
Butternut canker disease, discussed earlier, is similarly reshaping the effective range of butternut in the Northeast. Together, these two diseases illustrate a broader principle: the map of where a tree species “grows” can shrink rapidly when a new pathogen arrives, especially when stressed trees are already contending with drought or marginal soils.
Wild Walnut Forests and Why They Matter
Some of the most ecologically valuable walnut populations on Earth are the remaining wild forests of Central Asia, particularly the Arslanbob walnut forests of Kyrgyzstan. These represent one of the world’s largest natural walnut ecosystems, and they harbor genetic diversity that is critical for breeding programs.17Mitteilungen Klosterneuburg: Journal of Viticulture, Oenology, Pomology and Fruit Processing. Pre-selected walnut (Juglans regia L.) genotypes from the Arslanbob forest: Pomological characterization and identification of promising types Commercial walnut orchards worldwide rely on a narrow genetic base, the product of centuries of selection for large nut size, thin shells, and high yields. Wild forests contain genotypes adapted to drought, cold, disease, and a range of soil conditions that could prove invaluable as growing conditions change.
These forests are threatened by climate change and habitat loss, and their conservation is complicated by the fact that local communities depend on them for income. Walnut harvest is a major livelihood activity in the Arslanbob area, and finding ways to protect genetic diversity while supporting rural economies is an ongoing challenge. Similar conservation concerns apply to wild walnut populations in Iran and Afghanistan, which sit in regions where political instability can make long-term scientific monitoring and forest management difficult.
Genetic studies of these wild populations have produced some useful surprises. Researchers screening trees in the Arslanbob forest have identified individual genotypes with especially promising combinations of nut quality, disease tolerance, and growth habit that could be incorporated into breeding stock.17Mitteilungen Klosterneuburg: Journal of Viticulture, Oenology, Pomology and Fruit Processing. Pre-selected walnut (Juglans regia L.) genotypes from the Arslanbob forest: Pomological characterization and identification of promising types The forest is, in effect, a living library of walnut genetics, one that took millions of years to accumulate and could be lost within a few human generations if current pressures continue unchecked.
Why Walnuts Do Not Grow Everywhere
Given how widespread the genus is, it is worth noting the gaps. Walnuts are absent from tropical lowlands, sub-Saharan Africa, and the humid equatorial belt of Southeast Asia. The constraints are mostly climatic: walnut trees need either winter cold (in temperate species) or high-altitude cool seasons (in the Andean species) to complete their annual growth cycle. Persistently hot, wet conditions without a distinct dry or cool season are not favorable. Soil drainage matters too. Most walnut species grow best in deep, well-drained alluvial or loamy soils and perform poorly in waterlogged or heavy clay ground.
Salt tolerance is another limitation. Walnut roots are sensitive to saline soils, which restricts their range in arid and semi-arid lowlands even where temperatures would otherwise be suitable. This is part of why walnut cultivation in Central Asia tends to hug mountain valleys and irrigated piedmont zones rather than spreading onto the dry plains below. Altitude provides both cooler temperatures and better-drained, less saline soils, creating the conditions walnut trees evolved to exploit.
Human land use also shapes the practical map. Wild walnut populations were once more continuous across Central Asia and the Caucasus, and were gradually fragmented by agriculture and grazing. In the eastern United States, black walnut was probably more common in pre-settlement forests than it is today; early European settlers cleared vast tracts of bottomland hardwood forest, and much of what remains is regrowth on sites that are no longer ideal. The range of a wild species on a modern map often understates the range that species would occupy if left alone.