A walnut is the edible seed of a tree from the genus Juglans, most commonly the Persian or English walnut (Juglans regia). This seed is technically a drupe-like nut, enclosed in a husk that is discarded after ripening. Cultivation has a history spanning several thousand years, tracing back to Central Asia, particularly ancient Persia and India. The ancient Greeks and Romans valued the nut, with the Romans referring to it as the “royal nut of Jupiter.”
Global Centers of Walnut Cultivation
Walnut production is highly concentrated, with a small number of countries dominating the global supply chain. China is the leader in production worldwide, contributing a substantial volume to the international market. Major Chinese growing regions, such as Yunnan, Xinjiang, and Shaanxi, benefit from ideal climate and soil conditions for extensive cultivation. The United States consistently ranks as the second-largest producer globally, followed by Iran and Turkey, which are also major international players.
Iran holds a historically significant position, as the Persian walnut (Juglans regia) originated in its territory and was historically known as the “Persian Nut.” Iran’s production is notable for walnuts with thin shells and rich flavor, grown in provinces like Kerman and Hamedan. Turkey also contributes a large volume to the world’s supply, relying on its diverse climate and rich soil to produce high-quality nuts.
Other important international producers include Mexico, Ukraine, Chile, and Romania. Chile, in particular, has become a major global exporter, benefiting from its Southern Hemisphere location to supply fresh walnuts during the Northern Hemisphere’s off-season.
North America’s Primary Growing Regions
Commercial walnut production in the United States is highly centralized in a single state. More than 99% of the walnuts grown in the country originate from California’s Central Valley, the epicenter of domestic English walnut production. Commercial cultivation began in the mid-to-late 1800s after Spanish missionaries introduced the English walnut to California in the 1700s. The Central Valley provides the necessary microclimate, characterized by hot summers and cold winters, which allows for consistent yields.
California growers utilize specific, high-yielding varieties like ‘Chandler’ and ‘Hartley’ to meet the demand for large, light-colored kernels preferred by the export market. The vast, deep, and fertile soils of the valley allow the trees’ extensive root systems to develop without obstruction. The native Black Walnut (Juglans nigra) is primarily found across the eastern United States. This native species is harvested on a smaller scale, mainly for its distinctive flavor and highly prized wood.
Environmental Needs for Walnut Trees
Walnut cultivation depends on a specific set of environmental conditions. Walnut trees require a period of winter dormancy, satisfied by accumulating “chill hours,” or hours spent at temperatures between 32°F and 45°F. Most commercial varieties require between 400 and 1,500 chilling hours to properly break dormancy and ensure consistent bud formation and flowering in the spring. If the chill requirement is not met, the tree may experience delayed bud break, resulting in a reduced fruit set and lower production.
Temperature sensitivity is also a factor during the growing season. Late spring frosts are the greatest threat, as they can damage young buds and flowers, while extreme summer heat above 100°F can cause leaf scorch and compromise nut quality. The ideal soil for commercial production is a deep, well-draining loam that provides ample space for the large root system to penetrate, ideally at least three feet deep. Optimal soil pH ranges from slightly acidic to neutral, between 6.0 and 7.5.