Walnut and its close relatives are broadly distributed across North and South America, Europe, and Asia. Spanish missionaries first introduced English walnut (Juglans regia) to California in the 1700s and commercial walnut production began in the mid to late 1800s (Leslie and McGranahan 1998). Almost all walnuts grown for their nuts in the United States are produced in California, although a small proportion of walnuts are also grown in the Midwest. In 2012, 627,921 tons of walnuts were produced on 312,323 acres in California with an estimated value of $1,802,586,000 (CA Ag. Comissioners' Rpt 2012)).
Walnuts belong to the genus Juglans within the family Juglandaceae. Other well-known relatives in the Juglandaceae family include pecan, wingnut, and hickory. Most species within the Juglans genus can be hybridized, and are valued for their wood and/or nuts. The English walnut, (Juglans regia) also known as Persian walnut, is the most commonly grown walnut for nut production. The Northern California black walnut (Juglans hindsii) and the hybrid ‘Paradox’ (J. hindsii x J. regia) are used primarily as a rootstock for the English scion.
In California, walnuts flower in late spring, from April through early May. Walnut is a deciduous monoecious tree with separate male (catkin) and female (pistillate) flowers. Pollen is wind dispersed. Pollen dispersal and pistillate flower receptivity occur at different times on the same tree, and cultivars vary substantially in mean flowering times. As a result, nut set is improved by planting at least two cultivars with overlapping pollen dispersal and female flower receptivity. Nuts mature throughout the summer and are harvested in the fall from mid-September through early November.
Walnuts grow best in deep, well drained, fertile soils. Because walnut has deep roots, orchard locations should have at least 5-6 feet of permeable soil. Shallow soils, or areas with high water tables, should be avoided when planting an orchard. The English walnut thrives in a moderate, temperate climate and has limited tolerance to low temperatures in winter. Early fall frosts or severe winter freezing can kill entire branches and result in substantial damage. The structure of young trees can be severely damaged if the scaffolds are killed by cold temperatures. Late spring frosts can reduce fruit set by damaging flowers or young nuts.