The hazelnut is the fruit of shrubs or small trees belonging to the botanical genus Corylus. The species most often associated with commercial production is the common hazel, Corylus avellana, which is native to Europe and Western Asia. Hazelnuts are highly valued in global cuisine and remain a popular ingredient in confectionery, baking, and spreads worldwide.
Identifying the Hazelnut Source Plant
Hazelnuts originate from the Corylus genus, classified within the birch family, Betulaceae. The plants are typically multi-stemmed, deciduous shrubs, though they can be trained to resemble small single-trunked trees. Each plant is monoecious, producing separate male and female flowers on the same individual. Male flowers form long, pendulous catkins in the autumn and shed their pollen in the late winter or early spring.
The female flowers are largely inconspicuous, appearing as small buds. The resulting fruit is a hard-shelled nut that grows in small clusters. Each nut is encased in a protective leafy husk, known as an involucre. In species like Corylus avellana, the involucre covers a portion of the nut. The terms “hazelnut” and “filbert” are often used interchangeably, but filberts, such as those from Corylus maxima, are sometimes distinguished by a husk that fully encloses the nut. The nut typically falls out of its involucre about seven to eight months after pollination when it is fully ripe.
Global Cultivation and Climate Requirements
Hazelnut plants thrive in temperate climates characterized by cool winters and warm summers. A fundamental requirement is a sufficient period of cold temperatures, known as chilling hours, which is necessary for the plant to break dormancy and flower properly. The plants are also highly susceptible to damage from late spring frosts, which can destroy the delicate flowers and developing nuts.
Commercial cultivation relies heavily on specific geographical regions that meet these climatic needs, with wind being the primary agent for pollination. Turkey is the world’s leading producer. Significant production also occurs in other areas with Mediterranean climates, including Italy, and in the United States, primarily in Oregon’s Willamette Valley. Hazelnuts grow best in well-drained, loamy soils and require consistent moisture during the nut development phase, often necessitating irrigation in drier growing areas.
Key Varieties of Hazelnut Plants
While the genus Corylus contains numerous species, commercial production is dominated by a few specific types. The European Hazel (Corylus avellana) is the foundation of the modern hazelnut industry, prized for its large, flavorful nuts and for being the parent of many cultivated varieties. This species is typically grown as a multi-stemmed shrub, but its cultivars can be trained into a tree form for easier mechanical harvesting. It is widely used in global breeding programs despite its susceptibility to Eastern Filbert Blight.
Another species contributing to the commercial stock is the Giant Filbert (Corylus maxima), which is native to southeastern Europe. This species is known for its elongated nuts and a husked involucre that is significantly longer than the nut itself. Many of the older European cultivars are hybrids of C. avellana and C. maxima. North American species, such as the American Hazelnut (Corylus americana), generally produce smaller, thick-shelled nuts but offer superior cold hardiness and resistance to diseases like Eastern Filbert Blight. These native species are now being utilized in breeding programs to develop hybrid varieties that combine the large nut size of the European hazel with the disease resistance of the American hazel.