The hazelnut plant, belonging to the genus Corylus, is the source of the popular edible nut. Identification can be challenging because the term “hazelnut tree” encompasses species that grow in different forms, ranging from multi-stemmed shrubs to single-trunked trees. Hazels are deciduous plants native to the Northern Hemisphere and are members of the birch family, Betulaceae. Accurate identification requires examining its overall structure, the characteristics of its leaves and bark, and its unique reproductive features.
General Growth Habit and Size
The majority of commercially grown and wild hazelnut varieties, such as the common hazel (Corylus avellana), exhibit a multi-stemmed shrub form. These plants grow as bushes with several trunks sprouting from the base, often reaching a height between 10 and 26 feet. This shrubby growth pattern, which includes the production of numerous basal suckers, is the natural habit of many hazel species.
For commercial production, plants are sometimes trained into single-trunked tree forms. If left un-coppiced, the common hazel can reach heights of up to 49 feet. Some species, like the Turkish Hazel (Corylus colurna), naturally grow as a single-stemmed tree and can reach heights of up to 80 feet, functioning as true shade trees. Even in the tree form, the overall structure is broad, maintaining a relatively dense canopy.
Identifying Features of Leaves and Bark
Hazelnut leaves are a primary feature for visual identification, growing in an alternate arrangement along the stem. They are broad and nearly round, measuring between 2.5 and 4.5 inches long and across. A distinctive characteristic is the doubly serrated margin, meaning the edges have large teeth that contain smaller teeth.
The leaf surface is soft to the touch due to tiny, downy hairs on both the upper and lower sides. The underside is often a paler green, with prominent veins. The bark is generally smooth and grey-brown when young. As the plant matures, the bark may develop shallow fissures or a slightly peeling texture, but it does not form the deeply furrowed appearance of trees like oak.
Flowers and Nut Structure
The reproductive structures of the hazelnut are unique because the male and female flowers appear separately on the same plant, a condition known as monoecious. The male flowers, called catkins, are the most visible seasonal feature, appearing early in the year before the leaves emerge. These are long, pale yellow, pendulous structures that dangle from the branches, resembling lambs’ tails, and can be 2 to 5 inches long.
The female flowers are much smaller and less conspicuous, often looking like tiny buds. The only visible part is a cluster of bright red stigmas, which are receptive to wind-blown pollen and protrude from the bud. The fruit is the nut itself, which develops inside a leafy sheath called the involucre, or husk.
The appearance of the involucre is a key feature for distinguishing between different hazelnut species and cultivars. For the common hazel, the husk is short and leafy, enclosing about three-quarters of the nut. Other species produce involucres that are long and tubular, completely enclosing the nut. The nut itself is roughly spherical to oval, growing to about 0.5 to 1 inch in diameter.