The hazels (Corylus) are a genus of deciduous trees and large shrubs native to the temperate northern hemisphere. The genus is usually placed in the birch family Betulaceae, though some botanists split the hazels (with the hornbeams and allied genera) into a separate family Corylaceae.
They have simple, rounded leaves with double-serrate margins. The flowers are produced very early in spring before the leaves, and are monoecious, with single-sex catkins, the male pale yellow and 5–12 cm long, the female very small and largely concealed in the buds, with only the bright red 1–3 mm long styles visible. The seeds are nuts 1–2.5 cm long and 1–2 cm diameter, surrounded by an involucre (husk) which partly to fully encloses the nut.
The shape and structure of the involucre, and also the growth habit (whether a tree or a suckering shrub), are important in the identification of the different species of hazel.
The nuts of all hazels are edible. The Common Hazel is the species most extensively grown for its nuts, followed in importance by the Filbert. Nuts are also harvested from the other species, but apart from the Filbert, none is of significant commercial importance.
A number of cultivars of the Common Hazel and Filbert are grown as ornamental plants in gardens, including forms with contorted stems (C. avellana 'Contorta', popularly known as "Harry Lauder's walking stick" from its gnarled appearance); with weeping branches (C. avellana 'Pendula'); and with purple leaves (C. maxima 'Purpurea').