The hazels are well-marked deciduous trees and shrubs, with alternate, toothed leaves. Male and female flowers are borne on the same plant. The pendulous male catkins are borne in clusters of two to five; each scale subtends a single flower with four to eight bifid stamens and two bracts. Female inflorescences bud-like, the upper scales each subtending a partial inflorescence of two flowers, each with a bract and two bractlets; ovary surmounted by two free styles, only the red tips of which protrude at flowering time. Fruit a nut, surrounded by a leafy involucre made up of the much enlarged bract and bractlets.
In gardens the hazels are chiefly known as bearing edible nuts, viz., cobnuts and filberts. The common species have not much to attract planters for ornament alone, although in February when they are freely hung with the graceful, slender, yellow, male catkins, they have that charm in great degree which even the humblest flower possesses to some extent at that early season. The female flowers, too, sometimes give a quite effective red haze in sunshine. C. colurna is a handsome tree, and the newer C. chinensis is of similar although possibly not so robust habit. C. cornuta and C. sieboldiana var. mandschurica have remarkable fruits. The attention of those who admire purple shrubs may be directed to C. maxima ‘Purpurea’.
They all thrive well in a loamy soil, and are very suitable for chalky districts. The sorts grown for their fruit are most fertile on soil of moderate quality. In this country C. colurna needs some attention to ensure the formation of a good clean trunk by watching, and, if necessary, training up the leading shoot, and removing lower branches and suckers. As to propagation, most of the hardy sorts can be increased by taking off the suckers; if these do not form, layering should be adopted, and for the genuine species seed is usually obtainable. They bear transplanting well.
The species of Corylus are very much alike in leaf, and are best distinguished by habit and by the form of the husk.