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The 17 species of Corylus occur in the temperate northern hemisphere (Govaerts & Frodin 1998). Hazels are deciduous trees or shrubs with thin, grey bark, with no prominent lenticels, breaking into vertical strips and scales with age. The branchlets are often two-ranked; winter buds are ovoid, with several smooth scales. The leaves are alternate, petiolate and typically have double-serrate margins. Corylus is monoecious. The male inflorescences (catkins) are pendulous and are produced in small racemes on short lateral shoots. They are formed the previous season and exposed in winter; anthesis occurs before the leaves emerge in spring. The female inflorescences are much reduced and are composed of a small cluster of flowers, each within a bract, with only the style protruding from the bud. The involucral bracts are campanulate or tubular with lobed apices and are sometimes spiny; they expand to accommodate the mature fruit. The fruit is a subglobose or ovoid nut (Furlow 1997, Li & Skvortsov 1999).
The hazels are justifiably popular trees or large shrubs, appreciated both for their form and for their catkins in spring, with the hope also (where squirrels are not too abundant) of fruit in autumn. Corylus colurna has become an important street tree, valued for its neat shape and tolerance of urban conditions. It has been hybridised recently with one of the purple-leaved hazels (probably C. maxima ‘Purpurea’) to give progeny with intermediate characters. Of these, ‘Te Terra Red’ is probably the best, with the potential to form a striking purple-leaved tree of C. colurna shape. Another interesting horticultural development is a hybrid between a purple cultivar and the Corkscrew Hazel (C. avellana ‘Contorta’), resulting in ‘Red Majestic’, with both red leaves and twisted stems – sure to become popular with those who like curiosities. All hazels are tolerant of most soil types and are extremely hardy. Softwood cuttings can be attempted, but the success rate is not very good (Dirr 1998); layering is also an option, but most selected clones are grafted on C. avellana or C. maxima. Seed should be used where possible, but there is the risk of hybridisation.
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.