also called Hazel, any of about 15 species of shrubs and trees constituting the genus Corylus in the birch family (Betulaceae) and the edible nuts they produce. The former common name for the genus was hazel; various species were termed filbert, hazelnut, or cobnut, depending on the relative length of the nut to its husk. This distinction was found to be misleading, and filbert became the common name for the genus in the U.S. The term cobnut is limited to a commercial variety of one species; the Jamaican cobnut has a similar flavour but is an unrelated plant of the family Euphorbiaceae. The terms hazel and hazelnut, however, are still in popular use.
Filberts, native to the North Temperate Zone, are deciduous; their leaves are alternate, serrate, obovate, and hairy. The plants range from 3 to 36 metres (10 to 120 feet) in height. In late winter a profusion of yellow male catkins and smaller, red-centred clusters of female flowers appears on the same tree. The brown, roundish or oblong nut, usually one to four centimetres (1/2 inch to 1 1/2 inches) long, is partly or wholly enclosed in a husk.
Choice nuts are produced by two Eurasian trees, the European filbert (Corylus avellana) and the giant filbert (C. maxima), and by hybrids of these species with two American shrubs, the American filbert (C. americana) and the beaked filbert (C. cornuta), popularly called hazelnuts. The large cobnut is a variety of the European filbert; Lambert's filbert is a variety of the giant filbert. Nuts produced by the Turkish filbert (C. colurna) are sold commercially as Constantinople nuts. Barcelona nuts come from the Spanish, or Barcelona, filbert, usually considered a variety of the giant filbert. Turkey, Italy, and Spain are the leading commercial producers of filberts.
California, Chinese, Japanese, Manchurian, Tibetan, and Turkish filberts are valuable hedgerow and ornamental trees. Both C. avellana and C. americana are grown for their colourful autumnal foliage. An oil from the European filbert is used in food products, perfumes, and soaps; the tree yields a soft, reddish white timber, useful for small articles such as tool handles and walking sticks.
Filberts are deep-rooted, moderately shade-tolerant trees, which fruit best in well-drained soil and in full sun.