Seventeen species of the genus Corylus are found across the northern temperate hemisphere; they are all deciduous trees or shrubs. Members of the genus are monoecious (separate male and female flowers on the same plant). Corylus species have, justifiably, become popular garden and landscape plants; appreciated for their form, their female catkins in the spring, the hope of fruit in the autumn and their decorative male catkins through late winter.
Introduced to Britain in the mid-sixteenth century, Turkish hazel is native to south-east Europe and Asia Minor, but it was cultivated over a much wider area before 1580. Although quite slow growing, the Turkish hazel forms a large, single-stemmed conical tree, reaching up to 25 metres. As it matures, an attractive pale, exfoliating bark develops, making it ideal for dealing with particulate pollution in the urban environment, where it is valued also for its neat shape and tidy crown. Its pinkish-brown hardwood timber is of fine quality and is good for cabinet making; even the decorative root can be used for inlay work and veneers. Turkish hazel also makes an excellent shade tree, partly because of its very dense foliage and narrow crown.
The leaf is broadly oval in outline with a pointed apex, heart-shaped base and a margin that is coarsely double toothed or almost lobed. The upper surface is dark green, whilst the lower surface is downy along the midrib and veins. Early in the year the leaf stalk is covered in glandular hairs, which are lost later in the season. The edible fruits, which are true nuts, form in ball-like clusters, usually in threes. Each fruit is about two centimetres long, flat with a very thick shell and surrounded by a cup (involucre) of fleshy tissue. The involucre of the Turkish hazel is deeply cut and covered with many glandular hairs; the generic name means 'helmet', a reference to the involucre. Corylus nuts are used directly in cooking, whilst their oil is used for the manufacture of soap and paint, and nut-chocolate confectionary.
Corylus x colurnoides, a hybrid of our native hazel (Corylus avellana) and Turkish hazel, has a shrubby habit, but sometimes becomes a tree (20 metres in the Muenster Botanic Garden, Germany). The bark of the hybrid tends to be less corky, more fissured and only somewhat peeling on young plants compared to either of the parents. The hybrid is noted for its fine-tasting nuts.
Erdogan V and Mehlenbacher SA 2000. Interspecific hybridization in hazelnut (Corylus). 125: 489-497.
Whitcher IN and Wen J 2001. Phylogeny and biogeography of Corylus (Betulaceae): inferences from ITS sequences. Systematic Botany 26: 283-298.