Corylus is indigenous in the northern hemisphere and comprises about 15 species, which are cultivated as ornamental shrubs, as well as producing the well-known hazelnuts. C. avellana, the common hazel, grows wild in this part of the world. This is a deciduous, monoecious shrub or tree with tough, flexible young shoots (which are used, for example, for dowsing rods), and round to oval, bristly-toothedwith a heart-shaped base.
The male catkins develop early in autumn and remain on the branches in winter, while the female catkins are in the buds on the branches, and only the sticky, bright red stigmas are visible when it is flowering. C. avellana, common hazel, is a broad shrub, 3-5 m tall, with a smooth, brownish-grey bark, young, green shoots covered with red hair, spreading, hairy leaves, 5-15 cm long, which turn yellow in autumn, yellowish-green catkins and pale green fruits turning light brown.
It tolerates the shade. C. avellana ‘Aurca’ has yellow leaves which turn light green andlater. It has smaller fruits which should be cut off completely every 2/3 years; ‘Contorta’ is lower, and has twisting branches, shoots and leaves. C. colurna, Turkish hazel, is up to 20 m tall, with a broad, oval or pyramid-shaped crown, pale grey, cork-like bark which flakes off after a while, and brownish-yellow catkins early in spring. It is resistant to air pollution. C. maxima, filbert, has larger leaves and fruits; ‘Atropurpurea’, the brown hazel, is up to 6 m tall with beautiful, black, reddish-brown leaves, catkins and fruits. It is suitable for growing in slight shade, in normal soil with some lime. The species can be pruned back hard once a year, the cultivars should be pruned only by removing ugly shoots to retain shape. Propagate from (species), by and grafting.