a. Common Hazel or Corylus avellana
The common hazel is a small tree, native to Europe and Western Asia. It is characterized by multiple stems, and the younger stems are usually bronze in color, while the mature stems are brown. The buds are oval-shaped and green in color. The nuts of the common hazel can be oval or spherical in shape, and they can be about 12 to 20 mm broad and 15 to 20 mm long.
b. Filbert or Corylus maxima
The filbert looks quite similar to the common hazelnut. It is classified as a deciduous shrub and is native to Southeastern Europe and Southwestern Asia. The tree typically reaches a height of about 6 to 10 m. The leaves are almost round with a double-serrated margin, just like the leaves of the common hazel. The filbert can be distinguished from the common hazel by its fully-enclosed nuts and long involucres.
Varieties can be gathered in three groups, belonging to the three following subspecies:
1. Corylus avellana racemosa Lam.: Hazelnuts grouped in clusters, round and very voluminous, often netted. ‘Santa Maria de Jesús", ‘San Juan", ‘San Nicolás", and ‘San Cono’.
2. Corylus avellana glandulosa Lin.: Hazelnut with acorn shape, variable size, with a cone shape of the narrower base and acuminate apex. Not a very hard shell. ‘Ghiannusa, ‘Cannellina, ‘Minnulara, ‘Panuttara, Baccilara, ‘Muddisi, ‘Piattiddara, common hazel, ordinary hazel, hazel of Spain, etc.
3. Corylus avellana Máxima Lam.: Spheric or round shape, bulky or medium fruit, hard shell. Commonly known as Napolitano hazel. ‘Badara ubertosa, ‘Cerro, ‘Pigra, ‘Privitera, ‘Balzanotto, ‘San Giorgio, ‘Reganati, ‘Rizo, ‘San Elmo, hazel of England, netted hazel, etc.