The hazels (Corylus) are a genus of deciduous trees and large shrubs native to the temperate northern hemisphere. The genus is usually placed in the birch family Betulaceae,Germplasm Resources Information Network: CorylusChen, Z.-D. et al. (1999). Phylogeny and evolution of the Betulaceae as inferred from DNA sequences, morphology, and paleobotany. Amer. J. Bot. 86: 1168–1181. Available online.Rushforth, K. (1999). Trees of Britain and Europe. Collins ISBN 0-00-220013-9.Huxley, A., ed. (1992). New RHS Dictionary of Gardening. Macmillan ISBN 0-333-47494-5. though some botanists split the hazels (with the hornbeams and allied genera) into a separate family Corylaceae.Bean, W. J. (1976). Trees and Shrubs Hardy in the British Isles 8th ed., vol. 1. John Murray ISBN 0-7195-1790-7.Erdogan, V. & Mehlenbacher, S. A. (2002). Phylogenetic analysis of hazelnut species (Corylus, Corylacae) based on morphology and phenology. Sist. Bot. Dergisi 9: 83–100.

The shape and structure of the involucre, and also the growth habit (whether a tree or a suckering shrub), are important in the identification of the different species of hazel.

Species

There are 14–18 species of hazel. The circumscription of species in eastern Asia is disputed, with the Kew Checklist and the Flora of China differing in which taxa are accepted; within this region, only those taxa accepted by both sources are listed below.Kew Checklist: CorylusFlora of China: CorylusFlora of North America: Corylus The species are grouped as follows:

• Nut surrounded by a soft, leafy involucre. Multi-stemmed, suckering shrubs to 12m tall.
• Involucre short, about the same length as the nut.
• Corylus americana — American Hazel. Eastern North America.
• Corylus avellana — Common Hazel. Europe and western Asia.
• Corylus heterophylla — Asian Hazel. Asia.
• Corylus yunnanensis — Yunnan Hazel. Central and southern China.
• Involucre long, twice the length of the nut or more, forming a 'beak'.
• Corylus colchica — Colchican Filbert. Caucasus.
• Corylus cornuta — Beaked Hazel. North America.
• Corylus maxima — Filbert. Southeastern Europe and southwest Asia.
• Corylus sieboldiana — Asian Beaked Hazel. Northeastern Asia and Japan (syn. C. mandshurica).
• Nut surrounded by a stiff, spiny involucre. Single-stemmed trees to 20–35 m tall.
• Involucre moderately spiny and also with glandular hairs.
• Corylus chinensis — Chinese Hazel. Western China.
• Corylus colurna — Turkish Hazel. Southeastern Europe and Asia Minor.
• Corylus fargesii — Farges' Hazel. Western China.
• Corylus jacquemontii — Jacquemont's Hazel. Himalaya.
• Corylus wangii — Wang's Hazel. Southwest China.
• Involucre densely spiny, resembling a chestnut burr.
• Corylus ferox — Himalayan Hazel. Himalaya, Tibet and southwest China (syn. C. tibetica).