The nutty world of hazel names – a critical taxonomic checklist of the genus Corylus (Betulaceae)

Hazelnuts (Corylus L.) are the source of one of the globally most important nut crops. Despite their economic and cultural importance, taxonomic knowledge is poor, even the number of species is equivocal. Weak morphological differentiation, the inconsistent taxonomic treatment of horticultural selections and cultivars, and uncritical regional treatments generated a multitude of names. The situation is further complicated by an ancient history of use (at least 10 400 years), trade (at least 4000 years) and domestication (at least 2000 years). Here, we present an annotated checklist of the taxa in the genus Corylus based on an extensive literature review, electronic database research, and visits to some European herbaria. Full citations are given for all names, typifications are provided for the majority of them. Cultivars are listed if described under the rules of the ICN. We designate lectotypes and neotypes for 28 names, and discuss the identity of enigmatic C. maxima Mill., a taxon not known from the wild.