Corylus sieboldiana Blume

The east Asian equivalent of the American Beaked Hazel (Corylus cornuta) shares the bristly, ‘elephant’s-trunk’ husks which protect the ripening nuts, but has often slighly larger and more distinctly lobed leaves on slightly longer stalks (Bean 1981); the nuts also ripen later in autumn. The variety described here (var. sieboldiana) was the first to be described – by five years – and is a shorter-husked variant of the more widespread var. mandshurica, whose leaves are less likely to be heart-shaped at the base (Edwards & Marshall 2019).

Var. sieboldiana was introduced to the west in 1904 (Bean 1981), and has maintained a rather inconspicuous place in cultivation. (It should probably be born in mind that hazels labelled simply as Corylus sieboldiana may sometimes represent var. mandshurica, and that plants collected in Korea or Japan are not necessarily var. sieboldiana. A 15 m tree hazel at the National Botanic Garden at Glasnevin in Ireland labelled in 2012 as C. sieboldiana was obviously a case of misidentification (Tree Register 2023).)

Plants of known wild origin in big gardens in the UK include MK 91/28 and CU 95/215 at Howick Hall in Northumberland, a plant from Honshu (Japan) at the Sir Harold Hillier Gardens, and Japanese and Korean collections at the Royal Botanic Garden Edinburgh and its outstations (Tree Register 2023; Royal Botanic Garden Edinburgh 2023). The Linnaeus Garden in Uppsala, Sweden, has an example from seed collected in Japan (Royal Botanic Garden Edinburgh 2023). The Dawes Arboretum in Ohio has two plants of Japanese origin, accessioned in 1991 and 1996 (Dawes Arboretum 2023), while material from a joint expedition to Japan in 2005 by the Royal Botanic Garden Edinburgh, Howick Hall, the Sonoma Botanical Garden in California and the Polly Hill Arboretum in Massachusetts (BBJMT 40) has been shared between these institutions (Royal Botanic Garden Edinburgh 2023). Commercially available collections include BSWJ 11056 from Crûg Farm (UK) and CBJP 967 from Pépinière des Avettes (France) (Crûg Farm Plants 2023; Pépinière des Avettes 2023).

One peculiarity which this species shares with the European C. avellana is that later leaves in full sun can flush with a central irregular blotch of purplish anthocyanins, rather than the expected uniform pigmentation. The purple tint can also be seen in the husks of the ripening fruit in summer (Bean 1981). In the leaves of some seedlings from BSWJ 11056, the purple blotch can be particularly well-defined and decorative.

Corylus sieboldiana is one of the hardiest of hazels, and tends to show good resistance to Eastern Filbert Blight. A good range of material (of both varieties?) is grown at the National Clonal Germplasm Repository in Corvallis, Oregon (Molnar 2011), where plants have been experimentally hybridised in the search for hardy, disease-resistant new fruiting hazels (Erdogan & Mehlenbacher 2000). However, the small nuts of C. sieboldiana, which fall still protected by their prickly husks, do not lend the species to hazelnut production. In 1975, the South Korean Rural Development Administration began to select indigenous hazel stocks, and ten promising strains of C. sieboldiana (of whatever variety) were chosen, and were also experimentally hybridised with various European hazel clones (Molnar 2011). However, C. sieboldiana does not seem to have contributed any of its genes to the selections which are now used for nut production in South Korea.