Juglans is a genus of ca. 20 species. It is native in North and South America and from southeastern Europe to East Asia. Several species are widely grown as ornamentals, for timber or for their edible fruits (walnut).
In addition to the two species treated in detail below, a third species is possibly overlooked: Juglans cinerea L. It is obviously most closely related to Juglans nigra and also originating in North America. It is distinguished by larger terminal buds (12-18 mm long versus 8-10 mm long), more densely hairy leaflets, abaxially with stellate hairs (simple or 2-rayed hairs in Juglans nigra) and the presence of a well-developed terminal leaflet (obsolete in J. nigra). Juglans cinerea is planted and reported as an escape near Rhode-Saint-Genèse – Ophain (Triage des Sept Fontaines; comm. P. Dupriez 2008). This record should be further documented. A further species, Juglans ailantifolia was recently reported as an escape from Germany (Gausmann & al. 2017).
- Leaflets 13-23, toothed, pubescent beneath. Fruit ca. 30 mm long, not easily splitted at maturity === 1. Juglans nigra
- Leaflets 5-9, entire, glabrous. Fruit 40-50 mm long, easily splitted at maturity === 2. Juglans regia
De Langhe J. (2006) Determinatiesleutel voor de Juglandaceae. Studiedag Belgische Dendrologie 24/09/2006. Available online at: http://www.henriettesherbal.com/botany/jugl-keys.html.
Gausmann P., Haeupler H. & Adolphi K. (2017) Verwilderungen von Aralia elata, Fraxinus pennsylvanica und Juglans ailantifolia im Mittleren Ruhrgebiet (Nordrhein-Westfalen). Florist. Rundbr. 51: 108-127. [available online at:https://www.researchgate.net/publication/324029802_Verwilderungen_von_Ar...
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Wijnands D.O. (1989) Juglans. In: Walters S.M. & al. (eds.), The European Garden Flora, vol. 3. Cambridge University Press, Cambridge: 19-20.