Berberis L. (incl. Mahonia Nutt.)
Berberis is a large genus with more than 600 species. It is widely dispersed but its center of diversity obviously lies in eastern Asia (with about 200 species in China). As currently understood, Berberis includes Mahonia (Kim & al. 2004, Mabberley 2008). The latter was merely distinguished by pinnate leaves and the absence of spines. Its separation was essentially artificial and chiefly maintained by horticulturists. The existence of artificial intergeneric hybrids (for instance xMahoberberis neubertii C.K. Schneider, a putative hybrid of Berberis vulgaris and Mahonia aquifolium) also suggested their close relationship. Berberis vulgaris L. is locally a fairly common native species (especially in thermophilous valleys between rivers Meuse and Sambre in Wallonia). Records of Berberis vulgaris elsewhere in Belgium are most often escapes from cultivation or erroneous records (confusion with non-native Berberis-species).
The genus Berberis is very popular in cultivation. The most recent accounts of the genus in cultivation (Berberis + Mahonia) are provided by Chamberlain & Maxwell (1989) and Taylor & Knees (1989) and include more than a hundred species. The more regular escapes from cultivation are treated by Stace (1997). The taxonomy of some of the cultivated taxa has become complicated as a result of artificial crossings and selection. Some of the more popular taxa in cultivation nowadays are in fact complex hybrids. Especially the taxa related to Berberis gagnepainii and B. julianae form a difficult group and some of the records of these species might in fact pertain to related hybrids or cultivars (see also below). The same applies to taxa related to Berberis thunbergii (B. xottawensis and others).
In other western European countries (especially the British Isles; see Clement & Foster 1994) several additional species of Berberis have been recorded as escapes from cultivation. Its berries are very attractive to birds and are easily dispersed in the vicinity of plantations. In suitable habitats (for instance: coastal dunes, urban walls,…) new species are to be expected.
A useful, copiously illustrated account for the genus in cultivation in Spain is available here: http://www.arbolesornamentales.es/Berberis.htm.
1. Leaves pinnately compound with 5 to numerous leaflets, evergreen. Stem without spines === 2
1. Leaves simple, deciduous or evergreen. Stem with 1-3 partite spines in leaf axils === 3
2. Bud scales 4-8(-14) mm, deciduous; leaflet blades 1(-3)-veined from base. Distal end of each anther filament with pair of recurved teeth. Leaves thin and flexible, usually turning brownish in winter, with 5-9 (or slightly more) leaflets, with 5-21 teeth on either side === Berberis aquifolium
2. Bud scales persistent, conspicuous, 11-44 mm; leaflet blades 4-6-veined from base. Anther filaments unappendaged. Leaves thick and leathery, always green, with 9-21 leaflets, with 2-6 sharp teeth on either side === B. bealei
3. Leaves deciduous, herbaceous, entire or serrate with many spine-tipped teeth at margin and apex (spinose teeth < 1 mm long). Berry reddish === 4
3. Leaves evergreen, +/- leathery, with prominent spinose teeth at margin and apex (spinose teeth > 1 mm long). Berry bluish black === 7
4. Leaves 25-60 mm long, with many spine-tipped teeth at margin. Inflorescence a pendent, many-flowered raceme (usually ca. 12-25 flowers) (native) === B. vulgaris
4. Leaves 10-35 mm long, entire or with some spine-tipped teeth at margin. Inflorescence a few-flowered umbel or fascicle (usually (1-) 5-10 flowers) === 5
5. Spines tripartite. Young branches minutely hairy === B. wilsoniae
5. Spines simple. Young branches glabrous === 6
6. Leaves 10-15(-20) mm long, entire at margin. Berry 7-8 mm long. Flowers usually 1-5 === B. thunbergii
6. Leaves 15-35 mm long, usually at least with some spinose teeth at margin. Berry 8-10 mm long. Flowers usually 5-10 === B. x ottawensis
7. Leaves 10-20 mm long, with 1-2 pair of teeth on either side. Flowers 10-30 in racemes, orange. Spines short, ca. 5 mm long === B. darwinii
7. Larger leaves usually longer, with 4-20 pair of teeth on either side. Flowers in fascicles of ca. 2-15, yellow. Spines long, ca. 10-40 mm long === 8
8. Leaves glaucous-pruinose beneath. Branches always warty === B. xhybrido-gagnepainii
8. Leaves never glaucous beneath. Branches smooth or slightly warty === 9
9. Leaves usually longer than 60 mm, dull on upper side, narrowly elliptic (up to 10 mm wide). Flowers 2-5(-7). Ovules 3-4. Stem more or less terete === B. gagnepainii
9. Leaves usually less than 60 mm long, shiny on upper side, broadly elliptic (much wider than 10 mm). Flowers (8-)10-15. Ovules 1-2. Stem ridged === B. julianae
Literature
Ahrendt L.W.A. (1961) Berberis and Mahonia. A taxonomic revision. J. Linn. Soc., Bot. 57: 1-410.
Chamberlain D.F. & Maxwell H.S. (1989) Berberis. In: Walters S.M. & al. (eds.), The European Garden Flora, vol. 3. Cambridge University Press, Cambridge: 373-389.
Clement E.J. & Foster M.C. (1994) Alien plants of the British Isles. BSBI, London: XVIII + 590 p.
De Koning J., Van den Broek J.W., Van de Laar H.J. & Fortgens G. (2000) Nederlandse dendrologie (13e druk). H. Veenman & zonen, Ede: 585 p.
Duvigneaud J. & Saintenoy-Simon J. (1993) Le parc du château de Xhos à Tavier (prvince de Liège, Belgique) une belle station de plantes castrales. Nat. Mosana 46(3): 102-108.
Houtman R.T., Kraan K.J. & Kromhout W.H. (2004) Mahonia aquifolium, M. repens & M. xwagneri en hybriden - sortimentsonderzoek en keuringsrapport. Dendroflora 41: 42-71.
Jonsell L. (2001) Berberidaceae. In: Jonsell B. (ed.), Flora Nordica, vol. 2. The Bergius Foundation, Stockholm: 336-339.
Junsheng Y. (2011) Berberis. In: Wu Z.Y. & al. (eds.), Flora of China, vol. 19. Science Press, Beijing and Missouri Botanical Garden Press, St. Louis: 715-771. [available online at: http://flora.huh.harvard.edu/china/PDF/PDF19/Berberis.pdf]
Kim Y.D., Kim S.H. & Landrum L.R. (2004) Taxonomic and phytogeographic implications from ITS phylogeny in Berberis (Berberidaceae). J. Pl. Res. 117: 175-182. [available online at: https://www.researchgate.net/profile/Leslie_Landrum/publication/5432270_...(Berberidaceae)._Journal_of_Plant_Research_117_175-182/links/553a5fa90cf245bdd7642229.pdf]
Mabberley D.J. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.
Roloff A. & Bärtels A. (2006) Flora der Gehölze (2e Auflage). Ulmer, Stuttgart: 844 p.
Rounsaville T.J. & Ranney T.G. (2010) Ploidy Levels and Genome Sizes of Berberis L. and Mahonia Nutt. Species, Hybrids, and Cultivars. HortScience 45(7): 1029-1033. [available online at: http://www.ces.ncsu.edu/fletcher/mcilab/publications/rounsaville-and-ran...
Schneider C. (1942) Die Berberis der Section Wallichianae. Mitt. Deutsch. Dendrol. Ges. 55: 1-60.
Stace C. (1997) New flora of the British Isles, 2nd ed.: XXVII + 1130 p. Cambridge University Press.
Taylor N.P. & Knees S.G. (1989) Mahonia. In: Walters S.M. & al. (eds.), The European Garden Flora, vol. 3. Cambridge University Press, Cambridge: 371-373.
Van de Laar H.J. (1975) Mahonia en Mahoberberis. Dendroflora 11-12: 18-35.