Euonymus is a genus of ca. 130 species of northern temperate regions, mainly in Asia and Australia. One species, Euonymus europaeus L., is native in Belgium as well (originally rather locally and confined to calcareous soils but nowadays much commoner and more or less widely dispersed as an escape from cultivation). At least 28 species are cultivated as ornamentals in Europe (Vickery 1997) but only few are rather widespread.
Native Euonymus europaeus is an exceedingly variable species. Plants in cultivation (and found as escapes) possibly represent, at least in part, non-native races. The exact placement of such plants is uncertain and requires further research. At least some possibly belong with var. intermedius Gaudin (syn.: var. ovatus Dippel, f. intermedius (Gaudin) Borza), a native of Italy and Switserland (see also Sell & Murrell 2009). This taxon has much larger leaves (up to 13 cm long) and fruits (up to 18 mm across). It has been reliably recorded, for instance, between basalt blocks of river Leie between Comines-Warneton and Wervik in 2010. Some related species might have been overlooked, especially Euonymus hamiltonianus Wallich, the Asian counterpart of E. europaeus. It has longer and larger leaves (8-17 x 2,5-4,5 cm), reddish seeds (not whitish) and purple anthers (not yellow) (see also Lawrence 1955).
1. Young stems with distinct corky wings, these up to 5 mm wide (if wings absent, then leaves subsessile and deciduous) === Euonymus alatus
1. Stems unwinged, at most angular, never subsessile and deciduous === 2
2. Leaves evergreen, sometimes variegated === 3
2. Leaves deciduous, not variegated === 4
3. Plant trailing, rooting at the nodes (at least when young). Leaves usually less than 50 mm long, often variegated === E. fortunei
3. Plant erect. Leaves usually at least 50mm long, rarely variegated === E. japonicus
4. Flowers usually 5-merous (but sometimes some flowers 4-merous). Fruit with 5 lobes (but sometimes some with 4 lobes). Leaves 80-160 mm long and up to 70 mm wide === E. latifolius
4. Flowers usually 4-merous. Fruit with 4 lobes. Leaves 30-100 mm long and up to 35 mm wide (native) === E. europaeus
Literature
De Jong P. & Kolster H. (2013) Euonymus: een kleurrijk geslacht. KNNV: 144 p.
Lawrence G.H.M. (1955) Euonymus europaea, E. hamiltoniana and relatives. Baileya 3: 113-114.
Ma J. S. (2001) A Revision of Euonymus (Celastraceae). Thaiszia 11: 1-264.
Ma J.S. & Funston A.M. (2008) Euonymus. In: Wu Z.Y. & al. (eds.), Flora of China, vol.11. Science Press, Beijing & Missouri Botanical Garden Press, St. Louis:440-463.
Sell P. & Murrell G. (2009) Flora of Great Britain and Ireland. Volume 3. Mimosaceae - Lentibulariaceae. Cambridge University Press, Cambridge: XXVIII + 595 p.
Vickery A.R. (1997) Celastrus. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 5. Cambridge University Press, Cambridge: 178-179.