Hedera hibernica (Kirchn.) Bean (syn.: H. helix subsp. hibernica (Kirchn.) McClintock) (SW and W-Eur.) – A very rare but poorly known and hence probably widely overlooked escape from cultivation or garden throw-out. Known for instance from Koningsbos (coastal woodland) in Knokke and doubtlessly largely passing unrecorded elsewhere.
Hedera hibernica much resembles native H. helix and is obviously closely related. However, according to McAllister & Rutherford (1990) they are cytologically and morphologically distinct and have a different geographical distribution. All recent authors therefore accept both as separate species (see also Valcárcel & Vargas 2010). As a rule Hedera hibernica is more vigorous, has larger leaves, a stronger (often pine-like) odour,… The position and colour of the rays of the trichomes doubtlessly provides the best character: the rays are pale yellowish-brown and lay parallel to the leaf surface in Hedera hibernica (vs. whitish and at right angles to the leaf surface in H. helix). Pubescence appears to be very sparse in Hedera hibernica and rather dense and untidy in H. helix. This character is best assessed on the lower side of young leaves and is most obvious on and near the main vein (hairs readily disappear with age).
The usual garden plant (and escape) is cv ‘Hibernica’ (Irish ivy), characterised by larger leaves and a strong creeping habit (rarely climbing).
Hedera hibernica, especially its cultivated races, is a very vigorous grower and a potential invasive weed in Belgium. Invasive populations of Hedera in the Americas mainly belong with this species (Clarke & al. 2006).
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Selected literature:
Clarke M.M., Reichard S.H. & Hamilton C.W. (2006) Prevalence of different horticultural taxa of ivy (Hedera spp., Araliaceae) in invading populations. Biological Invasions 8(2): 149-157.
McAllister H.A. & Rutherford A. (1997) Hedera. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 5. Cambridge University Press, Cambridge: 375-380.
Metcalfe D.J. (2005) Biological flora of the British Isles. Hedera helix L. Journ. Ecol. 93: 632-648.
Valcárcel V. & Vargas P. (2010) Quantitative morphology and species delimitation under the general lineage concept: optimization for Hedera (Araliaceae). Am. J. Bot. 97(9): 1555-1573.