Hedera, commonly called ivy (plural ivies), is a genus of 12–15 species of evergreen climbing or ground-creeping woody plants in the family Araliaceae, native to Western Europe, Central Europe, Southern Europe, Macaronesia, northwestern Africa and across central-southern Asia east to Japan and Taiwan. Several species are cultivated as climbing ornamentals, and the name ivy especially denotes common ivy (Hedera helix), known in North America as "English ivy", which is frequently planted to clothe brick walls.
Description
On level ground ivies remain creeping, not exceeding 5–20 cm (2.0–7.9 in) height, but on surfaces suitable for climbing, including trees, natural rock outcrops or man-made structures such as quarry rock faces or built masonry and wooden structures, they can climb to at least 30 m (98 ft) above the ground. Ivies have two leaf types, with palmately lobed juvenile leaves on creeping and climbing stems and unlobed cordate adult leaves on fertile flowering stems exposed to full sun, usually high in the crowns of trees or the tops of rock faces, from 2 m (6.6 ft) or more above ground. The juvenile and adult shoots also differ, the former being slender, flexible and scrambling or climbing with small aerial roots to affix the shoot to the substrate (rock or tree bark), the latter thicker, self-supporting and without roots. The flowers are greenish-yellow with five small petals; they are produced in umbels in autumn to early winter and are very rich in nectar. The fruit is a greenish-black, dark purple or (rarely) yellow berry 5–10 mm diameter with one to five seeds, ripening in late winter to mid-spring. The seeds are dispersed by birds which eat the berries. The species differ in detail of the leaf shape and size (particularly of the juvenile leaves) and in the structure of the leaf trichomes, and also in the size and, to a lesser extent, the colour of the flowers and fruit. The chromosome number also differs between species. The basic diploid number is 48, while some are tetraploid with 96, and others hexaploid with 144 and octaploid with 192 chromosomes.
Ecology Ivies are natives of Eurasia and North Africa, but have been introduced to North America and Australia. They invade disturbed forest areas in North America. Ivy seeds are spread by birds. Ivies are of major ecological importance for their nectar and fruit production, both produced at times of the year when few other nectar or fruit sources are available. The ivy bee Colletes hederae is completely dependent on ivy flowers, timing its entire life cycle around ivy flowering. The fruit are eaten by a range of birds, including thrushes, blackcaps, and woodpigeons. The leaves are eaten by the larvae of some species of Lepidoptera such as angle shades, lesser broad-bordered yellow underwing, scalloped hazel, small angle shades, small dusty wave (which feeds exclusively on ivy), swallow-tailed moth and willow beauty. A very wide range of invertebrates shelter and overwinter in the dense woody tangle of ivy. Birds and small mammals also nest in ivy. It serves to increase the surface area and complexity of woodland environments.
Taxonomy and evolution
The following species are widely accepted; they are divided into two main groups, depending on whether they have scale-like or stellate trichomes on the undersides of the leaves:
Trichomes scale-like Hedera algeriensis Hibberd – Algerian ivy. Algeria, Tunisia (Mediterranean coast). Hedera canariensis Willd. – Canaries ivy. Canary Islands. Hedera colchica (K.Koch) K.Koch – Persian ivy. Alborz, Caucasus, Turkey. Hedera cypria McAllister – Cyprus ivy (syn. H. pastuchovii subsp. cypria (McAll.) Hand). Cyprus (Troodos Mts.) Hedera iberica (McAllister) Ackerfield & J.Wen – Iberian ivy. SW Iberian coasts. Hedera maderensis – Madeiran ivy. Madeira. Hedera maroccana McAllister – Moroccan ivy. Morocco. Hedera nepalensis K.Koch – Himalayan ivy (syn. H. sinensis (Tobl.) Hand.-Mazz.). Himalaya, SW China. Hedera pastuchovii G.Woronow – Pastuchov's ivy. Caucasus, Alborz. Hedera rhombea (Miq.) Siebold ex Bean – Japanese ivy. Japan, Korea, Taiwan. Trichomes stellate Hedera azorica Carrière – Azores ivy. Azores. Hedera crebrescens M. Bényei-Himmer et M. Höhn - Buda ivy. Central Europe Hedera helix L. – Common ivy (syn. H. caucasigena Pojark., H. taurica (Hibberd) Carrière). Europe, and widely introduced elsewhere. Hedera hibernica (G.Kirchn.) Bean – Atlantic ivy (syn. H. helix subsp. hibernica (G.Kirchn.) D.C.McClint.). Atlantic western Europe. The species of ivy are largely allopatric and closely related, and many have on occasion been treated as varieties or subspecies of H. helix, the first species described. Several additional species have been described in the southern parts of the former Soviet Union, but are not regarded as distinct by most botanists. Hybrids have been recorded between several Hedera species, including Atlantic ivy (H. hibernica) with common ivy (H. helix). Hybridisation may also have played a part in the evolution of some species in the genus. A well-known hybrid involving ivies is the intergeneric hybrid × Fatshedera lizei, a cross between Fatsia japonica and Hedera hibernica. This hybrid was produced once in a garden in France in 1910 and has never successfully been repeated, the hybrid being maintained in cultivation by vegetative propagation.
Evolution The genus is thought to have originated in East Asia, with the earliest fossil of the genus being known from Korea, dating to the Oligocene epoch. The genus later expanded westwards, reaching Europe during the early-middle Miocene epoch.
Uses and cultivation
Ivies are very popular in cultivation within their native range and compatible climates elsewhere, for their evergreen foliage, attracting wildlife, and for adaptable design uses in narrow planting spaces and on tall or wide walls for aesthetic addition, or to hide unsightly walls, fences and tree stumps. Numerous cultivars with variegated foliage and/or unusual leaf shapes have been selected for horticultural use. The American Ivy Society is the International Cultivar Registration Authority for Hedera, and recognises over 400 registered cultivars.
Problems and dangers
On trees
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