Ivy is a well-known plant, familiar to everyone. At the species level it is uncontroversial: Hedera helix L., Ivy. However, it becomes more complex when you look at subspecies and varieties, as it is a very variable plant and interesting forms have been collected and grown horticulturally. In Britain there are two native subspecies of H. helix (taxonomy here follows that of Stace 1997):
- H. helix ssp. helix, Common Ivy, is the commonest wild form with typically dark green (often marbled) leaves. It is diploid: 2n = 48 (Stace 2010).
- H. helix ssp. hibernica (G. Kirchn.) D.C. McClint., Atlantic Ivy, occurs along the western parts of the British Isles and has paler yellowish-green leaves. This plant is tetraploid: 2n = 96 (Stace 2010).
To confuse matters, a cultivated variety of H. helix ssp. hibernica, known as Hedera ‘Hibernica’, Irish Ivy, is widely grown in gardens and sometimes escapes into the wild. It has large, relatively pale green leaves and is only a weak climber. The subspecies are not always easy to distinguish, and many recorders do not bother. The best guide to accurate identification is probably Rutherford (1997).
Photography: A.J. Lockton
Hedera helix is a common and widespread plant almost throughout the British Isles, although not quite as widespread as claimed by Grime, Hodgson & Hunt (2007). It is not native in Shetland, although there are pollen deposits from an earlier interglacial period (Scott & Palmer 1987). Introduced plants occur only in gardens and they fail to establish themselves in the wild. It is also considered an introduction in Orkney except in one site. In Flora Europaea (Vol. 2) D.A. Webb describes H. helix as being widespread throughout Europe, as far north as Norway and eastwards to Russia. In Greece & Turkey the subspecies poetarum Nyman has yellow (not black) fruits; this subspecies has recently been recorded in several places in Britain. GBIF has limited data, but shows Hedera helix s.l. as occuring in the New World and the Pacific Rim, presumably as an introduction.
The most complete account of the ecology of Hedera helix is given by Grime, Hodgson & Hunt, but they did not separate the subspecies. It is a long-lived perennial (up to 400 years, apparently) with two distinct growth forms. Non-flowering stems grow horizontally across the ground, often in dense shade, and bear the typical ivy-shaped (palmately-lobed) leaves; meanwhile the vertically ascending fertile stems have ovate leaves and bear the flowers and fruit, often in sunlight or only half shade. Seeds are dispersed by birds, but in the far north flower buds rarely open (Scott & Palmer, op. cit.). Rodwell (1991) lists it as a component of most woodland types, excepting only the most upland and northerly communities (Scots Pine woods, juniper and alpine willow scrub). However, it is an important plant phytosociologically, because it is only abundant in secondary or otherwise disturbed woods where there are elevated light levels. This could be caused by elm death, for instance, and it is characteristic in, for instance, W8d Fraxinus excelsior woods; Hedera helix subcommunity.
- Origin: native throughout the British Isles except Shetland, although many populations are undoubtedly introduced.
- Rarity: common throughout, except in Orkney, where only one population seems to be considered native.
- Threat: in the New Atlas it is given a small Change Index of -0.65, but it is described as unchanged. The subspecies are very unevenly recorded and no analysis would be possible. Grime, Hodgson & Hunt (1997) claim that it is less frequent on dead trees and old walls than formerly.
- Conservation: no county recorder has yet listed H. helix or its subspecies an axiophyte, although it is conceivable that on the edge of their ranges they could be indicators of ancient woodland (e.g. in north Scotland) or of changing climate (e.g. ssp. hibernica in the east). More anecdotally, many people believe that ivy is a threat to trees and cut through the climbing stems, so it is often viewed as an invasive native plant.
The subspecies of Hedera helix are not well known, and perhaps it is now not possible to separate native and introduced populations, but it might be advantageous to study them towards the edges of their range to see if there is any habitat specificity. Whether ivy is increasing or decreasing in Britain seems to be anyone’s guess. There is little change at the 10 km square level, but at finer scales there could be quite profound changes. Certainly many people believe ivy to be a troublesome weed and attempt to control it. This is quite controversial, as it is said to create good habitat for birds, invertebrates and small mammals. What is the truth about the palatability of ivy? Grime et al. (op. cit.) describe it being collected for fodder, but claim it is poisonous in quantity and subject to extremely low rates of herbivory. In the New Atlas, Dines describes it as highly palatable and considers it limited more by grazing than by climate.
- Rutherford, A. 1997. Hedera. In Rich. T.G.C. & Jermy, A.C. Plant Crib 1998. Botanical Society of the British Isles, London, pp. 216-219.
- Scott, W. & Palmer, R. 1987. The Flowering Plants and Ferns of the Shetland Islands. The Shetland Times Ltd., Lerwick.