Hedera helix L., Ivy

Account Summary

Growth form and preferred habitats

Ivy is such a common and well known plant, its lobed, spear-shaped, evergreen leaf (although extremely variable) has become an ornamental emblem in western art and an ancient icon of the Christmas season, probably borrowed from paganism, while in classical times the Ivy leaf was a symbol of fidelity. The evergreen habit allows Ivy to photosynthesise throughout the year, and the plant climbs vertically by means of a clinging 'adhesive' double row of short, unbranched rootlets that are intermittently produced on the stem. Alternatively, it scrambles and sprawls over the ground in woods, thickets and on shady banks, its stems when growing horizontally, rooting at their nodes. Ivy is a woody-stemmed, perennial that often surrounds and overtakes old or slow-growing trees in moderately open or less-shaded parts of woods of many kinds (Metcalfe 2005; Strelau et al. 2018).

The very wide range of woods and scrubland types in which Ivy plays a part in B & I can be glimpsed by scanning the National Vegetation Classification (NVC), where H. helix features in 21 of the 25 defined communities of woody vegetation. It is recognised as a 'constant species' in four of them and is absent only from a few highland and montane woodland types (Rodwell et al. 1991 Volume 1).

Ivy can survive in deeper shade than any other woodland plant, managing to hold on even under the canopy of well-established Gorse (Ulex europaeus) and Bramble (Rubus spp.) thickets. It is also the last species to be excluded by the shade under Rhododendron ponticum (Rhododendron) and Taxus baccata (Yew), and it will climb even these for a metre or two if they are by the edge of tracks and are relatively open at the base allowing sufficient light to penetrate. Ivy tends to be most frequent in soils that are mesic with respect to moisture and pH, while preferring medium to high nutrient status. It avoids wetland habitats, waterlogged, very dry or very acid soil (scarcely ever below pH 4.0), exposed or heavily disturbed sites and, as mentioned above, higher, colder altitudes (Grime et al. 1988; Hill et al. 1999).

In Fermanagh, hedgerow Ivy really is omnipresent. Hedgerow trees and shrubs are more vulnerable to Ivy infestation than those in the darker environment of closed canopy woodland, since at the first sign of weakness in their growth, light penetrates the hedge immediately, giving the Ivy that crawls at their base the opportunity to launch itself upwards into competition for light and space. Hedgerow Fraxinus excelsior (Ash) is particularly vulnerable to Ivy invasion since its pale green, compound leaf casts a relatively light shade and it probably bears leaves for fewer weeks than any other tree species in the British Isles (Thomas 2000, p. 232).

The widespread modern practice of mechanical hedge-trimming using a tractor with a spinning disk saw, flails and mauls trees and shrubs so savagely that it must severely check their growth for considerable periods. This also facilitates the Ivy invasion of the hedgerow, initiating competition by the evergreen climber, leading to its eventual dominance of hedges 'managed' in this brutal and abhorrent manner.

In addition to climbing, Ivy can often equally well cover the ground in shaded or partially-shaded situations like a springy, evergreen blanket, 15–25 cm deep. Long-established Ivy can form clonal monocultures, almost completely dominating the herb layer of woods and scrub thickets in this manner. Although as detailed below, Ivy exhibits prolific seed production, its vigorous vegetative reproduction is chiefly responsible for its success as a species (Strelau et al. 2018). Crawling or climbing, the leathery, more or less glossy leaves arrange themselves in a leaf-mosaic with minimum overlap, thus capturing the maximum amount of sunlight (Melderis & Bangerter 1955).

Associated species

In woods and thickets the main competitors of H. helix are Brambles (Rubus spp.), Honeysuckle (Lonicera periclymenum), Gorse (Ulex europaeus), Bracken (Pteridium aquilinum) and other ferns with large fronds. Other close associates of H. helix are the pre-vernal and vernal patch- or carpet-forming herbs, such as Bluebells (Hyacinthoides non-scripta), Ramsons (Allium ursinum) and, in demesnes at least, Snowdrops (Galanthus nivalis), all of which often grow up through the Ivy regardless of its vigour. In Britain, this list would of course include Dog's Mercury (Mercurialis perennis), but not so in Ireland (see RSF's comment in the Allium ursinum species account on this website). It appears that subsp. hibernica is a better competitor than subsp. helix under conditions of woodland shade (McAllister & Rutherford 1990).

Fermanagh occurrence

In winter, in Fermanagh, as indeed elsewhere in W Ireland, Ivy is often the most conspicuous plant species in the countryside, being completely hardy in this part of the NW Atlantic coastline. It has been recorded in 483 Fermanagh tetrads, 91.5% of those in the VC. It is almost ubiquitous in woods, hedges, shaded cliffs and on neglected walls and buildings. It can become superabundant, burgeoning everywhere, growing up and shrouding its natural and man-made supporting structures.

Variation

Recent studies have determined that two forms of Ivy exist, a diploid (2n=48), variously referred to as H. helix, H. helix subsp. helix, or H. helix var. helix with the common name Common Ivy, and a tetraploid (2n=96), called either H. hibernica (G. Kirch.) Bean, or H. helix subsp. hibernica (G. Kirch.) D.C. McClint. and given the common name 'Atlantic Ivy'. McAllister & Rutherford (1990), who have done much of the work to establish the existence of these two forms and define their differences, give them species rank, but An Irish Flora (1996) reckons the distinction only merits recognition at the varietal level. The current author (RSF) prefers the compromise position proposed by Lum & Maze (1989) after their analysis of trichomes in the taxa and consider them as separate subspecies, a stance also preferred by Stace (New Flora of the BI 1997).

The subspecies are not always easy to separate, but a full account of their differences, written by A. Rutherford, appears in the Plant Crib (Rich & Jermy 1998). One of the differences is odour; Atlantic Ivy has a strong, sweet, resinous, pine-like smell, while Common Ivy has only a weak, disagreeable, usually rather acrid odour. Unfortunately, the current degree of clarity separating the Ivy subspecies was not available when the bulk of the Fermanagh fieldwork was carried out and there are not yet any records that distinguish them. The same situation pertains in the great majority of Irish VCs, and although Alison Rutherford (pers. comm., January, 2002) considers that the Atlantic Ivy is the prevalent form in Ireland and Stace (New Flora of the BI 1997) appears to agree with this notion. How the two taxa are distributed and their degree of overlap is unknown.

Effects of climbing Ivy

Since Ivy does not penetrate the living tissues of the plants it climbs or clambers over and it does not derive any water or nutrient from them, it is classed as an epiphyte rather than a parasite (Holmes 1979). Nevertheless, a perennial debate arises as to whether or not Ivy harms other plants or buildings with its vigorous growth and evergreen shading leaves.

Generally it does not become dominant on woodland trees until they become old and senescent, when their canopy opens and allows light to penetrate to lower strata. Undoubtedly the evergreen climber indirectly hastens the demise of trees it smothers, since they are much more likely to catch the wind in winter gales and be felled (Thomas 2000, p.232).

Direct harmful effects, such as branch breakage are sometimes caused on older trees by the physical burden of the climber and there could also be some degree of restriction on growth of the supporting trunk due to layers of the rope-like network of H. helix stems gradually accumulating on it. However, despite its specific epithet 'helix' (Greek, meaning 'anything that assumes a spiral shape') (Gilbert-Carter 1964), Ivy does not climb by winding itself around its support. Thus, while it is one of the few temperate representatives of a plant family otherwise exclusively tropical in its distribution, the behaviour of Ivy is not analogous to the strangler vines characteristic of tropical forests.

Climbing Ivy does not harm structurally sound walls and buildings but damage does occur if the ivy stem finds a weakness or an opening it can grow through, since it will then act as a living wedge; then it will also, given time, become very difficult and expensive to extricate.

On the other hand, ornamental cultivars of Ivy are valued as a means of covering unsightly buildings and it is reputed to be the only climber which does not make walls damp. The leathery evergreen leaves form a sheltering curtain, holding and absorbing rain and moisture, but do not transfer it to the underlying supporting structure (Grieve 1931).

Ivy in ancient and secondary woodlands

Although a plant of prehistoric woodland, Ivy is remarkably local in ancient woods (at least in SE England), being much more associated with secondary woods (ie woodland on ground that previously supported other vegetation or use), and with straightforward plantations or re-plantations (Rackham 1980). In his detailed study of well documented, managed ancient woods, Rackham found Ivy had problems of initial colonisation, but when it does manage to achieve establishment, it radiates inward from the woodland edge and invades along rides, or into disturbed areas within the vegetation boundary, for instance, where quarrying has taken place.

In secondary woods, Rackham suggests Ivy is associated with a Hawthorn dominated scrub stage in the succession towards woodland development (the NVC community W21, Crataegus monogyna-Hedera helix scrub of Rodwell et al. (1991)), or with more permanent Hawthorn scrub itself (Rackham 1980, p. 353). When Ivy arrives in ancient or well-established woodlands it can persist for centuries, but Rackham felt that it has little power of dispersal into existing closed woodland communities (Rackham 1980, p. 95) (see below on dispersal ability).

Fossil history

A Danish palaeobotanist has used the occurrence of fossil pollen to develop a technique to construct a thermal correlation graph which describes the climatic range of certain species in NW Europe over the last ten to twelve thousand years (Iversen 1944). He used Ivy and two other species of contrasting biology, Viscum album (Mistletoe) and Ilex aquifolium (Holly), as his three indicator species. It is perhaps a little surprising that Ivy, being an insect-pollinated species, produces and disseminates sufficient pollen to be followed accurately in the fossil record preserved in peat and lake mud sediments along with that of wind-pollinated species, but evidently this is the case (Godwin 1975).

Iversen found that the distribution of Ivy is governed by a double form of temperature control; it requires moderately high summer temperatures, together with winter temperatures not falling below -1.5˚C as the average for the coldest month. Climbing H. helix plants suffer severe damage at lower temperatures than this and seed production is also reduced, since growth of Ivy is then restricted to levels where adequate snow-cover gives protection from the worst winter weather (Godwin 1975). Iversen therefore reclassified Ivy as a eu-oceanic plant (ie 'true-' or 'truly'-oceanic), rather than sub-oceanic (Iversen 1944) and, indeed, the present distribution within B & I does reflect these findings, Ivy being most prominent in the Atlantic fringe areas of Europe, where the climate is wetter and milder throughout the year.

H. helix is not only present as fossils throughout the current interglacial period, but is also recorded in two earlier ones, called the 'Hoxnian' and 'Ipswichian' in Britain, the former referred to as the 'Gortian' in Ireland. The pollen record for these three interglacial warm periods in B & I sites accords well with those from Denmark (Andersen 1966), especially in that Hedera tends to have its maximum development early on in the interglacial cycle of rising and falling temperatures, in contrast with Ilex aquifolium. Andersen attributes this chiefly to the development of more acidic mor (ie peaty) soils in the latter part of each interglacial cycle, giving soil conditions that are better tolerated by Ilex than by Hedera. Ivy does extend south in Europe and Asia, however, reaching Asia Minor, Palestine and N Iran, and it is also naturalised in N America where it is commonly known as 'English Ivy' (Clapham et al. 1962).

Leaf and stem morphology

Ivy is dimorphic, having juvenile and mature growth forms of differing leaf shape (ie it is also 'heterophyllous') (Rose 1980). When creeping horizontally, it is always completely sterile and has 'juvenile phase' deeply lobed leaves in a great range of size, shape and pigmentation. Climbing Ivy, which some describe as 'arborescent' or tree-like, has quite different 'mature phase' adult growth form, with unlobed, or much less lobed leaves (Whitehouse 1992). Given sufficient illumination, the correct day-length regime and a temperature below 16°C, this 'adult' form of the plant will initiate and develop flowers in late autumn from September to November (Wallerstein & Hackett 1989). Since the length of stem internodes changes between horizontal juvenile stems and vertical mature ones, the latter being much shorter, hormonal control involving gibberelins in some way controls the change of growth phase, altering the species' response to light. The juvenile stem is negatively phototrophic, growing away from light into dark places, while the vertical stem is positive and grows towards a light source. At the same time the plant hormone initiates the ability of the plant to flower (Hopkins 1995).

Flowering reproduction

Flowers are produced on well-lit, aerial, adult shoots from about 10 years old (Clark 1983). The small, bisexual, lime-yellow flowers are borne in September to November in compact, globose, terminal umbels and have their parts in fives (although the five carpels are fused together and there is a solitary stigma). The five sepals are very small, so that the five greenish petals can be mistaken for the calyx. The flowers, which are strongly self-incompatible, can be either protandrous (ie male stage first), or homogamous (ie sexes maturing at the same time). Either way, the terminal, rounded umbels of flowers are cross-pollinated by flies (including hoverflies), Small Tortoiseshell, Peacock and other butterflies, plus moths, wasps and honey bees.

The abundant late-season pollen and plentiful, fully exposed nectar of Ivy flowers are of great importance to all these insect species, but especially so for honeybees which avidly collect and store supplies in the hive comb in order to feed next season's early brood. In late October, other hibernating insects, such as queen wasps, also frequent Ivy flowers to collect food (Knight 1962). A succession of further umbels may be produced below the terminal one from November onwards, and functioning flowers may even be found in December, but the chances of pollination decreases with advancing cold weather and consequent fewer active insect visitors (Knight 1962).

Since flower induction requires a minimum light level, H. helix never blossoms and fruits where the plant occurs most abundantly, ie in the deeper shade of woods and thickets (Whitehouse 1992). Despite the popularity of the flowers with insect visitors, examination of umbels reveals the levels of fertilisation achieved are far from ideal. Although the branch tips may bear 50 or more flowers, often only 40-50 % of these develop fruit, even in a mild winter (Knight 1962).

The fruit is a 6-8 mm diameter, green, turning black or deep purple, urn-shaped berry, which does not fully ripen until the following spring. Early ripening fruit may wither and abort if exposed to excessive cold (Snow & Snow 1988). The berry is smooth, succulent, thin-skinned and contains purple pulp and up to five rather soft seeds, although generally fewer than this are produced since some of the ovules abort. The main berry crop ripens in March and April when the weather is becoming milder and food supplies for birds are already plentiful.

Seed dispersal

Since Ivy berries are among the most nutritious wild fruit available in B & I, having an especially high fat content, they are keenly sought, eaten and transported by the members of the Thrush family (Simms 1978, pp. 108-11), plus Robins, Blackcaps, Starlings and Woodpigeons. Where the latter are common, a very high proportion of Ivy seed, perhaps up to 75% of the total production, is destroyed in the birds' stone-filled crop by being taken when the berries are unripe (Snow & Snow 1988, p. 33).

Since berries can contain more than one seeds, and birds often consume numerous fruits at a time, a single bird dropping may deposit numerous seeds and give rise to clusters of seedlings. As it grows, Ivy often continues to maintain its own company and many stems, often of different ages, may be observed together climbing trees, pillars and walls (Metcalfe 2005).

Toxicity

Although the bird species mentioned above eat the berry pulp without harm and Blackbirds feed the pulp to their young nestlings (Ridley 1930, pp. 408-9), all parts of the Ivy plant contain saponins which are broken down with loss of sugars to form toxic substances called 'hederins'. In sufficient quantity, these poisons are harmful to mammals including farm stock, producing vomiting, diarrhoea, muscular spasms, paralysis and even coma. The sap of the plant is also dangerous and can give rise to irritant and allergic dermatitis (Cooper & Johnson 1998).

Despite this, ivy has a history dating back to the ancient Romans of use as fodder (Grieve 1931; Troels-Smith 1960); it was collected in winter, when grazing became scarce, and fed to cattle in particular, but sometimes also to horses and sheep (Kelly 1997). There are plenty of folklore records attesting to this fact and, indeed, Ivy was also used as a medicine for ewes that were poorly after lambing (Vickery 1995). Quite apart from providing spring food for birds, climbing 'arboraceous' Ivy also provides ideal homes for nesting birds, the dense evergreen foliage giving them and their young dry, secure shelter.

Seed germination and seedling establishment

Ivy seeds are quite large, weighing on average 20.4 mg (more than twice the weight of Holly seed for example) and they can germinate immediately on being sown in the spring, the soil seed bank being only transitory (Grime et al. 1988, 2007). Germination is inhibited somewhat by light (Grime et al. 1981) and establishment from seed in woodland shade is probably a rather slow, prolonged process (Lang 1987, p. 134). A study of the behaviour of Ivy seedlings and the factors influencing establishment of the species from seed in existing woodland is sadly lacking and would be a very worthwhile project. Although quantitative measurements do not appear to exist, subjective observations suggest that establishment from seed is likely to be rare, and while bird populations do provide vectors for jump dispersal, the frequency of this happening is, as usual, completely unknown.

Vegetative reproduction

In comparison with establishment from seed, vegetative spread is very obviously significant once Ivy has arrived at a site, and it remains to be shown what factors influence the balance between and the effectiveness of sexual and asexual reproductive processes in Ivy. Ivy can invade garden hedges, including clipped evergreen privet. It does so from a previously established base on adjacent soils of more open or rocky ground, or sometimes from the base of adjacent walls. Grime et al. (1988, 2007) point out that this ability to extend over soil-less habitats from a base rooted locally in soil is unique within the flora of B & I, but the current author (RSF) suggests this is a matter of degree, since brambles can also behave in much the same manner.

H. helix climbs straight up trees, poles, walls and rock without twining, adhering by means of very short adventitious roots on the shaded side of the stem, which attach the stems to bark or any other surface irregularities by secreting a sticky substance; subsequently they keep their grip with great tenacity. The climbing stem obtains all its water and dissolved mineral nutrients in the normal manner, through a taproot in the soil (Brimble 1962). Climbing Ivy stems can be immensely long-lived, with reports of them developing trunks 90 cm in circumference (Simpson 1989), or greater 'than a fat man' (Rackham 1986, p. 117), and of being up to 400 years old (Rose 1980, p. 12). The weight of ivy removed from a single tree in Olympic Park, Washington was estimated to be 953 kg (Simon 2002). The typical climbing Ivy stem, however, branches very frequently to form a interlaced network of much narrower diameter than the giants just mentioned, the lower, older parts of the stem being clothed with a buff coloured 'fur' of adventitious rootlets, each about 2-3 mm long (Metcalfe 2005; Strelau et al. 2018).

Herbivory and disease

A number of insects, including a weevil and a moth, have Ivy as their principal host food-plant, but the most prominent feeder is the Holly Blue Butterfly, Celastrina argilus, which attacks the buds, flowers and fruit of Holly in the spring generation, and switches to those of Ivy in the summer generation (Asher et al. 2001). One insect, the aphid, Aphis hederae, feeds exclusively on Ivy leaves and stems and is, therefore, described as 'monophagous' (Peat 2002: Ecological Flora Database, York). Ivy leaves are also attacked by at least five species of microfungi which cause spots, the most common species probably being Colletotrichum trichellum (Ellis & Ellis 1985).

In NI, very rarely, Ivy roots host the total parasite Orobanche hederae (Ivy Broomrape) itself a flowering species which is more frequent further south in both B & I, and is chiefly coastal in the former (Perring & Walters 1976; Webb et al. 1996; Stace 1997; Preston et al. 2002). It has been suggested, perhaps somewhat tentatively since information is scarce, that Ivy Broomrape is more frequently found associated with Atlantic Ivy, H. helix subsp. hibernica, than with Common Ivy, subsp. helix, since the distributions of the two plants appear to be quite strongly correlated (Rutherford 1985).

British and Irish occurrence

H. helix is a widespread and abundant species throughout B & I, except in parts of the Scottish Highlands since it is essentially a lowland plant, reaching a maximum altitude of around 610 m in Isla and Atholl (Wilson 1949; Garrard & Streeter 1983). It reaches the same altitude in the Mourne Mountains, Co Down (H38)(Hackney et al. 1992). H. helix is recorded as introduced in both Orkney and Shetland (Preston et al. 2002).

European and world occurrence

Although hardy and apparently completely so in western parts of the British Isles, the distribution of H. helix is undoubtedly climatically limited in parts of Europe, where it is native in 30 countries. It declines to rarity in the N & C parts of Europe which suffer cold winters and it is entirely absent, for example, from most of Russia, E Poland, Iceland, the Faeroes and Finland. In SE Europe it reaches Ukraine (Metcalfe 2005).

H. helix has been introduced in India and South Africa and become naturalised in Australia, New Zealand, Hawaii, Brazil, Canada and the United States (Larocque 1999, cited in Grivet & Petit 2002, quoted in Metcalfe 2005).

Medicinal uses

The value of Ivy is not much regarded by many working in traditional, orthodox medicine today, but in the past it was used in cases of dysentery and jaundice, and a paste of Ivy boiled in butter was even use to relieve sunburn (Grieve 1931). More recent medicinal folklore suggests a poultice using boiled Ivy leaves is effective in treating corns, or again boiled in butter will reduce or remove burn scars. In view of the fact that Ivy can cause contact dermatitis, it is interesting that it has been recommended in herbal medicine for cooling eczema, and also for healing a scalp rash (Vickery 1995).

At present, Hedera species are being examined as a potential cure for the uncomfortable and sometimes fatal condition known as leishmaniasis (Ridoux et al. 2001; Abbasifar et al. 2017). This condition occurs in tropical and sub-tropical regions around the world and affects over 400,000 people annually. Ivy is most frequently used today in the homoeopathic treatment of a number of common conditions and diseases, and extracts of the plant are also being examined as possible ingredients in cosmetic creams and as a possible tumour reducing agent (Rose 1996). Studies have shown Ivy extracts to provide effective control against liver fluke (Julien et al. 1985). Hederasaponin B extract from H. helix can also be used as a novel drug candidate with antiviral activity against subgenotypes of enterovirus 71 (EV71), the foremost source of hand, foot and mouth disease (Song et al. 2014).

Garden use

Ivy has a very long history as a garden plant. The Classical Roman writer Pliny the Elder reported that Theophrastus, around 314 BC stated that Ivy did not grow in Asia Minor, but that Alexander the Great had come back from India wearing wreaths of Ivy because of its rarity (Rose 1980). Pliny also described the cultivated Ivies he knew, including the first mention of a variegated form and of a stiff, erect 'Tree ivy', which stands without a support (Rose 1980). Eleven species of the genus Hedera are in garden cultivation (Griffiths 1994) and Rose (1980) reckoned that about 200 cultivars exist, the great majority being forms of H. helix.

Names

The genus name 'Hedera' is an ancient classical Latin name for the plant (Gledhill 1985). In Greek legend, a crown of Ivy intermingled with flowers was first used by the God of Wine, Dionysus (or his Roman equivalent, Bacchus), such wreaths initially being reserved for the gods, but gradually their use spread to sacrificial animals, to priests and, finally, to the faithful worshippers themselves. In some cults, Dionysus bore the designation of 'kissoi' (Ivy), because legend said that as a baby he had been wreathed in Ivy by the forest nymphs (Baumann 1993). The connection with alcohol continued into historical times when English taverns bore over their doors or on a pole the sign of an Ivy 'bush', to indicate the excellence of their liquor: hence the saying 'Good wine needs no bush.' (Grieve 1931).

Considering how common and conspicuous a plant Ivy is, and how long it has been associated with man his animals and his dwellings, it is rather surprising that it appears to have accrued so few English common names. Grigson (1955, 1987) lists only seven, while Britten & Holland (1886) manage ten, including the one with which we are most familiar 'Ivy', which is derived from the Old English 'ifig' (Grigson 1974). There are several close variants of it among the English common names rounded up by Grigson, including 'Ivin', 'Ivery', 'Ivory', 'Eevy' and 'Hyven'.

An alternative suggestion for the origin of the name 'Ivy' is that it might be derived from the Latin 'ibex' meaning 'climber' (Rose 1980, p. 18). Several other names refer to the woody character of the plant and the idea that Ivy binds and bends around the objects it climbs, for example, 'Bentwood', Bindwood', 'Woodbind' and 'Benewith Tree', names it often shares with Lonicera periclymenum (Honeysuckle) (Britten & Holland 1886).

Threats

None.

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