Ilex aquifolium L., Holly
Account Summary
Native, common and widespread. Suboceanic southern-temperate, often cultivated beyond its native range.
1882; Stewart, S.A.; Co Fermanagh.
Throughout the year.
Fermanagh occurrence
Abundant and recorded in 441 tetrads, 83.5% of those in the VC, Holly is the fourth most common tree or shrub in Fermanagh in terms of record numbers and the fifth most widespread woody plant in tetrad terms.
It is common and widespread everywhere except on the most extremely exposed or the wettest ground. It appears in woods, scrub, hedgerows, riverbanks and loughshores, plus a lesser presence on cliffs, gullies on rock outcrops, limestone scree and in crevices in limestone pavement. Holly is also very frequently planted around habitation (see below under Fodder).
Growth form and preferred habitats
A suckering, much branched, evergreen small tree or shrub up to 15 m tall, with alternate leathery, often prickly leaves, while its blackish-green, foliage casts a heavy shadow, Holly itself is extremely shade tolerant and it is most typically found as a secondary canopy understorey shrub in woods. Like other evergreen-leaved species, the leaf lifespan is merely extended beyond a single year, the leaf eventually fading a greyish brown and dropping off the branch after about two years (Edlin 1964). Leaves drop at all times of year, but the main period of shedding is in mid-summer (Peterken & Lloyd 1967).
Thanks to suckering growth from its roots and occasional layering of pendulous lower branches that root adventitiously, and depending upon the degree of animal disturbance it faces, I. aquifolium can sometimes form ± dense clonal groves in mixed woodland along with Oaks, Ash, Rowan, Hazel, Hawthorn and Downy Birch (and previously Elm). Vegetative spread may be important for filling gaps in stands, but it does not appear to be an effective means of species dispersal and spread (Peterken & Lloyd 1967).
However, Holly also commonly occurs as isolated individuals, sometimes of more tree-like dimensions, in hedgerows, or along the banks of rivers or streams mixed among other woody species. More or less stunted Holly bushes are also frequent in crevices on cliffs, rock outcrops, screes or even in shady grykes, ie the deep, often dark, narrow fissures that occur in limestone pavement.
Holly grows in such a very wide variety of soils, acid to lime-rich that it appears almost indifferent to the normal range of soil pH and nutrient status it encounters. While generally it avoids very wet, cold soils and the extremely acidic nutrient-starved conditions of peat bogs, Holly can survive on the drier parts of fen alder-carr scrub around lakeshores and, in some areas of Britain, it manages to occupy soils that are waterlogged even in summer to within a few cms of the soil surface (Peterken & Lloyd 1967).
Soil improvement
Like birches and, indeed, all other trees, Holly has the ability to improve podsol conditions by the production of a deep leaf litter which slowly decays to form mull humus and thus the tree gradually generates a more fertile brown earth soil beneath it. However, the growth rate of birch is very much faster than Holly, so that any comparison of the two in terms of soil improvement could only be made in the extremely long term. In regions of high rainfall like Fermanagh and other western oceanic parts of B & I, leaching tends to override the mull-forming abilities of Holly litter, so that unfortunately the podsols persist and must thus limit the tree's growth (Peterken & Lloyd 1967).
Temperature limits
Holly is sensitive to prolonged winter frost and the natural distribution of the species in Europe falls within the -0.5oC winter isotherm for the coldest month, a feature which chiefly affects its northern and eastern boundaries (Iversen 1944, pp. 474-6). The species also has a threshold minimum requirement for summer temperature and this means that in B & I Holly is not found naturally in areas where the July mean does not exceed 12.8oC (= 55oF) (Peterken & Lloyd 1967). It is very likely that reasonably high summer temperatures are necessary for the successful formation of fruits on the female tree. Holly trees are susceptible to fire on account of their remarkably thin bark, a property which is probably also involved in the damage they suffer in frost (Iversen 1944; Peterken & Lloyd 1967).
Seedling and sapling growth and first flowering
Given good sunlight, young Holly bushes or trees first flower when they reach a height of between 1.5-3.0 m, generally reached when they are 8-15 years old. Bushes flower very poorly or not at all when growing in shade (Richards 1988). Seedling growth is exceedingly slow to begin with, achieving only approximately 1.0 cm per year for the first four or five years after germination. This is followed by a veritable growth spurt, when vigorous individuals can for a few years (obviously the duration variable with circumstances) put on more than 50 cm per year. Shaded, understorey plants, however, cease active height growth somewhere between 5-10 m, depending upon the dominant tree species and its level of shade; 17 m appears to be the absolute upper height limit achieved in woodland conditions. After the active growth spurt, height increase drops back to around 2.0 cm per year in trees over 30 years of age (Peterken & Lloyd 1967). The greatest recorded age of an individual Holly tree appears to be 254 years, but as this particular specimen was healthy and not conspicuously large, greater ages are considered certainly attainable (Peterken & Lloyd 1967). The tree expert, W. Dallimore (1908) reckoned that without doubt, Hollies could live for 250-300 years.
Flowering reproduction
Mature Hollies, in good light, flower reasonably freely most years, but berry production varies enormously, probably following a masting cycle similar to that in Beech and Oak (see the Beech species account on this website for a description of the controlling factors and possible masting rationale). The trees and flowers are dioecious, although there have been very rare reports of perfect flowers (ie hermaphrodite, bisexual – containing both sexes), which Richards (1988) suggests may really belong to a popular hybrid cultivar, I. × altaclerensis 'Pyramidalis'.
The sex ratio of trees is either a straightforward 1:1 or, in some populations, may be male predominant to some degree, although in semi-natural woodland shade a high proportion of non-flowering plants can obscure the picture in this respect (Richards 1988). One large Holly tree in Cologne was observed to change sex from female to male between 1910 and 1916 (Hegi 1924-7, 5), but the true identity of this specimen is unknown.
Flower buds develop in late summer on the current year's shoot growth, and often are produced only high on the crown of the bush or tree. The flowers typically open in May and June of the following season (ie on second-year wood) but, occasionally, they bloom prematurely out of season between September and mid-December, presumably due to a combination of inherent variation and local environmental conditions, although artificial lighting may well play a part through mimicking the day-length flowering trigger (Taylor 1992; Stace 1993; Wheeldon 1993; Nicolle 1995).
Small, white flowers, 6 mm in diameter, are borne in few-flowered axillary cymes with their parts in fours. In female flowers, nectar is secreted from tissue at the base of the ovary and bees and flies are attracted and serve as pollinators. The familiar bright red Holly 'berries' are in fact globose drupes, 1 cm in diameter, each weighing 130-150 mg and sometimes up to 60 may be tightly packed on a single twig. Towards December, the fleshy fruits ripen to crimson and attract birds such as Thrushes, Blackbirds and Finches that eat the pulp and void the small, hard, black seeds. Heavy berry crops are the result, not of hard winters as many of the public imagine, but of a sunny summer 18 months previous when the flower buds were laid down (Edlin 1964).
Each 'berry' (ie drupe) contains up to four seeds and each female twig on a tree may bear up to 50 berries. Potentially, even a small tree standing alone, 4.7 m tall and 3.7 m in diameter at the crown, might produce 120,000 seeds (Peterken & Lloyd 1967).
In common with other bird-dispersed stony fruits, germination is delayed and normally takes place in the second or third spring after formation. However, passage through the gut of a bird may speed up the weathering process, allowing the seed to germinate sooner (Hyde 1961).
Fossil history
I. aquifolium fossils are easy to identify both macroscopically by its wood, charcoal, seeds and leaves, and microscopically by its tricolpate pollen grains with distinctive sculpturing of the heavy exine which is unmistakable. The tree or shrub being entomophilous and having very much lower pollen productivity than wind-pollinated species, its pollen grains are unlikely to be subject to distant transport, giving the species high climatic indicator value.
Fossils from the glacial stages are absent except for scattered pollen grains, but there are plentiful interglacial records, macroscopic and pollen alike, from the Hoxnian onwards. The return of Holly in the Flandrian interglacial was delayed until the Boreal period (zone VI), when it first appeared in Ireland and S Wales. Afterwards, it slowly spread east into England, and Holly sites became very frequent in the wetter Atlantic period (zone VIIa) and still more abundant from then on in the Sub-Boreal (increased summer warmth) and Sub-Atlantic (milder winters). Fossil pollen frequencies in B & I in the early stages of this post-glacial have been highest of all in Co Kerry, SW Ireland. In the later historic stages of the Flandrian, pollen curves of I. aquifolium parallel those of weed and cereal pollen, marking the positive response of Holly to forest clearances and exploitation for fodder and other uses. However, this responsiveness to clearances and cutting tends to confuse any explanation of changing pollen abundance in climatic terms alone (Peterken & Lloyd 1967; Godwin 1975, p. 173-5).
In Ireland, there are no longer any pure stands of Holly such as are rarely encountered in Britain, eg in Epping Forest, or the unique ancient stand on shingle at Dungeness (Peterken & Hubbard 1972). These English stands are considered relicts of medieval planted Holly woods which are believed to have once been widespread in these islands, and particularly so in the west and north of Britain (Rackham 1980).
There is evidence that in past ages, Holly used to grow much larger trunks than it does at present; it is now rarely seen larger than a shrub. In Ireland, it once formed pure stands from which valuable timber was cut (Fitzpatrick 1933). At Inniskallen, Lough Leane, at Killarney, Co Kerry, in 1897 a 'celebrated' Holly tree was measured with a girth of 15 ft 4 in [4.67 m] (at 4 ft [1.22 m]) (Nelson & Walsh 1993). There are historical records of large Holly trees being cut down on river banks in Scotland and in Northumberland in the 1830s and 1850s, although no measurements are given (Peterken & Lloyd 1967).
British and Irish occurrence
Occurs throughout most of lowland Britain and Ireland and reaches 520 m in Argyll (VC 98) and 550 m in Co Kerry (H1 & H2). The distribution becomes more scattered northwards and westwards into Scotland and similarly, but to a lesser extent, it thins towards the W coast of Ireland. In both cases, this most likely reflects the very open country, the absence of woodland, unsuitable soil conditions, excessive altitude and exposure (Godwin 1975; New Atlas).
European and world occurrence
I. aquifolium has an oceanic and suboceanic distribution in Europe and adjacent parts of Asia. It occurs throughout NE, C & S Europe, SW Asia and N Africa but is often cultivated beyond its native area. It reaches 64oN in Norway where it is confined to the coast and the southern limit is at 34oN in Algeria and Tunisia. It is absent in the Mediterranean from Cyprus and Crete. An eastern outpost is said to occur in Heupeh province, C China (Peterken & Lloyd 1967; Hultén & Fries 1986, Map 1295; Sell & Murrell 2009). Holly is also introduced in N America and New Zealand.
Holly woods and use as fodder
Despite its prickly leathery foliage, Holly once provided important winter fodder for sheep and cattle (Radley 1961; Kelly 1997). For this reason, Holly was planted and encouraged in hedgerows near farm habitation, making it readily available for cutting whenever winter fodder was required. In the 8th century, in Irish Laws of the Neighbourhood, I. aquifolium was one of the seven 'Nobles of the wood', the high ranking undoubtedly due to its fodder value, but possibly also on account of its very hard, heavy, ivory white timber which was used for 'chariot shafts' and for cooking spits (Peterken & Lloyd 1967; Nelson & Walsh 1993; Kelly 1997, p. 382).
It is important in this context to realise that in Ireland hay-making was rare before the coming of the Normans. The Old Irish law-texts, which provide extraordinarily detailed information on farming routine, contain no reference whatsoever to hay-making and no term for 'hay' has been identified in the language of this period (Kelly 1997, p. 47). As a response to stock browsing, prickly leaves are concentrated on the lower part of Holly bushes or trees, and the relatively spine-free upper branches were preferentially cut for feeding to cattle and sheep. A 14th century Irish source also recounts how Holly leaves were roasted a little to soften their prickles before feeding them to cattle (Kelly 1997, p. 46). One might imagine that boiling the leathery foliage would even better serve this function.
Holly hedges withstand pruning or clipping extremely well provided the tool used is sharp enough, in the current author's (RSF) experience, and apart from animals being allowed to browse Holly repeatedly in woodland, there is evidence that some Holly stands were deliberately planted and actively managed as coppice or pollard to supply cut winter fodder (Rackham 1980, 1986, p. 120; Spray 1981).
The previous occurrence and distribution of these long gone managed Holly woods is hinted at chiefly through the survival of place-names. The Old Irish name for Holly is 'cuillen' or 'cuileann', while 'cullenagh' is 'a place producing Holly', so that many places still have 'cullen' or 'cullion' (anglicised from 'cuillionn'), as a word element, for example, 'Cullen Hill' near Ely Lodge in Fermanagh. Irish townland names, either called 'cullion', or with it as a word element, are very common throughout the whole island (Joyce 1968; Milner 1992).
Names, uses and folklore
No less than 16 English common names are listed by Grigson (1987), reflecting both the variety of uses (timber, quick-lime also known as bird-lime, fodder, medicinal and Christmas seasonal decoration), magical folklore beliefs (lucky and unlucky) and physical properties of this familiar woody species, especially its prickles. Medicinal uses included treatment for chilblains, arthritis, rheumatism and whooping cough. In Ireland, it was used for treating burns and for curing a stiff neck (Vickery 1995; Allen & Hatfield 2004).
Threats
None.