Dr Terry Mabbett takes a closer look at the well-known and resilient tree-climber.
THE nights are closing in and our thoughts turn to leafless landscapes as nature locks down for winter. Britain’s smattering of native evergreens offers some comfort, though the much-maligned common ivy (Hedera helix) offers something for all seasons with several intriguing anomalies along the way.
IVY’S ANOMALIES
Ivy will cling: Ivy is essentially a ground-cover plant that can to cling to solid objects. When these are vertical in orientation – whether trees, hedges, wooden fences or brick walls – ivy stems secure a hold and embark on rapid growth. The plant’s capacity to climb is due to adventitious roots equipped with clinging suckers. These have no absorptive function and are complementary to conventional roots in the ground, which absorb ivy’s water and mineral nutrient requirements.
At first sight, ivy has some of the attributes of an epiphyte, but cannot claim epiphytic status because water and nutrients are sourced from the soil via a classical root system. True epiphytes such as algae, lichens, mosses, liverworts and ferns growing on trees source their water and nutrients from the air or from moisture on the surfaces on which they grow. True epiphytes are also called ‘air plants’ because they do not root in the soil.
Last of the summer wine: Ivy’s second core anomaly relates to developmental events in its life cycle. Common ivy’s peculiarity is bearing flowers in autumn on non-clinging flowering stems at the apex of the plant, distinct from the creeping stems. And these stems have distinctly different leaves. So-called ‘juvenile’ leaves borne by the climbing stems are simple, but divided into three to five lobes. In contrast, the so-called adult leaves borne by fertile flowering stems in full sun, high in the crowns of trees and higher up in the hedge, are cordate (entire or un-lobed) leaves.
BENEFITS AND CONSEQUENCES
Broadened biodiversity: The consequence of common ivy’s propensity to climb trees is far-reaching and frequently cause for considerable concern. Its main plus point is extended biodiversity, including well-protected nesting sites for native birds. As a native plant, ivy can be expected to add significantly to the arthropod (insect, arachnid and terrestrial crustacean) population, particularly when in flower. When growing as ground cover in woodland, ivy insulates the ground against frost, allowing birds and mammals to forage.
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Whether the climbing capabilities of ivy have co-evolved with native tree species is a moot point. Ivy is most frequently seen on native English oak and common ash, both of which have a furrowed bark when at maturity, but considerably less so (if at all) on beech and hornbeam, with distinctly smoother bark. I cannot recall encountering ivy growth in any quantity on the exotic though naturalised sweet chestnut, although ivy is frequently found on horse chestnut with a similar set of characteristics.
Even conifers do not escape climbing ivy vines, with the more open-structured conifer species such as pines and larches frequently colonised by ivy. Irrespective of tree host, ivy can add to the aesthetics of the tree canopy and skeleton-like, semi-natural woodlands in winter, although clearly not everyone would agree. Ivy should be encouraged in semi-natural, broadleaf woodland, which is all the poorer without it.
Cut ivy down: One major downside is the widely held belief that ivy strangles and saps the strength of trees, although neither water nor nutrients is extracted. Ivy may levy an extra load bearing on already weakened boles and branches, though there is no evidence to show it kills healthy trees. However, there are long-held views, apparently rooted in Victorian times, that ivy debases trees, leading to equally long-held traditions of severing ivy vines on trees.
Once ivy is established on a tree, it becomes essentially impossible to strip off the vines without inflicting physical damage on the tree. There are few sights more ghastly than dead ivy vines wrapped around a tree. Cutting ivy at the base of the trunk is invariably a waste of time. The contractor goes away and within a couple of years the same ivy plant is growing back up from its established position in the soil.
If you don’t want ivy to grow up trees, then strip it off in the early stages – don’t wait until the ivy vines have colonised the canopy then cut to leave a sorry-looking mess for years to come. Ivy growth is out of the question in plantation forestry, because it can cause real problems for the felling, extraction and processing of timber. Ivy may not be wanted on landscape trees grown for their attractive bark, although little looks more attractive than the contrast of dark green ivy leaves against the white bark of the silver birch.
There some ‘conservationists’ who will strip ivy from its tree support despite overwhelming evidence of wider biodiversity benefits. These are the ‘lichen lovers’ who say ivy prevents these plant associations from establishing on tree trunks and branches. A lichen is not a single organism but a close symbiotic association between a fungus and an alga.
Mitigation of pollution: Not everybody will rush to sever ivy’s links with the tops of tall trees. In a contemporary quest to mitigate pollution in urban environments, innovative ivy screens are increasingly used to protect school children against traffic pollution. Evidence suggests ivy screens, typically seven feet tall, can improve air quality by filtering out lung-damaging particulates and absorbing pollutants from nearby streets.
A SHRUB FOR ALL SEASONS
Ivy offers something positive for all four seasons of the year. As spring arrives, ivy gives protection to nesting, resident songbirds such as blackbird, song thrush and robin. At this time of the year, broadleaf hedgerows lack foliar cover and are little more than skeletons and, as such, potential coffins for songbirds preyed on by avian corvids including magpies, jackdaws and jays.
Blue butterflies in spring: By April, ivy is flush with new growth and highly attractive to Celastrina argiolus, a beautiful blue butterfly called the ‘holly blue’. The first (spring) brood of this small azure butterfly is almost entirely on holly, which is food plant for the larvae at this time of the year, but switches to ivy for the second (summer) brood. One can only surmise that by August, holly leaves which emerged in spring are too tough and unpalatable for the larvae.
Drinking the last of the summer wine: Ivy continues to grow at a rapid rate through summer, with focus on flowering stems at the apex of the plant, covered with bunches of buds by August. On still September mornings you can smell and hear the results before you see them. Ivy flowers emit a heady and somewhat sobering scent which attracts a host of insects. The smell is overpowering and the humming incessant.
Humming of the Hymenoptera: The vast majority of insect visitors collecting and feeding on ivy nectar are bees and wasps. These members of the Hymenoptera home in on ivy to suck up the last of the summer wine. They include increasing numbers of a relatively new arrival in the UK, the ground-nesting ivy bee (Colletes hederae), first recorded in 2001 and steadily spreading northwards. The ivy bee nectars almost entirely on ivy flowers and is in flight from September to October, carefully coinciding with the flowering period of Hedera helix.
Autumn hedgerows covered with red hawthorn berries and blue sloes, side-by-side with ivy in flower, are a consequence of common ivy’s peculiar life style.
Nature’s winter famine reserve: Fertilised flowers grow into berries, small and green at first but swelling and darkening with speed. By February, hedgerows are covered with bunches of plump black berries, ready for taking.
Unlike the garish red and orange haws, rose hips and rowan berries, ripening early and easy for birds to see and find, the dark ivy berries remain hidden among the dull green foliage right until the bitter end. This usually comes in late February/early March when everything else has disappeared. Only the wit of hungry and near-starving native birds allows them find the plump, black ivy berries which are truly nature’s ‘winter famine reserve’.
Oh, a dainty plant is the Ivy green,
That creepeth o’er ruins old!
Creeping where no life is seen,
A rare old plant is the Ivy green.
From: ‘Ivy Green’ by Charles Dickens.
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