This is an episode from the predecessor of the column ‘5 questions’: QED. Each instalment centred on a single question.
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Ivy is said to make life difficult for the tree it grows on by stealing nutrients and blocking sunlight. But is that actually true?
They exist: people who set off on their bicycles with a saw or an axe, on the lookout for trees that appear to be buckling under ivy’s rampant growth. The idea is that ivy is a parasite that robs its host of nutrients, deprives it of sunlight, and sometimes even causes it to collapse under the added weight. In short: get the saw out! But does this line of reasoning actually hold up? ‘People certainly mean well, but most of them go about it rather aggressively, which regularly causes damage to trees’, observes biologist Dick van Hoffen, board member of Vereniging Bomenbelang Bronckhorst. ‘That is a shame not only for the tree – which is usually perfectly fine – but also for the ivy: there are few species as important for the environment as Hedera helix.’
According to Van Hoffen, that begins with the fact that ivy is one of the few late-flowering plants in the Netherlands. ‘In autumn it produces nectar that all manner of insects make grateful use of, including wasps, bees, hoverflies and butterflies. And that in turn attracts various predators, among them the spotted flycatcher. It really is worth standing next to an ivy plant in mid-October to enjoy all the activity’, Van Hoffen says enthusiastically.
That activity does not stop in autumn: because the plant carries leaves year-round, with ripe berries in March, it is a perfect shelter, nesting site and overwintering spot for birds. ‘All kinds of birds – from redwings and pigeons to woodpeckers and the tawny owl – get through the winter with the help of Hedera helix. The Eurasian blackcap eats the berries on its return in March. Taken together, that makes it an indispensable pillar of our biodiversity’, says Van Hoffen.
But does this line of reasoning actually hold up?
Then the key question: is ivy a parasitic organism that ultimately spells the end for trees? ‘Absolutely not’, says Van Hoffen emphatically. ‘In the tree, ivy has only small adhesive rootlets that give it support to climb higher. It does not strangle the tree – unlike honeysuckle, which does wind itself around branches – and it does not take nutrients from the tree either. Hedera simply has its own root system in the ground, at the base of the tree.’ Van Hoffen does not expect any competition with the host tree’s roots either. ‘Tree roots often extend beneath the entire crown projection and deeper, meaning they cover a much larger area.’
Yet everyone has seen them at some point: trees covered from top to bottom in ivy, with the plant seemingly intercepting precious sunlight. Surely that cannot be good? ‘You do sometimes see old trees becoming overgrown. Those trees grow somewhat more slowly, but they can remain like that for 25 years. And it is genuinely rare: most of the time Hedera climbs only as far as the thick branches, so light competition remains negligibly small.’
Traditionally, the advice was to keep trees free of ivy in order to carry out a proper safety inspection, but nowadays tree surgeons too are advocating leaving ivy in place. A visual summer inspection is sufficient. Are there really no drawbacks at all?
‘There are’, Van Hoffen concedes. ‘Tree species with soft wood can suffer from the weight or even be brought down by it – willow, elder or birch, for instance. Fortunately, ivy occurs mainly on sandy soils, where many hardwood species grow, such as oak and European beech. These can easily cope with ivy. In short, take action if you think ivy poses a risk because it is growing on a soft-wooded species in your garden. Otherwise, just let it get on with it!’
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