Does ivy kill trees?
I am sure I have mentioned this controversial topic before but it is still a bit controversial. And yet, ivy (Hedera helix) is not a parasite and it does not strangle trees- as honeysuckle can. It clings to surfaces with its aerial roots but these do not penetrate the bark. So that is clear then. Simple.
But things are never quite that black and white are they? ‘So’ it is argued ‘ why is that dead tree covered in ivy?’ It is obvious that the ivy killed it!
To explain this, and to slightly anthropomorphise the ivy slightly, we need to look at the curious morphology of ivy. Ivy has a juvenile and a mature phase. This seems slightly odd and is more commonly seen in acacias and eucalyptus but can be seen in holly in NW Europe. Young plants (from seed) of holly are often very prickly – the spine-edged leaves protect the plant from browsers – and when older and taller the leaves tend to be less armed. Ivy is a weak-stemmed creeper when juvenile, scrambling across the ground till it finds something to climb. It then scampers up any support, to get to the light. When the support ends and the shoots can’t cling, the growth changes dramatically. It becomes stouter and self-supporting, branching freely and it starts to flower. The flowers are produced in late autumn, providing nectar for insects and the berries turn black and mature in winter and early spring, providing food for birds. Few plants are as useful for wildlife. The dense foliage, overlapping like tiles on a vertical surface, provide shelter for insects and other creatures. This dense cover also keeps walls dry but as a form of insulation it needs care to control it and you can do damage if/when you have to remove it.
Ivy does not directly kill trees but it is often seen thriving on trees that are in decline. This is because ‘sick’ trees, with poor leaf cover, allow more light to get to the ivy and it will grow more quickly. A healthy canopy of tree leaves limits the vigour of the ivy. And when the tree dies, the ivy thrives, hoisted up into the light, and it will flower and berry freely. When the ivy gets big and bulky it will bring down dead branches as they rot.
This makes it sound as though I think ivy is perfect but I don’t and I vowed not to plant ivy in this garden. This is because I adopted a laissez-fair attitude in my last garden and let the seedlings, dropped by birds, start to climb the house walls. Before I knew it, they had scaled the wall and was out of reach, in the gutters and becoming a real nuisance. It is not easy to remove the ivy without damaging the pointing. It also stuck to the frame of a greenhouse, on an inaccessible shady side and sneaked between the glazing and shattered glass.
I have some wild ivy in the surrounding hedge and it can stay there. But I do remove any seedlings I find. Ivy makes good ground cover in shade and I may plant some in the future but I am resisting for now and am collecting vincas at the moment, which at least won’t climb trees. The little pots of variegated ivies that are so useful for winter pots are always tempting and they are lovely in containers in shady spots but remember that, if they get their roots into surrounding soil, they will revert to their wild habits.
But rules are made to be broken and I am now adding one ivy to the garden. The curious thing about the ‘adult’ parts of ivy is that, when you take cuttings of these, they retain their shrubby form and are sometimes sold as ‘tree’ ivies. They are not common but they make excellent shrubs for shady spots. Their shrubby habit and glossy leaves look wonderful in winter. They usually reach about 1m high before the weight of the branches gets too much and they start to spread rather than gain height and, of course, they flower and fruit. So I have bought (well I hope it arrives) ‘Poetarum’ the poet’s ivy, which produces golden yellow berries. It is usually called ‘Arborea Poetica’. All this is a throwback to my time at Myddelton House where there were ancient tree ivies planted in the deep shade of cedars, surviving along with Cyclamen hederifolium in the dense, dry shade.
Ivy is not an aggressive tree killer but it is a vigorous plant in the garden. It can be difficult to eradicate where not wanted but has many uses and, especially if it is allowed to bloom, is valuable for wildlife.
Honeysuckle does the same though. I mean that it does not kill trees directly. It just climbs them.
Oh, you mean that honeysuckle strangles young trees. That makes more sense.
Yes, honeysuckle twines round the supports and it can literally strangle trees as they increase in girth
Algerian ivy does not twine, but can branch and then fuse into a network around tree trunks, which can tighten as they and the tree trunk within grows. It rarely does so, and even when it does, it does not seem to damage the affected tree. I really do not know how it can do that. Strangler figs do exactly as their name suggests though.