First published in Crann, Ireland's Tree Magazine, Summer 2010
Luke Eastwood brings his experience to bear as a head gardener to arrive at some conclusions on this controversial issue.
It's one of those plants that prospers in our variable and largely damp climate while also being able to propagate itself very successfully. For this reason, ivy (Hedera helix), which is one of the most widespread plants in this country, is considered more as an undesirable weed rather than as an attractive ornamental plant. However, interesting variegated varieties of Hederas are now widely available in garden centres all over the world. Ivy is an unusual plant in that it has different characteristics as juvenile and mature plcliht - when young it often trails along the ground or climbs trees and hedges with the aid of its adventitious roots and in this stage it also exhibits the familiar 3-pointed leaves. In its mature state the stems become woody, much like a tree branch, growth habit becomes lateral instead of vertical and the leaf shape becomes elliptic, as is typical among many trees.
It is these very characteristics that have made ivy somewhat of a nightmare for the horticulturist and aboriculturist, something that I can testify to myself! Ivy is a valuable source of food to bees, so fertilisation is therefore no problem; the subsequent fruit are usually eaten by birds and the ingested seeds are distributed far and wide. Although ivy seedlings are easily removed they have an annoying ability to germinate within the root space of other plants, thus making them often hard to spot and remove. The biggest problem, though, seems to me to be the sheer quantity of seedlings that can appear in often quite a small area of ground. As ivy is a very hardy plant, often resistant to weedkillers, and very fast growing, it is possible for unnoticed plants to become quite invasive in a short time. In the case of trees, if ivy germinates next o a sapling it can overrun the sapling in a couple of years causing the tree to grow in a very distorted and restricted manner.
As Risteard Mulcahy points out in his book 'For the Love of Trees' (which is primarily about ivy), the invasiveness of ivy has become a serious problem in our woodlands and hedgerows. Opinions vary as regards the level of hazard that it poses to our trees, especially as there has been very limited serious research on the subject. In the absence of empirical evidence one must rely on personal experience and what our own eyes tell us. In this respect I am with Professor Mulcahy 100% in my belief that ivy is indeed a serious problem as regards the premature death of mature trees and the disabling effect it has on immature trees. One needs only look at a typical hedgerow in almost any county to see the plethpra of ivy that fills hedges and covers the mature trees within it. In the case of hedges, the ivy tends to reduce leaf coverage of the 'host' that it climbs up as well as competing for resources such as water, light and nutrients. Over time, if not remedied, growth ofivy can severely impair the "edge, leading it to look weak and straggly,
As far as trees are concerned, there are several negative effects, becoming more severe with the passing of time. As with hedges, trees suffer from the competing effect of ivy, although this is generally not of any great significance if the tree is far older than the ivy parasite, As the ivy matures its stems thicken and intertwine, often forming a constrictive lattice around the tree trunk that puts severe pressure on the tree. There is also the possibility of ivy providing a good environment to insects that damage the tree bark and also the weight of the ivy in its advanced stages may distort the bark and perhaps constrict the flow of the Important fluids through the tree (xylem and phloem).
In its mature phase the lateral growth of ivy along and between branches can often block out light and restrict the photosynthesis of the tree, particularly with deciduous trees as ivy is evergreen, I have seen many examples of mature trees with sparse leaves mainly at the ends of their branches due to the massive spread of ivy that covers most of the tree.
So, having established that ivy can be a severe problem in arboriculture, how then do we deal with the problem? From my own experience I can say that it requires both persistence and determination to gain control and keep it in check. In the case of seedlings and young plants, one needs eagle eyes to spot and remove them before they get a chance to do damage; this, of course, is time consuming and labour intensive. With the more advanced specimens it is generally necessary to use loppers or a saw such as a ‘Bushmab' to cut the limbs all around the tree.
This can quite often be difficult as, over time, layers of ivy branches of differing thickness can build up and are usually obscured by the leaves. After stripping away the lighter branches or the leaves it is then posslble to see the extent of the ivy branches and where they have become intertwined. Often branches can grow into each other so that unless all of the branches are severed the plant continues to prosper. A good example of this is when I cut the ivy from a mature oak at work, the stems were between 1 inch and 3 inches thick due to the long neglect of the tree! I thought I had cleared all of it around the circumference of the tree but, upon returning a few months later, I was surprised to see the ivy was not dead,
On closer inspection I found that I had missed two minor stems that were growing into the lattice of stems and these two were sufficient to keep the whole plant alive. Even when the ivy is completely severed from its roots, the plant can take months to die, particularly if the weather is mild, Eventually, after death, the branches will rot and begin to fall away from the tree. Once this has begun it becomes a lot easier to manually remove the dead material. Although it is probably not desirable to completely remove ivy from the Irish woodland environment, as it is valuable to wildlife, careful control of this almost omnipresent plant is essential for renovation of degraded woodland and also for the maintenance of healthy woodlands and hedgerows.