In Britain there is always a debate about the desirability of allowing ivy to grow on trees. Many people consider that it is harmful to allow it or, at best, that it spoils their appearance.
Ivy is Britain's only native climbing evergreen and is very widespread. It can tolerate deep shade. If it were an exotic plant imported from far away it might be widely considered a great boon for gardeners. As it is, we frequently think of it as a weed. Is it actively harmful? It can of course be annoying, invading shady borders where tidy gardeners might not want it. It can radically change the appearance of a tree, removing the beauty of a clear trunk or its classic appearance. Long term experiments have shown however that the effect on the growth of a tree of allowing free rein to ivy is negligible. Only aged trees are likely to be affected by the reduction of light caused by the ivy leaves or the greater profile offered to the wind. Unlike parasitic mistletoe, ivy does not take anything from trees as it grows, it merely uses them as a means of support. In the process it provides a wonderful habitat for birds to nest, insects and invertebrates to live and bats to roost. Flowering late in the year ivy gives pollen and nectar at a time when there is little else around. These flowers produce berries in March and April which are an excellent food source for thrushes, pigeons, starlings and overwintering blackcaps.
Many people will however decide that the appearance of a cherished tree is being spoiled by ivy and will decide to remove it. This is is often easily done but keep in mind that cutting the stems of a large ivy plant on a tree will result in brown unsightly remains and these may take a year or two to fully remove. If the ivy is removed during the spring or summer it is important to realise that this may disturb nesting birds or roosting bats, both of which activities are against the law. If you still want to remove your mature ivy from a trunk it is best done in early autumn when these problems are less likely to arise.
In the end the decision to remove ivy from your trees is probably more of a personal matter. It is likely to be about neatness or the pleasure derived from seeing the uncluttered tree. In their natural situations trees and ivy, at least in Britain, have achieved a balance which provides a very valuable habitat.
