Alys Fowler: ivy

Frost on Hedera colchica ‘Sulphur Heart’. Photograph: Gap Photos

Apart from a brief spell over Christmas, ivy gets a bad rap. It is thought to be invasive, and to harbour slugs and snails; many are sure it strangles trees, too. But for every complaint, there’s another side.

On the ground under trees, ivy protects insects and the soil from hard winter frosts; above ground, it gives spiders a home and hides small birds. Its flowers are rich in nectar; its berries rich in protein. I am not advocating a garden of the stuff, but a little goes a long way. And there’s a much wider world than the straight Hedera helix: there are pretty ones (H. helix ‘Glacier’), wavy-edged ones (‘Ivalace’ or ‘Parsley Crested’), rampant ivies (H. colchica ‘Sulphur Heart’ or ‘Dentata Variegata’) and smaller, delicate ones (H. ‘Congesta’ or ‘Erecta’).

There are two stages to ivy’s life: first comes its sterile, juvenile phase, when it romps around uninvited and eats too much. At this point the leaves are distinctly lobed and it grows aerial rootlets that cling to anything and everything. Next is the lofty and fertile arborescent stage: the stems don’t have clinging aerial rootlets, the plant grows vertically and the leaves are rounder, often heart-shaped. It is in this stage that ivy flowers and fruits.

At the juvenile stage, the plant is easy to propagate; any node will give rise to new roots. Ivy can stay permanently in its juvenile stage. Light seems to affect whether it moves from one stage to the other, so if your ivy is in deep shade, it may never flower.

If you want rid of ivy, there’s only one effective, environmentally safe option – dig it out strand by strand. Often it’s easier to give it something to grow up, which makes it seem less invasive once it can climb. Also, if the ivy is growing up a tree and you cut it off around the base, it tends to grow back, and in the meantime you are left with unsightly dead ivy.

If you are growing a tree for its fine bark or attractive trunk, a coating of ivy is not ideal, but at no point is ivy parasitic. Those aerial rootlets merely support the plant; its ground roots do the feeding and watering. If the ivy appears to be growing into the trunk, this is often because it is exploiting the rotting trunk of a dead or old tree.

The canopy can become infested with too much ivy, which often happens in open-crowned trees such as ash. Ivy can act as a sail in high winds or add weight to an ailing tree. In these cases, it may be necessary to thin the ivy. Mostly, however, it’s not a problem; trees have been supporting ivy for far longer than we have taken issue with it.