Ivy – Knowing when to leave it, knowing when to intervene

Glossy ivy leaves on a tree – great food a shelter for wildlife but not so great for the tree in the long term – click here for photo source/link

Ivy (Hedera helix) is England’s only evergreen climber / liana and provides year round foliage and cover for a variety of wildlife. This is why, along with Holly, it is revered in folk lore for its capacity to remain green even in the very depths of winter. However, Ivy is not just for Christmas!

Butterfly on Ivy

Comma Butterfly on Ivy Flowers. – Click here for photo source/link

It flowers late in the season, September, and being rich in pollen and nectar is a vital ‘fuelling up store’ for a wide range of overwintering invertebrates including Red Admiral, Tortoiseshell and Comma butterflies (shown here), numerous hoverfly and bee species. Ivy foliage is also a vital cover for brimstone butterflies whose underwing shape and vein configuration resembles the individual leaves of the plant amidst which it hibernates. The clamorous exit of wood pigeons as they fly from the canopy-level boughs is a noisy indication that ivy also provides important and important roosting and feeding point for this bird, which will gorge on the purple ivy berries, given half a chance. Other avian species like blackbirds, thrushes, robins and wrens also exploit the cover of ivy’s heavy drapes of foliage for their nesting sites. Numerous UK bat species use ivy clad trees as a roost – mostly for the summer months but a few through the autumn, winter and early spring months too.

Ivy – the canopy of the young ash tree in the left of the picture still has light and room for growth. In the middle trees intervention to cut the ivy is urgently required if the trees are to survive, and it’s too late to bother doing anything to the extreme right – the oak tree is already dead.

Whilst ivy undoubtedly provides benefits for numerous animal species, its merits for other members of the plant kingdom, particularly trees, are more debatable. Ivy is not parasitic (a misconception many have, including myself in earlier years) it’s tiny hook-like clingers are just to maintain a vice-like grip on the bark  but do not actually draw sustenance from the tree itself. However as they continue to grow and coil new, ever-thickening stems around the tree, the ivy constricts the vital capacity of the trunk and branches to expand – something known as ‘secondary growth’ (primary growth being the annual extension of shoots at the tips of leaders and twigs) and this can cause the cambium layer, so important for the tree’s continuing growth and health, to lose its vitality and the stem ceases to expand. The sapwood becomes constricted and the timber becomes less flexible and more brittle. During high wind conditions, largely in the winter months, when deciduous trees should have lost their leaves, the evergreen ivy acts like a sail and can bring down the limbs and even whole tree itself if it occupies much the canopy.

Noctule bat emerging from a tree hole roost, ivy helps to conceal and maintain a more even temperature for such roosts. Click here for photo source / link

So, like any other aspect of multi-purpose forestry, if a balance in the ivy population is to be maintained it needs to be managed. All UK species of bat come under the European Protected Species regulation which means that their habitat, as well individuals of a population are protected. So thus the timing, extent and severity of intervention to control ivy growth on trees is crucial to make sure that no bats (and ideally no other species) are harmed or compromised in any way as part of the process – go to the Bat Conservation Trust’s webpage for further help and guidance and useful downloads in this respect.

A bat survey – using a light, low intervention endoscope probe to inspect cracks and crevices behind ivy stems – click here for photo source/link

If large areas of woodland management are planned including felling of bigger trees a ‘damage minimization’ approach may be justified where particular compartments, hedgerow / boundary corridors of trees and even just significant individual ivy clad trees within the woodland are retained whilst work goes on around them. A bat survey of individual trees may be necessary in order to establish their presence or absence and measures will need to be put in place to ensure that such trees are ‘retained or refrained’ in the programme of work until it is certain that the bats have vacated that particular roost.

Within this framework a ‘rotation’ of ivy on trees may therefore be encouraged and a useful indicator of the time where intervention against continued ivy growth is required is the extent to which it has penetrated the crowns of individual trees and the quantity and density of trees. As a useful rule of thumb, the larger and older the tree that has ivy, the more it should be retained, ivy and all, as the greater its contribution to wildlife as a bat roost, shelter and nesting site for other species ; unless it can be demonstrated that the health of the tree and its timber value or the safety of those that walk and work below it may be compromised and/or there are numerous trees of a similar character within the same location (ie a 12 metre radius).

A simple hand tool such as a sharp bow saw, pruning saw or even a pair of loppers is sufficient to help cut the stems of ivy from around a tree. Cut out and remove a clear section of all the ivy stems visible around the trunk of the affected tree being careful not to cut into the tree itself.

Removal of ivy from around a tree trunk using pruning saws – click here for photo source link

Trees and the 'After-life'
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