Thursday, 13 February 2014

To Cut or Not to Cut - The Ivy Debate

Over the years I've heard many times the same reaction by people when talking about ivy and trees -

"Ivy smothers, strangles and kills trees"
In this blog, with help from Brian Muelaner - Ancient Tree Advisor for the National Trust - I will be exploring the facts to find out the truth, once and for all.

The Ivy Cutters

Ivy which has been cut to reveal the stem of a tree growing close to a popular tourist destination.

There seems to be a common held view that ivy (Hedera helix) will grow up a tree, smother it with dark green leaves and a myriad of thick woody stems, eventually causing the death of that same tree. The danger of this opinion is that people might cut away ivy from trees, with good intentions but without knowing the damage they could actually be doing.

Brian says:

"Ivy is much maligned and it is mistakenly thought that ivy strangles trees, this is totally untrue. Ivy sends up a vertical main stem and then puts out side branches which hug the tree as opposed to spiralling around it. Ivy's hugging branches open up as the tree grows"

Only recently I walked around an important ancient wood in the Lake District and was horrified to see tree after tree with dead ivy - all had neat sections of their woody climbing stems cut out (to stop the cut pieces fusing together again) and the stark sight of dead ivy leaves over the tree stems.

This in my view was an act of vandalism, and was akin to cutting trees down for no reason. How could this happen and why?

The Real Deal

Let's take a look at the facts...

Ivy is a climbing plant

There are 5 woody climbers in Britain which are ivy, clematis, honeysuckle, dog-rose and woody nightshade.

Adapting to climbing takes time to evolve, with a specific beneficial purpose in mind. The plant will use energy producing the structures it needs to climb with, in this case very fine roots to fix the ivy stems to trees or buildings. There has to be a real advantage to climbing which will aid the plant to survive and reproduce, within that particular habitat. On ivy, only the branches that reach sunlight (often high up in the tree canopy) will produce flowers.

"It uses trees and walls simply as scaffolding, clamping itself on
by means of a mat of adhesive suckers. Only when these
encounter soil or deep crevices does it put out true, feeding
roots." (Mabey 1996)
By using this ash tree as support, the ivy has grown up into the tree canopy where it'll get more light and produce flowers.

Ivy is eaten by deer and sheep

Holly and ivy are the only British evergreens that are both edible and non-poisonous to livestock (Rackham 2006).
I pointed out to a colleague that the trees around the edge of his car park all had ivy on and that must make safety inspections difficult. Looking deeper into the woodland his trees were bare. This I believe was because deer were too afraid to venture to the edge of the wood and up to a busy car park to nibble on the ivy, instead staying deep in the woodland and browsing away on all they could eat.
A browse line - where deer eat all they can stretch their necks up to - is often seen on ivy in woods and even gardens.

In a woodland garden I found this scots pine which had a very clear browse line on it. Deer will be the culprits.

Ivy is shade tolerant

The glossy, dark green leaves of ivy are similar to other evergreen plants and shrubs, such as holly, which can tolerate a certain amount of shade and so be found growing under the canopies of trees. Leaves that grow in sunlight contain more chlorophyll and are a brighter shade of green than leaves growing in the shade, although both types of leaves can be found on the same tree or plant.
Ivy is not restricted to shady places - it can be found growing in full sun on the edge of a wood or even on top of walls, where it will flower in abundance.
It's now thought that ivy will grow all year round especially in the warmer south, and certainly making the most of early spring sunlight before the tree leaves start to unfurl.

All around the world, plants that grow in shade will have larger leaves than those which get full sunlight.

Ivy provides a habitat

Ivy can support a healthy population of insects, which will also support a healthy bat and bird population (Read 2000).

Ivy is a flowering plant and like all others provides nectar, especially in autumn, for flying insects such as hover flies. Ivy is also valuable for woodland birds such as the tree creeper and the blackbird.

Here Brian pointed out that:

"Ivy plays an extremely valuable role in providing late nectar and a protein rich berry when most other food sources have disappeared"
He also added:
"Several species of bat use ivy’s thick cover to safely roost and some will even use ivy as a winter roost if the ivy cover is dense enough"

Ivy will increase the effect of wind 

The effect that wind has on a tree is lessened when the leaves have fallen. If a large tree were to be covered in ivy then the wind 'sail effect' would be greater than if the tree has no ivy. If the tree was decaying, then this added weight and sail area might be enough to cause a collapse, but only a little sooner than otherwise.

The increased surface area of the ivy leaves will also hold a huge amount of rain water, and in winter storms this may cause the tree crown to collapse.

I recently saw a section of dry-stone wall in Ullswater blown over in strong winds due to a thick covering of ivy.

Ivy will shade the stem of a tree

Sun can have a scorching effect on the stems of trees and especially those with thin bark such as beech. Ivy on the south side of a tree would help to reduce any stress or damage caused by prolonged intense sunlight. If this ivy was then suddenly removed the tree could suffer and die.

The stem of this woodland edge tree is shaded by the thick glossy ivy leaves.

Ivy is not a parasite

Beneath the soil surface in a woodland is a vast array of roots from trees, plants and fungi. The trees and plants will be competing with each other for water and nutrients and an underground battle will be going on. 

Brian on this matter said:
"It is mistakenly thought that ivy is a parasite, that the little suckers coming off the branches of ivy are taking water and nutrients from the tree, whereas they are just anchoring points to improve its hold on its host.  Ivy gets its sustenance from the ground just like the tree, so there is a small degree of competition with the tree, but a healthy tree can easily obtain all the nutrients and water it needs even with ivy growing up its trunk"

Ivy will add weight to stems and branches

Climbing plants cannot know if their chosen tree is in full health or suppressed and dying (in a woodland situation), if it is the latter then a climber might bring on the demise only a little sooner through its weight. If parts of the tree start to decay and the strength reduces then the added weight of the ivy might cause quicker collapse than normal.

However in a healthy tree the added weight of ivy will trigger reactive growth in the branches, making them stronger and able to hold up the extra weight.

Ivy is a native plant

Ivy (Hedera helix) is a common plant of dry oakwoods here in the Lake District. We must protect and encourage all our native flora and fauna, especially those from protected and important habitats such as native broadleaved woodland. Native wildlife will rely on or benefit from all our native plants.

When to Cut

Lichens can be very important and if there is a chance that ivy could smother rare or important lichens on the stem of a tree then control might be needed. Light grazing is the best way to achieve this. (Read 2000)

Buildings often have ivy or other climbers growing up them. Important buildings could be damaged especially those with soft lime mortar, paintwork or buildings which would suffer from damp. On these occasions, provided a bat survey has been carried out then I would be happy to see the ivy removed.

BRE Digest 416, Bird, Bee and Plant Damage to Buildings states:
‘Ivy is a serious threat to buildings. Its stems produce short adventitious roots which grow in search of moisture and darkness.’

‘If these aerial roots secure a foothold in cracks or open joints they will inevitably cause damage. In time, they will disrupt the wall by forcing stones apart as they grow and thicken.’
 Interestingly, cutting ivy on buildings might make matters worse:
"When ivy is cut off at the roots it tends to try and use
adventitious roots within holes in the walls to provide alternative
moisture sources – thus causing more damage". (Viles 2009 appendix 7.2)

Tree safety is the one area where I admit I have purposefully severed thick ivy stems around the base of a mature tree, because the tree was in such a high risk area (busy roads & houses) I could not be confident that the tree was safe without seeing - after the ivy had died - the condition of stems and branches.

If ivy has to be cut then great care must be taken.
"If the tree...has a thick covering of ivy, it should ideally be surveyed for bats by an experienced bat worker prior to any work. Contact your local bat group to request a survey" (Read 2000)
Brian Muelaner, Ancient Tree Advisor for the National Trust said:

"It is important that ivy isn’t stripped off the tree…I recommend the cutting of one stem a year until all of it is cut, which spreads the impact over many years giving wildlife time to relocate"
Brian also pointed out a time when cutting ivy might save an important tree:

"When trees like oak go into ancientness around 600 years old they will naturally retrench: their upper crown will start to die back which exposes their inner crown to light and here new buds develop. If the branches are totally encased in ivy when the tree begins to retrench then the new buds are not stimulated and the tree would die"
A tree in full health has its leaves above the growing ivy.


From the evidence above we can be sure of these facts:

  • Ivy is not a parasite, it doesn't intentionally kill a tree it uses for support.
  • If a tree is already in decline or decaying then ivy may simply speed up the death or collapse of a tree.
  • The shading effect of ivy can protect trees from strong sunlight.
  • Ivy is a superb native habitat providing food, shelter and deadwood. Important insects, protected bat species and declining woodland birds all benefit from ivy.
  • The lack of ivy can indicate too much grazing in a woodland.
  • Cutting should only be done for public safety, protection of important buildings and conservation of epiphytes such as rare lichens, mosses and liverworts.
After this, I now feel that there isn't a good reason to cut ivy if tree health is your goal. There are exceptions as I mentioned earlier, but after seeing this blog I hope readers will correct anyone who says...."ivy kills trees"!

What You Can Do....

  • Don't cut ivy unless you really have to, and seek expert advice.
  • Plant more native ivy (Hedera helix) in your garden, near to trees.
  • Pass the message on that ivy is a great place for wildlife!

Ben Knipe
Woodland Ranger

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