IVY
- FRIEND OR FOE
Andrew Cowan N.D.Arb.
First Published: 2000
The issue of ivy and trees is one of those matters
where it is all down to personal opinion. To some, it
is a pernicious weed that smothers the natural form
of trees and on which constant war must be waged. To
others, it is an essential part of the wildlife habitat,
providing shelter and food for a diverse range of different
organisms. In reality, it will all depend on where and
under what circumstances the tree is growing. Context
is the key word here, and what might be fine in the
middle of woodland may not be so desirable in formal
parkland or a residential front garden.
Ivy is very well adapted to living in woodland, which
represents it’s natural habitat. It’s growth
characteristics enable it to survive where light levels
are low, on the ground and up trunks of trees whose
dense foliage shade the woodland floor. Ivy’s
attributes of shade tolerance and evergreen foliage
have proved invaluable in our gardens, where it has
been used for attractive evergreen coverings for north
facing walls and to provide ground cover in dark corners.
Although the common ivy Hedera helix appears the most
frequent, a variety of cultivars and other species are
available for horticultural use.
Hedera helix, is the only native British evergreen
climbing shrub. It has a habit known as dimorphism,
whereby two forms occur within the same species. The
juvenile growth, with its characteristic lobed ‘ivy
shaped’ leaves, is adapted to living in low light
conditions and is found creeping along the ground or
climbing up walls and tree; while as the plant matures,
it can throw out bushy branches and flowering shoots
with very different, elliptical leaves (lanceilate to
ovate). This adult form will only develop where the
light conditions allow, and it is mostly found on the
climbing section of the plant, only rarely on the ground.
Ivy has a very bad reputation and it is commonly thought
that it kills trees. Contrary to popular belief, ivy
is not parasitic and does not directly affect the health
of the trees it climbs. Unlike true parasitic plants,
(such as mistletoe, whose roots tap directly into the
resources of the host plant) ivy has its feeding roots
anchored in the ground and simply uses the tree as a
support to get to where it wants to go. The masses of
tiny, hair-like roots sprouting from the under surface
of the stems, are simply designed to provide support
and allow the plant to climb. Although these roots provide
almost immovable adhesion to the rough surfaces of tree
trunks and walls, they are not used for feeding, and
at worst only penetrate the outermost layer of bark
on host trees.
It is primarily in terms of competition for natural
resources that ivy affects the health of trees, particularly
where light is concerned. If ivy has become established
on a tree, it is more likely to be a sign of stress
than a cause of it. A heavy infestation of ivy, particularly
in the upper crown, is usually an indication that the
tree is in a natural state of decline; most healthy
crowns will let insufficient light through for the ivy
to grow vigorously. Ash, Fraxinus excelsior, is an exception
as the crown tends to be thin and open. This allows
major infestations to occur, thereby restricting photosynthesis,
but it is still unlikely that the life of a healthy
tree will be shortened. In the case of a diseased or
dying tree, where the it’s growth rate and vigour
may be slow or in decline, the ivy’s more vigorous
growth allow it to smother the tree. The bushy adult
growth will then have a tendency to make the tree top
heavy, making it more likely to fall, particularly during
adverse weather conditions.
One of the most important aspects when contemplating
the removal of ivy from a mature tree, is its enormous
wildlife value. The dense mass of foliage and intertwining
stems around the trunks of trees, provide shelter for
birds to build their nests, and dark nooks and crannies
where bats can roost through the day. In Autumn, ivy
flowers are an important source of pollen and nectar
for wasps, butterflies, bees and a hose of fly species.
Over winter, ivy protects woodland soils from full snow
cover and frost. This enables ground foraging birds
such as blackbirds, robins, dunnocks and thrushes to
continue feeding;. while a sheltered habitat is also
provided for small mammals and insects.
The berries, which ripen in March / April, have a high
fat content and, although poisonous in large quantities,
they provide both native and migrant birds with an invaluable
early energy resource. Woodpigeons, starlings, resident
and migrant thrushes and newly arrived summer migrants
such as blackcaps feed on them. Some species of invertebrate
are known to feed on the foliage of ivy, and several
species of beetle bore the mature stems, while spiders
spin their webs to catch others that fly in to shelter.
It should rarely be considered necessary or appropriate
to remove ivy from trees within a woodland setting,
where it is an integral part of the native ecosystem.
An experiment was carried out, from 1890-1942, where
ivy was cut on half the trees in a wood, and left to
its own devices on the rest. When the wood was felled
in 1942 there appeared to be no difference in the height,
average girth or cubic content of the trees. On the
other hand, in parks and gardens where conditions have
allowed it to grow unchecked, it can become quite a
problem; choking the crowns of ornamental trees, swamping
less vigorous shrubs and smothering walls and rockeries.
Although rarely a problem to the tree, a dense covering
of ivy over the trunk and throughout the crown of a
mature specimen can inhibit essential safety checks,
by limiting a visual inspection of the trunk and main
branches. Where mature trees are growing in residential
gardens often close to dwellings or public open space,
it is important to be able to complete regular hazard
assessments and monitor the decay of old wounds. In
such circumstances it becomes essential to remove the
ivy.
From a practical viewpoint , the most effective way
of removing ivy is to cut it near to the base of the
tree. When doing this, it is necessary to remove a section
of all the stems around the entire circumference of
the tree’s trunk. Once cut from it’s roots
the ivy growth, up the trunk and branches, can be left
to die on the tree, and when sufficiently dry and brittle
it can be removed. The best time to consider this work
is over the winter when the host tree is likely to be
bare of leaves, and visibility while completing the
task is much improved. Very dense ivy can sometimes
be used by bats to hibernate through the winter. To
avoid unnecessary disturbance of hibernating bats it
is best to cut the stems of ivy in the late summer or
autumn, so the foliage dies before the winter months.
The removal of ivy during the summer should be avoided
where possible, because of it’s likely use by
nesting birds and roosting bats.
It is an offence under the Wildlife and Countryside
Act 1981 (as amended), to intentionally damage or destroy
a wild bird’s nest, whether in use or under construction.
The use of the ivy by bats for shelter and roosting
must also be considered. A bats roost is protected both
under the Wildlife and Countryside Act 1981 (as amended),
and The Conservation (Natural Habitats etc.) Regulations
1994, which make it an offence to damage or destroy
a breeding site or resting place of any bat, and it
does not require the offence to be intentional or deliberate.
Furthermore, under an amendment made within the Countryside
and Rights of Way Act 2000, it became an offence to
recklessly damage or destroy a bat roost, and it could
be reckless not to consider possible use of ivy for
roosting.
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