Pruning Deciduous Broadleaf Trees
In the wild, trees manage well enough, so why do we prune
them? The first reason is simple: safety. Trees are big and when
pieces fall off, they can do considerable damage.
Another reason, particularly for fruit trees, is to stimulate
flowering and fruit production. Occasionally, pruning may be needed
to remove disease or malformed branch work. Rarer still, pruning is
sometimes needed for timber production, such as veneer wood. But,
the most common reason of all is to control the growth and
appearance of the tree for aesthetic reasons (particularly in
cultivated varieties) or to control a perceived nuisance.
This leaflet refers only to deciduous broadleaf trees in
general, as evergreens and conifers often have different
requirements. Also, it should be noted that even the trees dealt
with here may have their own very specific requirements. The
following is no more than a guide; if in doubt, seek expert
advice.
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THE TREE AS A VISUAL AMENITY
Deciduous broadleaf trees are the most commonly found in England.
They range from our native Oak to exotic species such as the Tulip
Tree, from America. They all lose their leaves in winter.
A mature tree that has never been pruned will have a complicated
arrangement of branches which may be seen best in winter.
The shape of the tree is dictated by the structural
framework of the main branches. If those are pruned, the
shape of the tree will be changed.
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A mature deciduous broadleaf tree without pruning
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An extreme form of Lopping and Topping
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In urban areas, trees used to be pruned by lopping and
topping, a technique now generally frowned upon.
It leaves an unsightly pole of a tree. The regrowth often leaves
weaknesses where branches could snap away and the risk of rot in
the main stem is increased. Lime trees can tolerate this treatment
better than most, but the result is often unattractive to look at.
Other trees, such as Beech, can be killed by this treatment.
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Crown reduction gives a far more pleasing
result where the overall size of a tree has to be reduced. Even
trees that have been subjected to dramatic pruning in the past can
sometimes be restored to a reasonable shape by this procedure.
Crown reduction involves the overall reduction of just the crown of
the tree, so there is little change in the overall height.
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Such reductions are usually quoted as percentages, the usual
maximum, being 30% (in the USA the limit is set at 25%). Greater
reductions usually entail changes to the structural framework of
the tree and can cause undue stress to the tree Only exceptionally
will a larger reduction be justified. Most reductions will be in
the 10% – 20% range, if the tree is amenable to such works in the
first place.
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Crown Reduction and Thinning |
Where crown reduction would leave a dense mass of branches,
thinning may also be carried out. Again, a guideline maximum is
30%.This procedure is best carried out in winter when it is easier
to select the branches to be pruned. An exception to this is
thinning selectively to decrease shade (see summer pruning).
Crown reduction in excess of 30% is often referred to now as
pollarding, so-called after the old practice of pruning trees at
2-3 metres, still seen in riverside Willows. Nowadays, pollarding
is often applied to any heavy reduction work that changes the
structural framework of the tree.
Amenity Pollarding can be used to describe a
modern alternative to the discredited lopping and topping. While
not the procedure of choice, it is often the only option available
for trees that have been badly managed in the past. Unless there
are essential reasons, it should not be initiated on a tree, except
as formative pruning when young.
If amenity pollarding is the only viable option, care must be
taken to avoid creating a tree that is more eyesore than amenity.
High pollarding is one such mistake. The resulting tree looks quite
unnatural. However, it does have a place in a formal avenue,
particularly of Planes, or Limes. This is more usually seen on the
continent, as are other specialist pruning techniques, such as
pleaching, annual pollarding, and umbrella pruning.
A last major category of pruning is crown
lifting. As its name implies, this technique is used to
increase the bare stem height below the crown.
Crown lifting may be done for a number of
purposes. The most common is to obtain clearance above highways (up
to 6 metres) and footpaths (up to 2.5 metres). It is also quite
frequently employed in garden renovation, to free up ground below
the tree, either for access, or to increase ambient light. None of
these works should normally require removal of structural branches.
More controversial reasons are when the tree is on a development
site, or if the work is to increase vistas. Both these cases often
require the removal of large low limbs of considerable age.
It is often hard to justify works to a mature tree just for a
view, particularly when the pruning may require the loss of major
branches. This can result in a poor looking tree and the wounding
involved may unnecessarily shorten the life of the tree.
A Mature Tree Crown lifted
for Highways, For light and access,
Excessively
There are a number of cases where pruning requirements are for
particular problems. A few examples are given below.
Clearance pruning may be required when the
canopy of a tree is very close to a building, especially to the
north side. If the wind and sun cannot get to the building, the
exterior becomes permanently damp and this encourages algae and
moss to grow, which in turn increases the dampness. Clearance
pruning should always be just of young growth.
Depending on the individual circumstances, 1 – 2 metres
clearance between a building and the tree canopy should allow a
reasonable airflow. This pruning may be required at regular
intervals. There should not be an aesthetic problem here as the
tree and building would be viewed in their totality, rather than as
individual elements in the landscape.
Formative pruning is the procedure carried out
on young specimen trees to remove foreseeable problems, such as
tightly forked and rubbing stems; and to create the structural
framework for the mature tree. In some ways it is akin to shrub
pruning, but on a grander scale. There is no reason why such
pruning should not be carried out from an early age until the
desired final shape of the tree forms. Trees heal far better when
young. This is when choosing a good specimen from a nursery pays
dividends.
BRACING
Another past
practice was bracing to prevent the forces of the wind taking their
natural course. The problem is that once a tree is braced, by wire,
chain or rod, it may be impossible to assess how safe it will
remain. There is, however, a new flexible form of bracing that
allows some natural movement in the tree, but reduces the worst
effects of the wind. It also allows the controlled fall of a branch
or stem if one does snap.
Fruit pruning is highly specialised and is
beyond the scope of this leaflet. Useful information should be
found in the appropriate gardening books.
Note that non-commercial fruit trees are no longer exempt from
the legislative controls for Conservation Areas. This may be
related to the fact that people grub out old fruit trees on the
basis that they are no longer productive, ignoring their amenity
value.
When pruning out dead branches it is important to realise that
the tree was aware of the dead branch long before it became visible
to humans. The tree has probably already taken the necessary
compensatory steps to protect further dieback. It follows that dead
wood removal should be just that. Never cut beyond deadwood without
taking expert advice, or you could undo all of the tree’s good
works.
WOUND PAINT
The issue of wound paint is simple: trees have
been around for millions of years losing branches and stems in
storms and to the attentions of animals. Popular twenty years ago,
it is now recognised that wound paint probably does nothing except
promote disease by maintaining a humid atmosphere for fungi and
bacteria to thrive in.
Summer pruning is recommended for some trees
that weep copious amounts of sap. Species prone to this include
Walnut, Hornbeam, Mulberry, Birch, Lime, Acers and some Poplars. It
is also recommended for those trees that could otherwise suffer
from infections prevalent in springtime, such as silver leaf of
Prunus. It can also be useful when thinning for ambient
light.
It should, of course, be remembered that most trees can be
pruned in the summer. The main obstacles to work being carried out
at this time is the tree surgeons ability to see the structure of
the tree, the fact that wood is difficult to cut when the sap is
flowing and, last but by no means least – birds and bats may be
using the tree for nesting or roosting! This is not something to
take lightly as disturbing nesting birds or a bat roost is an
offence. Trees and wildlife will be the subject of another
note.
However, as far as the health of a tree is concerned, the only
time pruning should never be done is during the
spring growth period. Generally, this is March to June when the
buds are swelling, bursting and then the new leaves are growing to
their full size. The tree is investing stored energy at this time
and can ill-afford to have it wasted.
IVY DOES NOT STRANGLE
TREES
Ivy is very beneficial for wildlife. It is a natural component of
the countryside providing food, roosts, nesting and hibernation
sites for a wide variety of wildlife.
Ivy can cause damage, however, if it establishes in the crown or on
limbs. The tree has developed in tune with its own weight. When Ivy
gets high up into the tree, the extra loading it puts on branches
can cause them to snap, particularly in high wind. More
importantly, in the long term, Ivy can cover up problems in the
stem of a tree. A good rule of thumb is that Ivy needs controlling
once it appears as a mass in the crown.
The precise methods of pruning for homeowners are dealt in
another leaflet, but if you are seeking information now, the
keyword for searching in books, or on the internet is ‘target
pruning’. This is a technique that complements the tree’s natural
processes.
The local authority often gets asked to recommend tree surgeons.
We do not do this as it interferes with free competition. However,
it is strongly recommended that you always turn down the ‘today
only’ offer from the uninvited caller, however much a bargain you
may seem to be offered. More often than not the result is expensive
butchery. Any tree surgeon worth his salt does not need to tout
door-to-door for business.
If you need to trim trees near overhead cables,
always employ a professional tree surgeon.
Bringing down a telephone cable may be expensive, but shorting out
electricity cables may be deadly.
We hope that you have found this information helpful. This is
one of a series of leaflets on trees and wildlife. While we have
tried to cover the broad outlines, there maybe something that we
have missed. If you wish to comment on this leaflet, please contact
Nick Delaney on 01304 872481, e-mail nick.delaney@dover.gov.uk
Please note that this information is for guidance only. The
terms and techniques used are to aid tree owners, no more.
Contact
Telephone: (01304) 872205
E-mail: conservation@dover.gov.uk
