Epping Forest Conservation Volunteers

SHEAR DELIGHT! A GUIDE TO TREE PRUNING

BY KEVIN MASON

The what, why, when, where, how and who of tree pruning.

1. What is a tree?
   Tree pruning is all about maintaining the health of the tree, so before pruning starts it is necessary to consider how a tree works. Understanding the processes involved in tree growth will help us appreciate the ideas involved in carrying out pruning successfully.
   A tree is a dynamic living organism with a self-supporting woody stem. The tree converts carbon from the air into sugars by a process known as photosynthesis. These sugars are used to make the building blocks of cellulose and lignin which are required to sustain the trees self-supporting structure.
   The inner bark area of the tree known as the phloem provides the mechanism to transport these sugars to all areas of the tree where they are used. Any sugars which are not immediately required are stored by the tree in the trunk, branches and roots.
   Water and other essential minerals and nutrients are absorbed from the soil by the tree roots. Once in the tree they are then carried to the leaves along tubes known as xylem. The tree produces flowers, fruit and seeds from these minerals and the sugars produced by photosynthesis; this ensures the next generation of trees can be produced.

2. Why prune your tree?
   Before carrying out any pruning operation ask yourself the following questions
   Will pruning leave the tree open to an increased risk of disease?
   Will pruning leave a large wound on the tree?
   Does the pruning cut out a large leaf-bearing branch?

If these questions can be answered 'no' then move on to the final question

Assuming you have an answer to the final question then pruning can proceed.

The reasons why trees need pruning are many and varied. They could include allowing more light in, removing dangerous branches, improving the structure or size of the tree or if the tree has grown to close to a neighbouring building. Care must be taken when pruning not to remove too much in one go. This will reduce the vitality of the tree and disease can enter the tree through the open wound left behind. It is better to take smaller cuts over a number of years if the tree has grown particularly large.

Removal of branches can be a dangerous business especially if the branch cannot be safely cut with both feet firmly on the ground; the safest option if this is the case is to employ a qualified and insured tree surgeon.

At this point it should be recognised that in most circumstances it is always best to prune trees as little as possible, unless there is a serious risk of failure of the tree that needs to be dealt with. Large scale pruning removes wood and the foliage necessary for the tree to photosynthesise and can put the tree under significant stress leading to infection and death of the tree.

1. Where to make the pruning cuts and how to do it.

   

   A branch of a tree is formed from a growth bud which became a twig and eventually has grown into a branch.

   Each year the tree produces an annual growth ring and so does the branch. This strengthens the joint of the branch to the trunk of the tree. Depending upon the species of the tree the attachment point will produce a strip of raised bark which is known as the branch bark ridge on the top and sides of the branch and a branch collar on the underside.

   The British Standard for tree work (BS3998: 2010 Tree work) recommends that the final cut should not be more than one third of the diameter of the parent stem or branch. This will keep the wound as small as possible and help keep disease out.

   To make the pruning cut a sequence of step cuts should be carried out. The first cut should be a shallow cut on the underside or branch collar, this will prevent the bark from tearing when the branch is cut off. The second cut should be on the topside or branch bark ridge and further away from the main stem than the initial underside cut. If the tree does not form ridges and collars then the cut should be made in the same sequence leaving a small diameter wound. It is also advisable to leave a stump and not cut too close to the main trunk as this will decrease the chances of infection entering the main trunk.

   Cutting off too much in one go and leaving a large wound increases the risk of disease entering the tree. Certain bacteria and fungi can enter the tree through these wounds and cause the woody structure to decay faster that it would normally.

2. When to prune.

   If the decision has been made to prune or remove a branch then what time of year is best?

   In general the best time to prune a tree is when it has reached full leaf which is late spring early summer dependent upon species. At this time of the year the tree is at full production of sugars has plenty of energy and will be able to heal over the wound quite quickly.

   There are however other considerations for cutting at this time of year as it is likely to be bird nesting season and it is illegal to deliberately cut or tamper with an active birds nest so check the tree first for nesting birds or any other wildlife e.g. bats before work commences.

   Some species for example birch, maples and walnut will bleed sap when cut at this time of year and risk losing valuable sugars if they are pruned too early in the year. So pruning of these species should be kept to when this risk is low, so late summer or mid winter.

   The Prunus genus of trees such as cherry rely on producing resin or gum to heal over wounds and protect against infection so these trees are best pruned in summer when production of resin is at its height.
   As a general rule pruning should avoid times of year when the open wound will be left open to severe conditions such as frost, drought and the height of the fungus season, early autumn.

   The table below gives an appropriation of the best time for the pruning cuts to be carried out by the most commonly found species it also shows which trees respond well to cutting and which ones don’t.

This guide was first published as part of the EFCV Newsletter in Autumn 2016.