An introduction to Pruning
The pruning of a tree depends very much what species the tree is. Different species are pruned according to the time of year, due to sap rising and risk of certain diseases. The amount of branches that can be removed from the tree depends on the circumstances in which the tree is growing, but on average no more than 20% of the photosynthetic area (the crown of the tree) should be removed unless the trees in dormancy. Most pruning that is carried out is due to encroachment of property and highways or to reduce the likelihood of failure to a branch or tree (eg. crown reduction to reduce the sail, so the resistance experienced from high winds is limited). Pruning can be carried out to maintain the size of the tree, or to increase the amount of sunlight available to a garden or property. Pruning of fruit trees is achieved by reducing the water shoots, this will cause the tree's chemicals to make these buds swell into fruit or spurs.
Removal of branches: the size of the wound should be left as small as possible & rarely exceed 20cm though BS3998 stipulates that the wound should be no larger than half the diameter of the stem. This is to limit the amount of decay and disease that could enter the tree. Removal of large limbs should be avoided where possible and reduction of the branch should be considered instead. Small and young trees will benefit from formative pruning at an early stage to limit the size of wounds and the amount of work to be carried out in the future.
Branch collar: the tree has natural chemicals and barriers contained throughout the working system which help prevent the spread of decay and disease. Pruning cuts should be made so the wounds can seal quickly (usually cutting the shortest distance across a branch), leaving the swelling behind (contains many of the chemicals and barriers) close to the trunk. Stubs should be avoided and cutting flush with the trunk should not be done. Branch bark ridges are where the branch is attached to the tree, only by the bottom (which is why when a branch is pulled downwards it will break) and these ridges help to show where to prune if reducing a branch. If a branch is pruned at 45 degrees to this ridge this will provide a guide line as to where to prune the branch collar. If you are attempting to prune a branch yourself, then always place an undercut before cutting from the top and make your first cut at least 30cm away from your final pruning cut of the branch collar. This is to avoid tearing of the bark onto the trunk. As a 'rule of thumb' always cut the shortest distance.
Crown & branch reductions: usually carried out all over the tree or to a desired section of the canopy. The main objectives are to reduce the size of the tree, thus reducing the sail, so that resistance experienced from high winds is limited and to remove some of the weight held up by the limbs. This significantly reduces the likelihood of failure to individual branches or entire stems of the tree. Other benefits include the amount of sunlight available to a garden, property or swimming pool, the opening of a view or the increased reception to an aerial. The amount of branches to be removed depends on the tree – usually branches removed on a small tree are only around 1m in length and on large specimens can be anything between 2-3m in length, depending on the tree's branch structure. All pruning carried out should leave branches of a 1:2 ratio behind (this means the branch left is half the size of the limb it is attached to) or 1:3. This leaves a natural branch flow and look to the crown, and means that the branch will support the limb with enough nutrients. A crown reduction usually needs to be repeated every 4-5 years. If carried out with the installation of a cobra bracing system, large trees that would otherwise have to be felled can be retained or until a replacement tree has been planted and is well established. This means that continuous green cover can be provided.
Crown thinning: This involves the pruning of end branches all over the tree to achieve a less dense canopy. The size and shape of the tree will remain similar but its benefits are that it allows wind to flow through the canopy and dappled sunlight to reach the ground. A percentage of 20% is normally stipulated.
Coppicing: felling certain deciduous trees and leaving a stump a foot or so high allows the tree to re-shoot from dormant buds. Coppicing is an ancient form of woodland management to produce fire-wood and other timber materials. It is a process that has been practised for centuries and by re-coppicing every 3-6 years a regular supply of set sized timber can be achieved. It can be a way of retaining the benefits of a tree that has been planted in the wrong situation, or a tree that would be problematic if allowed to grow too large if close to a property or drains.
Remedial pruning: often carried out when the tree has established too many branches within the crown, that have grown from dormant buds on the main stems. It occurs naturally on some species but is usually caused by over-pruning or when the tree is under stress eg. from drought. Some of these branches need to be left behind because on a hot day the leaves on the outer canopy close their stomata and stop performing photosynthesis and the branches in the cooler part of the tree then start to work. It can also be a sign that the tree is trying to create a new inner crown to replace the existing canopy when it dies.
This can be a sign of root disease, root damage from mechanical digging, compaction, chemical and oil spills or can simply the age of the tree.
Careful selection of branches to be left behind should be made so that the inner structure of the tree does not interfere with existing limbs or crotches and this will help to reduce the growth of dormant buds re-occurring.
Removing all of the branches within the crown must be avoided, as this can cause many dormant buds to grow over the next year or two. Then the process would have to be repeated regularly at a high cost to the owner. Also the branches help to draw the nutrients along the limbs and if removed can cause the ‘lion tail effect’ where branches break off due to exposure to high winds or lack of moisture and minerals. Over-pruning such as topping, can cause an abundance of dormant buds which must then be thinned out to select dominant leaders to re-creating the tree’s natural structure.
Crown lift: this involves the removal of the lowest branches. The objectives are to allow sunlight to reach plants and can give an impression of more space and less size. If the lower limbs of the trees are too large to be removed then some of the limbs secondary branches can be removed instead. This form of pruning is also carried out to open up views, create access and to prevent encroachment on a highway or property.
Fruit tree pruning: Pruning of fruit trees is achieved by reducing the water shoots, this will cause the tree's chemicals to make these buds swell into fruit or spurs the next growing season. It is best to carry out the pruning while the tree is in dormancy because if pruned any other time of year the fruiting buds (flowers or fruits) will be removed and there will be less fruit for that season. The inner crown will need the removal of crossing and diseased branches. This also allows sunlight to reach through the crown and ripen all sides of the fruit.
Pollarding: Is a viable way of maintaining a trees size.It is the removal of all branches during dormancy, leaving the structure and shape of a tree. Usually carried out to street trees, the pruning should be carried out every 3 years because after this time the branches start to store the energy and this will be lost if the stems are removed. Species normally pollard are London Plane, Lime, Willow and Poplar. Pollarding is not to be confused with topping,which should only be adopted for hedging (eg. Cypress, Beech hedges). Topping leaves wounds that are too large to seal and removes large amounts of energy stored within the stems. Mature trees can suffer if heavily pruned though some may benefit to a veteran pollard if in decline. Once a tree is pollard it is important to repeat this process, otherwise the tree is more susceptible to branch or stem failure at the point of pollard.
Chris Baxter Arborist Telephone 07763 155061