There are several answers to this question, I am going to briefly discuss them all however, there is one answer I particularly want to discuss and that is 'never'.
Trees are managed for a variety of purposes including: timber production, fruit production, nursery stock, amenity, ecology, heritage, and each aspect of tree management is it's own individual industry.
Trees grown for timber production, forestry, or silviculture, are generally planted in tight grown groups at regular spacings. This introduces an early competition for light between the trees and therefore generates a crop that is straight and tall, perfect for harvesting in order to make the most of the product. Trees are generally harvested in groups. Pruning is not generally undertaken and weaker specimens or trees of poor form are naturally suppressed.
Trees grown for fruit production in orchards are similarly planted in uniform rows, at a greater spacing than for forestry, and are pruned regularly in order to promote flower and therefore fruit production. Pruning of a lead or apical shoot stimulates multiple side or lateral shoots to develop. Pruning also permits easy harvest of the fruit by keeping it within reach of the pickers.
You may even have heard of people beating trees. This again is a practice associated with fruit production; trees under stress anticipating decline or death will utilise all of their available energy to reproduce. Beating a tree therefore causing wounding may induce this state and therefore increase crop yield. (This method is not promoted.)
Nursery stock involves the cultivation of trees for sale onto all other tree related industries, and the general public, and incorporates the most important aspect of tree pruning, formative pruning. Trees develop new growth at their tips therefore the form of a young tree is same as that of the tree in maturity.
Early identification of structural defects, for example poor branch unions where the lead branch and a side branch are very close together at their point of origin, allows for formative pruning to be undertaken at a time where removal of a branch will result in a small wound easily sealed over or occluded by the tree. If the branch were to be left it would thicken along with the rest of the tree.
As the tree matures and the branch gains weight and size it may be susceptible to failure. If the branch does not fail naturally and is identified as a defect in maturity removal will result in a large wound which takes more time and energy for the tree to occlude and increases the chance of infection by providing a large area of exposed wood for pests and diseases to enter.
Formative pruning could conceivably save thousands of pounds of tree work required for trees permitted to mature with structural defects. It could also prevent the failure of thousands of trees for the same reasons. When purchasing nursery trees their form should be considered carefully and the tree, as it is, envisaged in maturity.
Trees managed for amenity covers much of our municipal tree stock including roadside trees and public park trees. Trees grown within urban settings are often subject to pruning regimes undertaken to permit traffic and pedestrians to pass unimpeded under mature canopies. Trees are also commonly pruned to permit site lines for security purposes whether this be CCTV cameras of patrolling police or security. Utility companies also regularly undertake pruning of trees to provide adequate clearance of cables, railway lines and waterways.
Trees managed for ecology and heritage are generally under a scheme of non interference save where over maturity or damage by storm render them in need of remedial works. There are also pruning techniques which encourage ecology that can be employed when remedial works are necessary in an attempt to mitigate the loss of part or all of a tree, or group of trees, which may provide valuable habitat.
The bottom line though is trees are self optimising structures designed to disperse forces evenly over their entire shape. They do this in order to effectively withstand wind, loading by means of fruit, leaves, and snow, and to successfully maintain their often enormous size in maturity. Pruning interferes with and influences that self optimised shape and can lead to and promote branch or tree failure.
Trees bending in the wind on a stormy day are dispersing forces through a graduation of branch size from the main stem and branches down to their shoot tips.
So, if you are privileged enough to have trees within your management whether they be at a place of work or within your own garden, remember to consider the full consequences of your actions. Always try and start from the viewpoint that you would rather not prune the tree at all and take it from there.
If pruning is necessary try and avoid wounding any trees during spring and autumn, when the tree is using much of its finite energy resources to either produce leaves or drop leaves.
Finally trees do develop deadwood. Stormy days, stress, disease, and many other factors, may result in the development of deadwood of varying sizes. Unless the deadwood is of a large size located over an area of high public usage it should be retained. The tree may still be utilising energy stored within the deadwood, and as development of deadwood can often indicate a state of stress further wounding may compound the issue. Deadwood also provides a valuable habitat for a multitude of insects, plants and mammals.
If you do think your trees need pruning always seek the advice of a professional arboricultural consultant. They usually offer advice for free and will be able to advise you whether further investigation is required and what if any remedial actions you should or can undertake in order to address your concerns. They will also be able to recommend a good arboricultural contractor to undertake any proposed tree works. Contractors are not all consultants and tree works and tree consultancy should be treated as separate areas of expertise.
N.B. Many arboricultural contractors have a good consultancy knowledge however this should not be assumed.
Trees should be inspected regularly by a qualified arboricultural consultant, particularly large, mature trees. It is recommended that trees are inspected every 3-5 years however, a consultant will be able to give advice on inspection frequency appropriate to your specific circumstances.
Welcome to The Green Man Arboricultural Consultancy
This website supports the consultancy business based in North Wales and which operates throughout the United Kingdom providing a wide range of arboricultural services including: home buyers tree reports, tree condition reports, development site surveys and reports, woodland assessment and management plans and general advice relating to trees.
For further information or to discuss your requirements please contact us on: 01978 821 851/ 07981 912 162 or via firstname.lastname@example.org