Pruning trees when young (formative pruning) may reduce the need for major pruning in middle age and in maturity.
The need to prune middle aged and mature trees is frequently determined by their position in the landscape. There are occasions when changing land use (e.g. building or highway development) may make extensive pruning necessary
It is worth considering the removal of trees that need frequent major pruning and their replacement with a more appropriate species.
Pruning cuts should, wherever possible, be made at a fork or at the main stem to avoid stumps, which can die back, and dense regrowth of shoots. Removal of large branches should only be carried out when it is unavoidable and wounds from such work should be kept as small as possible.
Cuts into live wood should be avoided when removing dead branches and stubs. When a branch collar is present the final cut (figure 1) should be just outside it. When there is no collar the angle of cut should be the mirror image of the branch bark ridge.
Pruning with either a handsaw or a chainsaw should be done in stages so as to avoid splitting of the tissues and irreparable damage to the tree. The construction of a chainsaw may make accurate positioning of the cut difficult, especially on small diameter branches, and the use of a handsaw is frequently preferable.
Formative pruning should aim to produce a tree which in maturity will be free from major physical weaknesses. Unwanted secondary leading shoots and potentially weak forks which could fail in adverse weather conditions, e.g. strong wind or snow, should be removed.
Note: Failure to remove such shoots may necessitate premature felling to safeguard people and property.
When growth within a tree crown results in crossing branches that may rub together causing loss of strength or possible fracture in adverse weather, one of the branches should be removed.
Crown reduction and /or reshaping
Note: Some trees can be reduced in height and /or spread while preserving a natural tree shape (see figure 2) by crown reduction and/or reshaping.
Crown reduction and/or reshaping should be carried out by cutting back to a side bud or branch to retain a flowing branch line without leaving stumps. All cuts should be made just outside the line of the branch bark ridge and branch collar (see figure 1) of the retained branch.
Very substantial crown reductions should, ideally not be made during a single growing season since severe loss of leaf area and multiple wounding may impair a tree’s defences against diseases and decay.
Reshaping should be a ‘once only’ operation to make a tree safe or to bring it to a desirable condition or shape.
With a few species it may be appropriate to reshape a crown by careful pruning. This technique has a place in urban area management programmes for existing mature trees which have previously been pollarded.
Regular crown reduction may be harmful and may make a tree unsafe, in the same way that regular pollarding can render a tree unsafe. It is worth considering the removal and replacement of such a tree with a more appropriate species.
Crown lifting, which involves the removal of the lower branches to a given height above ground level (see figure 3) should be achieved either by the removal of whole branches, or by the removal of only those parts which extend below the desired clear height.
Crown thinning, which involves the removal of a proportion of secondary and small, live branch growth from throughout the crown to produce an even density of foliage around a well spaced and balanced branch structure should usually be confined to broadleaf species. Crossing, weak, duplicated, dead and damaged branches should be removed.
The percentage of crown to be removed should be stated, but the leaf area removed should not normally exceed 30% of the original coverage.
Note: Crown thinning can stimulate many tree species into producing epicormic shoots and a dense crown will frequently form again.
Pollarding, which in some circumstances has been a traditional form of management, should not be used on trees that have not previously been pollarded, as the large wounds created initiate serious decay in mature and maturing trees.
Note: Very heavy pruning may kill some species (e.g. beech) while others will be stimulated to produce a proliferation of very dense regrowth of shoots from around each wound. Such shoots grow vigorously and have weak attachments to the tree making trees potentially dangerous unless recutting is done frequently. This risk is smaller for very young trees, but it is better to plant an appropriate species for the site rather than to restrict the size of an unsuitably wide spreading or all growing species.
Further guidance and recommendations can be found in BS 3998 (2010) "Tree Work - Recommendations".