Deciding Where and How Much to Prune

Spring is in the air and it is time to think about getting back into the garden. Tree work should always be planned to maintain, as far as possible, the aesthetic and ecological value of the tree; and should retain enough leaf-bearing twig structure to ensure that the tree remains healthy and able to resist disease and decay.

 

For most forms of crown management, pruning cuts should normally be made at a branch union so as to avoid the retention of stubs, which can die back, inhibit wound occlusion, or give rise to an undesirable proliferation of new shoots, e.g. in Lime trees. Stubs may, however, be retained in order to encourage the formation of shoots in old trees, lapsed pollards or damaged trees with conservation value.

 

Branch Pruning and Removal

In order to minimise the potentially adverse effects of pruning, the cross-sectional area of the cuts should be restricted, as follows:

a) each final cut should be kept as small as possible,

b) the diameter of the final cut should be less than one-third of the diameter of the parent branch. If the tree is old or declining, the maximum diameter of individual cuts should be even smaller, to allow for the relatively small proportion of sapwood and the slow rate of wound occlusion,

c) the number and size of cuts should generally be limited so that their total cross-sectional area does not exceed one-third of that of the stem, when measured at 1.5 m above ground level. An example is shown in Table 1, where the stem diameter is 600 mm and the cuts are all equal in size,

 

Table 1: Stem Diameter of 600 mm

Size (diameter) of cut (cm)  Max. number of cuts per size Total area of
wood exposed (cm2)
% of 600mm stem diameter area exposed
5 48 3,770 33.33
10 12 3,770 33.33
15 5 3,535 31.25
20 3 3,770 33.33

d) If a branch is to be shortened, the cut should be made distal to a union or group of unions where one or more healthy lateral branches bear enough leaf-bearing twig-structure to sustain the retained branch. If there is only one such union near the intended cut, the lateral branch should have as large a diameter as possible (i.e. at least one-third and preferably more than half that of the removed branch), and

e) in species which lack durable heartwood or which have particularly weak defences against wound-induced decay (e.g. birch); the exposure of older central wood should be avoided where possible.