Spring is the principle season for pruning evergreen trees. In adopting a correct approach to the task, the gardener should be focusing on two separate but connected matters. On the one hand, we are interested in the tree growing in the desired manner, all the while recognizing that pruning is liable to seriously affect the future long-term health and survival of the tree. In this regard, we should never forget that the tree stands unmatched as the single most significant and precious feature in the garden.
Pruning trees for shaping depends mainly on the natural growth habit of the species concerned. At one extreme, there are the plants whose natural shape is so strongly defined that pruning, at least for shaping purposes, is unnecessary. Two examples are Palms and Cypress trees. At the other end of the scale are trees such as Hawthorn or Elm, which tend towards a wild, untidy habit. Many, if not most garden species, require at least some pruning.
A golden rule for shaping purposes is to avoid shortening branches because this “stops” the natural direction in which the branch is growing. Instead, limbs that are earmarked for removal should be cut back to the trunk or thicker branch to which they are attached. In time, it appears that nothing has been pruned at all. This does not have to apply though to young stems that in some species shoot forward as long, but thin leaders. Such a growth pattern is common amongst citrus trees for example, and there is no harm in clipping these leaders, in order to encourage lateral growth.
It is important to remove at the juvenile stage, those stems that are clearly liable to be troublesome when they thicken over the years into mature branches. The most obvious candidates for early removal are stems that grow parallel to the trunk, or whose angle to the trunk is too small. Pruning out a young stem is often a matter of a quick snip with the secateurs. Attempting to saw a thick branch however, is not only time-consuming (the lesser problem by far) but will almost invariably result in a pruning wound which will become a source of rot and decay.
It is natural to believe that our hands are the principle part of the human anatomy by which we prune trees. This is utterly wrong! Professional gardeners do not prune with their hands, but rather with their eyes. Thought as usual precedes deed. Always have a clear idea as to which branches are to be pruned before even touching the saw or secateurs. Secondly, after removing one branch, do not proceed to the next, but put the tools down, step back from the tree and look at what you’ve done, revising your initial plan if necessary.
From the angle of the plant’s health, two crucial points should be recognized. Firstly, removing excessive material at one session can seriously reduce the energy level of the tree. Arboriculturists have reduced the whole complex of tree care to a matter of maintaining a positive energy gradient within the specimen. As a rule of thumb, one may remove, as an absolute upper limit, one-third of the volume of the tree. To be safe, however, I recommend pruning no more than half that figure. If there are many branches to prune, then it is best to stagger the work over a couple of seasons.
Secondly, the pruning wound should be as small as possible in relation to the width of the trunk. Large pruning cuts do not heal properly, even if the wound appears to have completely calloused over. The result is bacterial or fungal infections that lead to rot and decay within the heart of the tree. In cases where the branch to be removed is too thick in relation to the trunk, it can be shortened to a stub of a about a meter in length, (3 feet) and sliced back further every few months, as though it were a salami or cucumber. This has the effect of retarding the thickening of the branch, and while the trunk continues to thicken over a few years, its diameter remains the same. Consequently, when the final pruning cut is made, the wound will be of an appropriate size relative to the width of the trunk.