Another interesting article about pruning tree , in the book called “THE PRUNING BOOK” by “LEE REICH”. Hers is an excerpt from the book
Broadleaf evergreen trees and bushes
Generally,do what little pruning is necessary to bring out the natural form of a broad leaf evergreen.The right plant in the right place needs minimal pruning.Techniques for pruning evergreen trees and shrubs are similar to techniques for pruning their deciduous counterparts. Because evergreens are clothed in year round greenery,though,the arrangement of their branches is never prominent,so we prune these plants for pleasing three dimensional form,as well as for plant health,strength(in the case of trees),and perhaps flowers.
When needed,direct growth just as you would on any other plant. To remove a wayward stem without causing regrowth, cut it away at its point of origin or back to one of its side branches.Where you want denser growth,shorten a stem while the plant is dormant or pinch a shoot as it is growing.Wait to prune early- flowering plants until after their blossoms fade if you do not want to miss their show.Other specifics with respect to timing of pruning are noted in the Plant List.
Except where otherwise indicated,do most pruning of these plants around the time that growth is beginning.Dormant pruning can be done anytime that a plant is dormant,but if you wait until just before growth commences,new growth will quickly hide the pruning cuts.Also,
winter damage is more evident once new growth begins,and it is less likely to occur with marginally cold-hardy plants pruned at this time.
Prune a bush to give a pleasing form and sufficiently dense growth.
Most important with a broadleaf evergreen tree is to train the branches to a sturdy framework when the plant is young.Begin as soon as you plant by allowing only a single main stem,which will become the future trunk and central leader of the tree.If two or more shoots are competing for the role of central leader,cut away all but one.If a branch is turning upward and threatening the leader’s dominance,slow growth on that branch by pruning it back partially or by bending it down,with weights or string to a more horizontal position. Throughout the early development of your your tree,keep that central leader upright and dominant.
This single-leader“rule”is not hard and fast.Because the distinction between trees and shrubs is vague with evergreens, many plants that are naturally bushy can be trained to small trees(and many of the trees can be bushlike).Small trees are not threatened by collapse,and many of them look attractive with multiple trunks originating from ground level.Develop this form by selecting two or three trunks as the plant grows;alternatively,let the plant grow as a bush,then reduce the number of stems growing from ground level and trim off branches from the stems that remain to make them into trunks.
Small branches growing off the central leader of a developing tree will,in time, become main scaffold limbs of that tree. Begin selecting future scaffold limbs when you put the tree in the ground. Choose branches that are spaced far enough apart along the developing leader so that they will not be crowded even when the tree reaches old age.A distance
of 6 in.to 18 in.is adequate,with the closer distances reserved for smaller
trees. Successively higher branches should arise in a spiral fashion up the
trunk so that each has adequate space to develop and is not being starved by another branch directly and closely beneath it.So that the trunk can envelop and firmly hold the base of a branch,that branch must be thinner than the trunk.If a well-placed branch is too thick in proportion to the trunk,slow its growth by pruning part of it away,especially near the end of the branch.Strength that comes from good scaffold-limb choice and development becomes increasingly important with larger trees.
Leave any other branches growing off the leader as temporary branches.These branches will help strengthen the developing leader and trunk.Only remove those branches that are crowding the permanent scaffolds.Leave temporary branches only for about three years,and prune them back in the meantime if they start getting too big. Choose side branches growing off scaffold limbs in a similar way as you chose
the scaffolds themselves.Retain side branches that are not crowding each other or growing too near the central leader,then prune to suppress the growth of any that threaten to grow thicker than the limb from which they spring.