This is the third in our series on issues related to pruning of arid landscape trees. As has already been covered (AZTimes 2:6, 1995 & 3:11, 1996) the primary goals of tree pruning are to compliment the natural shape of the tree and to promote healthy, vigorous growth. Pruning can have a number of effects on trees. It can compensate for root loss, aid in maintaining health and appearance, control plant size, influence vigor and re-invigorate stagnant trees. Two things should always be appreciated when pruning: 1) improper pruning can serve to stimulate additional unwanted growth; and 2) each pruning cut creates a wound in the tree bark. Proper and selective cuts will enhance the appearance and health of trees and leave wounds that will heal rapidly. The angle and position of pruning cuts greatly influences the overall success of a pruning program and dictate how quickly wounds will "heal". Pruning cuts should be made close to, but not beyond, the branch bark ridge and the collar at the base of the branch (see diagrams 1).
Sharp tools that make clean, smooth edged wounds will heal the quickest. These wounds don't "heal" like animal wounds. Instead trees produce callus tissue that essentially "re-cover" the area. Wounds heal from the edges. This is easily seen by observing the ring of raised or swollen bark surrounding the edges of the wound. The bark tissue forms the callus, giving the edges of the cut this raised appearance. Over time, with growth and the subsequent increase in the branch diameter, the old wound is closed completely. Dull pruners and saws leave ragged edged cuts. Such cuts develop callus more slowly delaying the healing process. Wounds that are slow to heal can be sources of oozing sap (that can stain hardscape elements and patio furniture) and serve as points of entry for insects, bacteria and fungi. These pest can cause additional damage and further delay healing.
Another common mistake is leaving short stumps instead of pruning branches off just above the collar. Aside from being extremely unsightly (and unprofessional looking) these stubs can sunburn, dry out, cause die back and serve as entry sites for some wood boring insects, bacteria and fungi. Tree borer damage is often misidentified as being caused by tree stress or general decline when in fact it is the result of stub pruning. Secondly stubs can snag clothing or the skin of pedestrians. Most importantly, such cuts generate additional unwanted branches by stimulating both lateral and adventitious buds (bud arising from previously woody tissue) to produce numerous new branches. This proliferations of branches must ultimately be removed by additional pruning.
Removing the branch at the collar serves to direct the subsequent growth towards the terminal of the remaining branch without excessively stimulating other buds to produce additional branches. Similarly, when heading back a branch, pruning back to a lateral bud seems to direct the branch growth through that bud making it the new terminal bud (see diagram 2 center (B) image for correct cut angle).
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