It is all together too common for the natural growth characteristics of arid region trees to be completely ignored when these species are pruned. Trees native to mixed conifer and hardwood forests have a generally upright growth habit, typically dominated by a strong central leader. In these forests competition is primarily for light. The ability to grow straight and tall give those trees a significant competitive advantage over trees with other growth habits. In the southwestern desert competition is primarily for water. In this habitat trees are typically highly dispersed and tend to grow nearly as wide as they are tall producing branches that extend to the ground. This unique growth habit must be considered when developing pruning practices for desert region trees. The goals of pruning desert trees must be to promote tree vigor and health and to enhance and compliment the natural form of these native species.
An overview of pruning methods for arid region trees has been covered before (AZTIMES 2:6, 1995). A significant portion of the structure and form of nursery grown trees is usually firmly established before trees are offered for sale. For these trees all that is required is maintenance pruning and thinning to compliment and reinforce the established form and remove potentially problematic branches. It follows then that, in locations where standard trunk, upright trees are required, the desired ultimate form should be well established at the time trees are purchased and installed. Attempting to substantially modify the structure of desert species (e.g. pruning a low branching multiple trunked tree into a more standard trunk form) is almost never successful. The typical result is trunks that have a cork screw or highly twisted shape with large gaps between branches and numerous, large and unsightly pruning scars.
PRUNING PHILOSOPHY: It is ALWAYS preferable to regularly remove many small branches than to periodically remove a few larger ones. Removal of crossing and parallel branches and branches that pose hazards to foot traffic should be removed first. Both crossing and obstructing branches are best removed when they are relatively small twigs. By starting with "clean-up pruning",( the removal of small branches), the general form of the tree is more apparent. Stop periodically and step back from the tree, like an artist working on a painting, and take in an overall view of the tree from ALL sides. Identify problem areas then begin pruning again. Repeat this process several time while pruning. Recall that improper or inappropriate pruning can act to stimulate additional unwanted growth. Corrective pruning can stimulate desired growth or reduce and better control growth and form.
The most common point where tree branches fail is at the junction of two or more co-dominant or adjacent branches. This failure usually is from an included bark branching juncture or from lion tailing the trees branching structure, over burdening the branching connection points. Included bark is bark embedded or a bark ridge turning inward between opposing branches, a branch and a main trunk or two co-dominant branches creating a structurally weak point in the tree Included bark prevents strong attachments of branches, often causing a crack at the point where branches meet. An inward bark ridge line usually develops where they join and, more importantly, the included area declines or dies from to the cambium of both branches being squeezed and killed, weakening the branch or trunk. Trees with co-dominant leaders tend to have included bark and are more likely to split and ultimately fail. Included bark may be remedied by removing the smaller of the two branches or the one supporting less of the overall mass. Branches with wider or U-shaped angle of attachment should be retained. Good branch attachments have a raised ridge line or collar at the point where branches meet.
PRUNING METHODS: It is well documented that sharper pruning tools make cleaner cut that generally heal rapidly. Keep pruners and saws sharpened clean. Use the appropriate tool for the size of branch being removed. "Fine toothed" saws can be used on larger branches to finish a pruning process, leaving a smooth cut surface that will quickly heal. Removal of small branches can be done almost any time of year. Fall and winter have the advantage of giving the individual a better view of the structure of branches as leaves are shed. Clearly the objectives of any pruning program must be to foster tree health and vigor in concert with the natural form and character and to compliment the landscape design. In our January, 1997 issue of Arid Zone Times we will review the do's and don'ts when it comes to making pruning cuts.
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