Spring Pruning Update 2016
Alfred, Lord Tennyson once wrote “In the spring a young man's fancy lightly turns to thoughts of love.” Were Tennyson a landscape professional, instead of a poet, he might have ended this phrase with the word “pruning.” More often than not, the sad truth is that most landscape trees would live longer, be more vigorous and look more naturally attractive if we thought more and pruned less.
It’s easy to forget that trees are the products of millions of years of evolution.and have only been victim to man’s efforts to “control” them for a few thousand. As we welcome the spring and the beginnings of the growing season for southwestern trees, let’s review some basic principles of tree pruning.
Less is More: The objective of any pruning program must be to compliment and reinforce the natural form and structure of the tree, not reinvent it. Pruning always has some adverse effects on trees and should be evaluated from the prospective of its impact on the tree's growth and vigor. Removing branches serves to limit the total surface area available for photosynthesis, lowering the plant’s capacity to manufacture "food" (studies indicate that as much as 60% of all photosynthesis in trees like Blue Palo Verde occurs on the surface of young branches). Further it reduces the total amount of stored carbohydrates (particularly with trees transplanted while dormant) and results in less vigorous growth. Damaged or broken branches can and should be removed to facilitate and accelerate wound healing.
The native form of most desert adapted species is multiple trunk or low breaking branches (the first branch originates within 18” of the soil). Attempting to substantially modify the structure of desert species (e.g. pruning a low branch multiple trunked tree into a more standard trunk form) is 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.
Failed Attempt to Make a Multiple Trunk into a Single
The 80/20 Proposition: Periodic light thinning is the most desirable method of pruning. Frequent, light thinning is especially important in June, July and August. Selective thinning, prior to the monsoon months, can reduce the number of wind damaged branches and prevent uprooting of trees. Removing more than 30% of the total tree’s canopy, during any one pruning session, can lead to aggressive, unwanted branch re-growth, limited root development and increased vulnerability to sunburn injuries. Sun injury can lead to colonization by wood boring insects.
Sun Injury Due to Poor Pruning
Ideally, no more than 20% of the tree’s foliage should be removed in any single pruning session. These pruning sessions should be concentrated within the top most 80% of the new growth (the outer third of the canopy). Any additional thinning should be directed to removing succulent ("water") growth, crossing and damaged branches ONLY!
80% and 20%
The Lion’s Tail Tale: Pruning exclusively from the inside of the canopy working outwards toward the branch tips, can produce Lion’s Tails, tufts of small branches and leaves at the top edges of the canopy. This compromises both the appearance of the trees and interferes with the development of taper and strength of the branches. It is ALWAYS preferable to regularly remove many small branches than to annually 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 (less than 1 inch diameter). By starting with "clean-up pruning" (the removal of small branches), the natural form of the tree is more apparent. While pruning, stop periodically, 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.
Classic Lion's Tale
Repeat this process several times while pruning. Recall, improper or inappropriate pruning can act to stimulate additional unwanted growth. Corrective pruning can stimulate desired growth or reduce and better manage growth and form.
Excessive Growth Caused by Improper Pruning
Parting Thoughts: Never hedge or head back desert tree species, as this will only stimulate excessive branching. Always use clean, sharp tools that are regularly cleaned in a fresh solution of 10% bleach. Bleach acts as a sterilizing agent and helps prevent the spread of diseases within and between trees being pruned.
A Young Tree Developing Good Branching Scafolding
Proper Branching on a Mature Tree East West View
Proper Branching on a Mature Tree North South View
Some desert species are slow to exhibit the effects of freeze damage. Branches that do not immediately bear leaves in spring may not necessarily be damaged. Delay pruning suspect branches until the tree has begun producing new leaves. Realize that early season pruning can reduce spring flowering.
Never Forget: Even a brief session of poor pruning can reverse years of proper pruning and maintenance and cause damage to landscape trees that cannot easily be repaired. If you are not sure what to do, get a certified arborist's professional advice. Fortunately, extensive pruning is not required for most desert tree species when they are appropriately positioned in landscape designs. Proper tree placement and growth management can significantly reduce the amount of pruning desert adapted trees require. When deciding where to place trees in the landscape, consider their mature height and spread. If in doubt, mildly exaggerate the mature size rather than underestimate and risk having trees ultimately too close. Plant trees strategically around structures, play areas, pedestrian traffic and surrounding trees so that mature trees can provide maximum shade with minimal interference. Also, consider how mature trees will interact with other components and uses of the landscape. 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
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The above is intended as an overview of some very basic dos and don’ts of seasonal pruning trees. For a more detailed discussion of pruning, including illustrations, consult “Pruning Practices for Desert Trees-2005” on our website.
