Logan, Jeremy. New Life Journal, June. (2009)
Over the centuries, tree pruning has evolved beyond simply cutting trees to generate firewood or to clear land for crops. Today, the term "pruning" brings as many different images to mind as there are reasons to do so. Whether you're trying to improve fruit and flower production, prune for health or structural integrity, provide utility pruning, or are just looking to enhance a tree's natural beauty, there are a few simple guidelines that will ensure positive results.
At the foundation of proper pruning is an understanding of how trees respond to wounds. Since trees lack the ability to regenerate damaged cells, they must compartmentalize or seal off wounds to prevent the spread of decay and disease. This process -- commonly referred to as CODIT, or the compartmentalization of decay in trees -- should primarily influence the size of branches or stems being removed from the tree. The larger the pruning cut, the longer it will take the tree to seal off that wound. The bottom line is if you can achieve your objective by removing several one or two-inch diameter stems rather than removing a stem that might be six or eight inches in diameter, the tree will be able to seal those wounds quicker and prevent potential decay from occurring. With this in mind, we can begin to explore when, where and how much pruning is appropriate for a tree.
When: Certain types of pruning, such as fruit tree pruning, are best accomplished in the dormant season, January-February. However, most trees can tolerate pruning year round as long as there are no extreme amounts of stress, such as; drought, heat waves, or excessive root loss. Dead, dying, broken or diseased branches, as well as vines, should be removed at the earliest convenience.
Where: Trees have a highly developed system of checks and balances within their structure. When a tree develops a central stem with an undamaged terminal bud, that tip bud has the job of sending signals to the rest of the tree, in the way of hormones release, that tell the tree it has a leader and a direction to follow. When that terminal bud is a damaged or removed, new signals respond by releasing dormant buds into service to replace the missing canopy. Trees that have been 'topped' or have had their entire canopies removed exhibit extreme examples of this response. To avoid this unsightly, counterproductive reactionary growth, it's necessary to prune trees using lateral-reduction cuts. When stems are removed or reduced to a lateral branch at least 30 percent of the diameter of the parent stem being removed, the remaining stem should be established enough to take over the hormonal signaling role and direct growth energy into the remaining stem. It's also important to place pruning cuts just outside the branch collar or swell at the base of a stem being removed. A smooth, round cut will compartmentalize quicker than a long, oval or jagged cut. Pruning cuts should be left exposed to air flow and sunlight. Research has shown that 'tree paints', while popular in the past, actually inhibit the CODIT process and act to harbor insects and decay.
How much: In a healthy shade tree, and appropriate amount of pruning should not exceed 15-20 percent of the total canopy, or leaf-bearing stems, per year. More sensitive trees, such as ornamentals, old growth and stressed trees, will respond best to a much lighter pruning or removal of live tissue, around 5-10 percent of the total canopy. When major corrective pruning is needed over the 20 percent limit, a five- or ten-year plan may be warranted. You can prune to a maximum percentage once a year or every other year until the desired structure is achieved. When thinning trees, it's desirable to leave some interior branches in the canopy, as they're important for cooling the trees during times of extreme heat. In younger trees, low branches along the trunk help with trunk taper establishment. These branches should be allowed to remain as temporary branches during early development.
By following these basic principles, you can help your trees grow into healthy, attractive trees to enjoy for years to come.
Logan, Jeremy. New Life Journal, June. (2009)
