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This is first in a two-part series on fertilization. Part 2 will appear in our October 2002 issue.
Ed Gilman is professor in the Environmental Horticulture Department at the University of Florida in Gainesville. He has been studying and teaching pruning since 1986. Sharon Lilly is director of educational goods and services for ISA. She is the author of many publications and training materials in the field of arboriculture.
Experience and observation teach the truth in Alex Shigo’s observation: "Pruning is one of the best things an arborist can do for a tree but one of the worst things we can do to a tree." Pruning is a double-edged sword, either helping or hurting—depending on where, when, how, and why it is applied. Arborists must understand the biology of trees and how trees respond to pruning in order to optimize the health and structure of trees through pruning.
When pruning is properly executed, a variety of benefits are derived. Benefits include reduced risk of branch and stem breakage, better clearance for vehicles and pedestrians, improved health and appearance, enhanced view, and increased flowering. When improperly performed, pruning can harm the tree’s health, stability, and appearance. Several consequences occur when pruning is not performed at all. These consequences include development of low limbs; weak, codominant stems; defects such as included bark; and accumulation of dead branches. Formation of codominant stems and defects such as included bark can lead to increased risk of breakage.
One of the most common defects in planted trees is formation of large, low limbs. They could overextend and break, or they may droop under their own weight and have to be removed later, leaving a large pruning wound. Removal of large branches and those more than about half the trunk diameter is more likely to initiate decay than removal of smaller branches. Therefore, measures should be taken when a tree is young to minimize occurrence of this defect. On mature trees, live branch removal is less desirable than it is on young trees, but sometimes it is necessary. However, cleaning the crown by removing dead, diseased, or broken branches is a highly recommended practice on mature trees.
It is essential first to evaluate the tree and the customer’s needs to determine which objectives should be accomplished with pruning. Appropriate pruning types can be chosen to meet these objectives. The arborist then enters the tree and makes appropriate pruning cuts for the chosen pruning types. This decision is based on an understanding of branch attachment and tree biology.
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International Society of Arboriculture
