Peter J. Bedker, Joseph G. O’Brien, and Manfred M. Mielke – USDA Forest Service
The pruning of large trees is best left to qualified tree care professionals who have the proper equipment. However some common pruning practices exist that may cause excessive harm to trees. If these poor pruning practices are being applied to your trees, halt the job and seek help from a qualified individual or firm.
Trees should be pruned at the appropriate time of year. Although conifers may be pruned any time of year, pruning during the dormant season may minimize sap and resin flow from cut branches.
Hardwood trees and shrubs without showy flowers, however, should be pruned in the dormant season to easily visualize the structure of the tree, to maximize wound closure in the growing season after pruning, to reduce the chance of transmitting disease, and to discourage excessive sap flow from wounds. Dead branches can be removed any time of the year.
Topping and Tipping
Topping and tipping are pruning practices that harm trees and should not be used. Crown reduction pruning is the preferred method to reduce the size or height of the crown of a tree, but is rarely needed and should be used infrequently. Topping, the pruning of large upright branches between nodes, is sometimes done to reduce the height of a tree. Tipping is a practice of cutting lateral branches between nodes to reduce crown width. These practices invariably result in the development of epicormic sprouts, or in the death of the cut branch back to the next lateral branch below. These epicormic sprouts are weakly attached to the stem and eventually will be supported by a decaying branch.
Improper pruning cuts cause unnecessary injury and bark ripping. Flush cuts injure stem tissues and can result in decay. Stub cuts delay wound closure and can provide entry to canker fungi that kill the cambium, delaying or preventing wound-wood formation.
Proper tools are essential for satisfactory pruning. The choice of which tool to use depends largely on the size of branches to be pruned and the amount of pruning to be done. Tools should be clean and sanitized as well as sharp. Although sanitizing tools may be inconvenient and seldom practiced, doing so may prevent the spread of disease from infected to healthy trees on contaminated tools. Tools become contaminated when they come into contact with fungi, bacteria, viruses and other microorganisms that cause disease in trees. Most pathogens need some way of entering the tree to cause disease, and fresh wounds are perfect places for infections to begin. Microorganisms on tool surfaces are easily introduced into susceptible trees when subsequent cuts are made. The need for sanitizing tools can be greatly reduced by pruning during the dormant season.
If sanitizing is necessary it should be practiced as follows: Before each branch is cut, sanitize pruning tools with either 70% denatured alcohol, or with liquid household bleach diluted 1 to 9 with water (1 part bleach, 9 parts water). Tools should be immersed in the solution, preferably for 1-2 minutes, and wood particles should be wiped from all cutting surfaces. Bleach is corrosive to metal surfaces, so tools should be thoroughly cleaned with soap and water after each use.
Tree sap, gums, and resins are the natural means by which trees combat invasion by pathogens. Although unsightly, sap flow from pruning wounds is not generally harmful; however, excessive “bleeding” can weaken trees.
When oaks or elms are wounded during a critical time of year (usually spring for oaks, or throughout the growing season for elms) — either from storms, other unforeseen mechanical wounds, or from necessary branch removals — some type of wound dressing should be applied to the wound. Do this immediately after the wound is created. In most other instances, wound dressings are unnecessary, and may even be detrimental. Wound dressings will not stop decay or cure infectious diseases. They may actually interfere with the protective benefits of tree gums and resins, and prevent wound surfaces from closing as quickly as they might under natural conditions. The only benefit of wound dressings is to prevent introduction of pathogens in the specific cases of Dutch elm disease and oak wilt.
ANSI Z133.1. 1994. Safety standards. American national standard for tree care operators. Washington, DC: American National Standards Institute.
ANSI A300. 1995. Standard practices for tree, shrub, and other woody plant maintenance. Washington, DC: American National Standards Institute.
Fazio, J. R. ed. 1992. Don’t top trees. Tree City USA Bulletin No. 8. Nebraska City, NE: The National Arbor Day Foundation.
Harris, R.W. 1994. Clarifying certain pruning terminology: thinning, heading, pollarding. Journal of Arboriculture 20:50-54.
ISA Performance Guidelines Committee. 1994. Tree-pruning guidelines. Savoy, IL: International Society of Arboriculture.
Ryan, H.D.P. III. 1994. Arboricultural pruning methodologies. Arborist News Volume 3(4):33-38.
Shigo, A. 1991. Modern arboriculture. Durham, NH: Shigo & Trees, Associates.
Shigo, A. 1989. Tree pruning: a worldwide photo guide. Durham, NH: Shigo & Trees, Associates
Pruning Trees and Shrubs Mike Zins and Deborah Brown, Extension Horticulturists http://www.extension.umn.edu/distribution/horticulture/DG0628.html