Prune to the standard
The arboriculture industry has developed a variety of progressive pruning standards in recent years, the culmination of which is the current Standard Practices for Tree, Shrub and other Woody Plant Maintenance, published by the American National Standards Institute (ANSI A300-1995). This standard, which industry experts developed by consensus, is intended to replace all previous pruning standards for woody plants.
Two significant predecessors remain useful because they contain similarly progressive information but with notably different presentations: Pruning Standards, published by the Western Chapter of the International Society of Arboriculture Certification Committee in 1988, and Tree-Pruning Guidelines, published by the International Society of Arboriculture in 1995. Now that the industry has developed meaningful, well-accepted standards, we need to refine the resulting work to ensure it complies with what we know about tree physiology and biology.
Pruning standards are only as meaningful as your interpretation and application of them in the field. Misinterpretation and misapplication, regardless of the good intentions of the practitioner, remain prevalent in tree care and result in serious and often irreparable damage to trees.
Interpreting pruning standards Our current pruning standards are clearly much more about what not to do than about what to do. Pruning standards are our industry's effort to take the inherently unnatural practice of pruning and produce results conducive to the natural patterns of trees. Maintenance of individual trees often is performed by many different practitioners with varying interpretations of proper tree care. Thus, developing an industry standard plays a critical role in promoting consistent tree management over sporadic maintenance.
Much of the current pruning standards is not new information. The transition from innovative concept to industry standard, and eventually to a widespread basic understanding by consumers, is slow to progress. Let's consider some of the most basic, current pruning concepts.
Work with trees' natural characteristics Trees in their natural environment don't require maintenance to thrive. Trees require maintenance when we put them where they don't belong or alter the environment of existing trees. Trees grow into all kinds of shapes, sometimes bizarre, because of their genetic makeup, surrounding environment and the impact of various events during their lifetime. Just remember that trees grow the way they do for a reason, and we must be cognizant of that when we prune. All of your pruning should be conducive to the natural characteristics of each species. Consider these characteristics in your pruning practices and remember that trees in their natural environment don't require pruning to thrive.
This concept is apparent in our current pruning standards: * ISA Pruning Guidelines: "A thinning cut...helps retain the tree's natural shape." * ANSI A300: "To obtain the defined objective, the growth cycles of individual species as well as the type of pruning to be performed should be considered."
While progressive, this concept is basic to comprehending pruning techniques and certainly not new. Remarkably, this concept was published as early as 1868 in a book by John Grigor, entitled Arboriculture: "...to display the peculiar outline and ramification of each species to the greatest advantage, they should be left to their own efforts, either standing singly or in masses without anything being done to them beyond the removal of a shoot on the young plant when it happens to be too bushy or the amputation of a decayed branch in the more advanced state of the tree."
In 1888, James Brisbin mentioned it once again in Trees and Tree Planting: "To prune a tree so as to serve the purpose for which it is wanted, observation of its natural habit will soon teach the planter how much or how little is required to be cut away."
Such basic concepts are simply the result of observing and striving to understand the responses of trees to our pruning efforts and techniques. These predictable responses constitute the realities of pruning.
The realities of pruning Pruning standards are nothing more than a tool-just as arborist certification is only a tool-and a tool is only as valuable as the skill of the craftsman. Neither certification nor pruning standards provide all the tools we need to provide beneficial tree pruning. Progressive pruning requires additional information as well as skill, judgment and discretion.
So what exactly is "proper" or "progressive" tree pruning? Proper tree pruning is pruning that provides the maximum benefit while doing the least amount of damage to the tree. Pruning inherently damages trees. It creates wounds and removes functional tree parts that serve a purpose. Removing foliage reduces carbohydrate production, wounding live tissue increases carbohydrate consumption and removing wood results in all of that plus a loss of storage capacity and possibly structural integrity. Considering these effects, we need to justify every cut by defining its purpose and potential benefits and weighing them against the damage that will occur.
The possible purposes and benefits of pruning include: * Structural improvement * Architectural improvement * Reduction of hazard potential * Aesthetic improvement * View enhancement * Providing clearance (pedestrians, vehicles, buildings) * Disease or pest control * Increased sunlight penetration * Size control * Enhancing or inhibiting flowering or fruiting * Invigoration of "stagnant" plants (a debatable practice) * Increase in value (Christmas trees, nursery plants, bonsai specimens) * Other practical purposes include keeping fruit within reach, discouraging unwanted wildlife, mitigating neighbor relations, etc.
Negative impacts of all pruning include: * Reduced photosynthesis * Creation of entry points for pathogens * Overall dwarfing * Shoot elongation * Consumption of stored carbohydrates (via compartmentalization, woundwood development and shoot growth).
Negative impacts of improper pruning are: * Minimizing the benefits and maximizes negative impacts of pruning * Enhancing existing and creating new structural and architectural defects * Enhancing insect or disease problems * Increasing the frequency of maintenance needs Reducing tree longevity.
In summary, appropriate pruning practices maximize the benefits and minimize the negative impacts. Poor pruning minimizes the benefits and maximizes the negative impacts. Therefore, my personal philosophy for application of this perspective is do not remove any live tissue without a good reason-and then consider the benefits gained vs. the negative impacts.
Like other progressive concepts, this also is not new. From Richard Fenska's 1943 publication Tree Expert's Manual: "There must be a reason for every cut made in a pruning operation. This requires a knowledge of tree growth. Severe pruning may affect the food supply of the tree by removal of too much leaf surface and upset the balance between roots and top."
Target pruning cuts vs. flush cuts The pruning-cut technique known as the target pruning cut method emphasizes preserving the branch collar and avoiding invasion of the trunk tissue. This concept is basic to progressive pruning practices. * ISA Pruning Guidelines: "Flush cuts should be avoided because they result in a larger wound and expose trunk tissues to the possibility of decay." * ANSI A300: "...the final cut shall be made in branch tissue close to the trunk or parent limb, without cutting into the branch bark ridge or collar or leaving a stub."
This concept is mentioned in print as early as 1868, in John Grigor's book Arboriculture: "The evil consequences of cutting off large side branches from timber trees require to be stated, as nothing can more readily deteriorate their value, particularly if the branches are cut close to the trunk. This creates a large, unsightly wound...and rottenness penetrates into the trunk, and although the wound will collapse, and repeated layers of wood in course of time will cover it over, yet the timber remains unsound and much deteriorated."
Thinning cuts The current, generic definition of a thinning cut might be a proper pruning cut that removes a branch or portion either at its point of origin or to a lateral of sufficient size to assume the terminal role. * ANSI A300: "The lateral remaining should be large enough to assume the terminal role." * ISA Pruning Guidelines: "If it is necessary to reduce the length of a branch or the height of a leader, the remaining branch should be no less than one-third the diameter of the branch being removed..."
Some conflict exists over what is "large enough to assume the terminal role." The Western Chapter ISA Certification Committee (WCISA) released its publication Pruning Standards in 1988, which more conservatively described "large enough to assume the terminal role" as, "When shortening a branch or leader, the lateral to which it is cut should be at least one-half the diameter of the cut being made." If we adhere to the concept of doing the least amount of damage when pruning, then the smaller the pruning cut in relation to the branch that remains, the better. The diagram on the right illustrates the significant differences among these various specifications.
Regulating the volume of foliage removed Limiting the volume of foliage removed within a given time frame is a critical application of what we know about tree physiology. Preserving foliage is paramount to the long-term physiological viability of any tree. * ANSI A300: "When a branch is cut back to a lateral, not more than one-fourth of its leaf surfaceshould be removed," and "Not more than one-fourth of the foliage on a mature tree should be removed within a growing season." * ISA Pruning Guidelines: "When thinning the crown of mature trees, you should seldom remove more than 1/4 (one-fourth) of the live foliage."
Brisbin addressed this progressive concept as early as 1888 in his book Trees and Tree Planting: "Care, however, is necessary that it be not pruned to such an extent as to weaken or check its growth, nor should the whole of the branches prunable be cut off at once, as some, which it is ultimately requisite to trim away, may at that particular stage of growth be beneficial to the tree of which they are members."
Richard R. Fenska later addressed this concept a little less flamboyantly in his 1943 book Tree Experts Manual: "Severe pruning may affect the food supply of the tree by removal of too much leaf surface and upset the balance between roots and top."
Branch distribution (architecture) We now know that the placement and spacing of branches substantially influences the tree's structural integrity. Current pruning standards have addressed this issue in clear terms: * ANSI A300: "...one-half the foliage should remain evenly distributed in the lower two-thirds of the crown and individual limbs." * ISA Pruning Guidelines: "At least one-half of the foliage should be on branches that arise in the lower two-thirds of the tree."
John Grigor also discussed this concept, with exactly the same proportions, in 1868: "A good proportion in a tree of 30 feet in height is 20 feet of top to 10 feet of bare trunk."
"The skillful forester observes at a glance whether the tree is possessed of a trunk stout in proportion to its height, and, as in thinning, regulates the pruning accordingly. Where height is required, he subdues the side branches; where girth is required he preserves them as the speedy means of obtaining girth."
Basically, then, we must evaluate and apply all maintenance techniques to result in maximum benefits for the tree as well as for practical purposes, while minimizing the negative impacts.
Be cognizant of tree physiology when pruning. Use current pruning guidelines and standards, but also read between the lines. Strive to perceive the concepts behind them and their intended goals rather than only a literal application. The result will be no less than an urban forest that will thrive for generations to come.
Torrey Young is president of Treescapes Inc. (Oakland, Calif.).
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