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Little Known or Interesting Factoids About Trees and Tree Physiology

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HOW to Prune Trees
Table of Contents

Introduction                                                    Reasons for pruning
Pruning  approaches 
Crown thinning 
Crown raising
Crown reduction 
Pruning cuts  
Pruning living branches  
Pruning dead branches  
Drop crotch cuts
Pruning practices that harm tree   
References 
Authors

Introduction
The objective of pruning is to produce strong, healthy, attractive plants. By understanding how, when and why to prune, and by following a few simple principles, this objective can be achieved.


Figure 1
Reason for pruning

Reasons for pruning The main reasons for pruning ornamental and shade trees include safety, health, and aesthetics. In addition, pruning can  be used stimulate fruit production and increase the value of timber. Pruning for safety (Fig. 1A) involves removing branches that could fall and cause injury or property damage, trimming branches that interfere with lines of sight on streets or driveways, and removing branches that grow into utility lines. Safety pruning can be largely avoided by carefully choosing species that will not grow beyond the space available to them, and have strength and form characteristics that are suited to the site.

Pruning for health
(Fig. 1B) involves removing diseased or  insect-infested wood, thinning the crown to increase airflow and reduce some pest problems, and removing crossing and rubbing branches. Pruning can best be  used to encourage trees to develop a strong structure and reduce the likelihood of damage during severe weather. Removing broken or damaged limbs encourage wound closure. 

Pruning for aesthetics (Fig. 1C) involves enhancing the natural form and character of trees or stimulating flower production. Pruning for form can be especially important on open-grown trees that do very little self-pruning.

 

All woody plants shed branches in response to shading and competition. Branches that do not produce enough carbohydrates from photosynthesis to sustain themselves die and are eventually shed; the resulting wounds are sealed by woundwood (callus). Branches that are poorly attached may be broken off by wind and accumulation of snow and ice. Branches removed by such natural forces often result in large, ragged wounds that rarely seal. Pruning as a cultural practice can be used to supplement or replace these natural processes and increase the strength and longevity of plants.

Trees have many forms, but the most common types are pyramidal (excurrent) or spherical (decurrent ). Trees with pyramidal crowns, e.g., most conifers, have a strong central stem and lateral branches that are more or less horizontal and do not compete with the central stem for dominance. Trees with spherical crowns, e.g., most hardwoods, have many lateral branches that may compete for dominance. To reduce the need for pruning it is best to consider a tree's natural form. It is very difficult to impose an unnatural form on a tree without a commitment to constant maintenance.

Pollarding and topiary are extreme examples of pruning to create a desired, unnatural effect. Pollarding is the practice of pruning trees annually to remove all new growth. The following year, a profusion of new branches is produced at the ends of the branches. Topiary involves pruning trees and shrubs into geometric or animal shapes. Both pollarding and topiary are specialized applications that involve pruning to change the natural form of trees. As topiary demonstrates, given enough care and attention plants can be pruned into nearly any form. Yet just as proper pruning can enhance the form or character of plants, improper pruning can destroy it.

Pruning Approaches
Producing strong structure should be the emphasis when pruning young trees. As trees mature, the aim of pruning will shift to maintaining tree structure, form, health and appearance.

Proper pruning cuts are made at a node, the point at which one branch or twig attaches to another. In the spring of the year growth begins at buds, and twigs grow until a newnode is formed. The length of a branch between nodes is called an internode.

The most common types of pruning are:

1.Crown thinning, primarily for hardwoods, is the selective  removal of branches to increase light penetration and air    movement throughout the crown of a tree. The intent is to maintain or develop a tree's structure and form. To avoid unnecessary stress and prevent excessive production of epicormic sprouts, no more than one-quarter of the living crown should be removed at a time. If it is necessary to remove more, it should be done over successive years


Fig 2.  Crown thinning - branches to be removed are shaded iblue; pruning should be maid at the red lines. No more than one forth of the living branches should be removed 


 

Branches with strong U-shaped angles of attachment should be retained (Fig 3A). Branches with narrow, V-  shaped angles of attachment often form included bark and should be removed (Fig. 3B). Included bark forms when two branches grow at sharply acute angles to one another, producing a wedge of inward-rolled bark between them. Included bark prevents strong   attachment of branches, often causing a crack at the point below where the branches meet. Codominant stems that are approximately the same size and arise from the same position often form included bark. Removing some of the lateral branches from    codominant stem can reduce its growth enough to  allow the other stem to become dominant.
Lateral branches should be no more than one-half to three-quarters of the diameter of the stem at the point of attachment. Avoid producing "lion's tails," tufts of branches and foliage at the ends of branches, caused by removing all inner lateral branches and foliage. Lion's tails can result in sun scalding, abundant epicormic sprouts, and weak branch structure and breakage. Branches that rub or cross another branch should be removed.

Conifers that have branches in whorls and pyramidal crowns rarely need crown thinning except to restore a dominant leader. Occasionally, the leader of a tree may be damaged and multiple branches may become codominant. Select the strongest leader and remove competing branches to prevent the development of codominant stems.


Fig 4. Crown raising - branches to be removed are shaded in blue; pruning cuts should be made where indicated with red lines. The ratio of live crown to  total tree height should be at least two-thirds.

Crown raising is the practice of removing branches from the bottom of the crown of a tree to provide clearance for pedestrians, vehicles, buildings, lines of site, or to develop a clear stem for timber production. Also, removing lower branches on white pine scan prevent blister rust. For street trees the minimum clearance is often specified by municipal ordinance. After pruning, the ratio of the living crown to total tree height should be at least two-thirds       (e.g., a 12 m tree should have living branches on at least the upper 8 m).

On young trees "temporary" branches may be retained along   stem to encourage taper and protect trees from vandalism and sun     scald. Less
vigorous shoots should be selected as branches and  should be about 10 to 15 cm apar tlong the stem. They should be pruned annually slow their growth and should be removed eventually   



Fig 5. Crown reduction - branches to be removed are        shaded in blue;pruning cuts should be made where         indicated with red lines The ratio of live crown to total    height should be at least tow-thirds

Crown reduction pruning is most often used when a tree has grown too large for its permitted space. This method, sometimes called   drop crotch pruning, is preferred to topping because it  results in a more natural appearance, increases the time before pruning is needed again, and minimizes stress (see drop crotch cuts in the next section).     

Crown reduction pruning, a method of last resort, often results in large pruning wounds to
stems that may lead to decay. This method should never be used on a tree with a pyramidal growth form. A better long term solution is to remove the tree and replace  it with a tree that will not grow beyond the available space. 
 Pruning cuts should be made so that only branch tissue is removed and stem tissue is not damaged. At the point where the branch attaches to the stem, branch and stem tissues remain separate, but are contiguous. If only branch tissues are cut when pruning, the stem tissues of the tree will probably not become decayed, and the wound will seal more effectively.
1. Pruning living branches (Fig. 6 A.B)
To find the proper place to cut a branch, look for the branch collar that grows from the stem tissue at the underside of the base of the branch (Fig. 6A). On the upper surface, there is usually a branch bark ridge that runs (more or less) parallel to the branch angle, along the stem of the tree. A proper pruning cut does not damage either the branch bark ridge or the branch collar.


Fig 6 A. Targeting the cut

A proper cut begins just outside the branchbark ridge and angles down away from the stem of the tree, avoiding injury to the branch collar (Fig. 6B). Make the cut as close as possible to the stem in the branch axil, but outside the branch bark ridge, so that stem tissue is not injured and the wound can seal in the shortest time possible. If the cut is too far from the stem, leaving a branch stub, the branch tissue usually dies and woundwood   forms from the stem tissue. Wound closure is delayed because the woundwood must seal over the stub that was left.


 Fig 6 B-C. Branch Collar


The quality of pruning cuts can be evaluated by examining pruning wounds after one growing season. A concentric ring of woundwood will form from proper pruning cuts
(Fig. 6B). Flush cuts made inside the branch bark ridge or branch collar, result in pronounced development of woundwood on the sides of the pruning wounds with very little woundwood forming on the top or bottom (Fig. 7D). As described above, stub cuts result in the death of the remaining branch and woundwood forms around the base from stem tissues.

When pruning small branches with hand pruners, make sure the tools are sharp enough to cut the branches cleanly without tearing. Branches large enough to require saws should be supported with one hand while the cuts are made. If the branch is too large to support, make a three-step pruning cut to prevent bark ripping      (Fig. 6C).

1.The first cut is a shallow notch made on the underside of the branch, outside the branch collar. This cut will prevent a falling branch from tearing the stem tissue as it pulls away from the tree.

2.The second cut should be outside the first cut, all the way through the branch, leaving a short stub.

3.The stub is then cut just outside the branch bark ridge/branch collar, completing the operation.

2. Pruning dead branches (Fig. 6)

Prune dead branches in much the same way as live branches. Making the correct cut is usually easy because the branch collar and the branch bark ridge, can be distinguished from the dead branch, because they continue to grow (Fig. 6A). Make the pruning cut just outside of the ring of woundwood tissue that has formed, being careful not to cause unnecessary injury (Fig. 6C). Large dead branches should be supported with one hand or cut with the three-step method, just as live branches. Cutting large living branches with the three step method is more critical because of the greater likelihood of bark ripping.

3. Drop Crotch Cuts 

(Fig. 6D) A proper cut begins just above the branch bark ridge and extends through the stem parallel to the branch bark ridge. Usually, the stem being removed is too large to be supported with one hand, so the three cut method should be used.

1.With the first cut, make a notch on the side of the stem away from the branch to be retained, well above the branch crotch.     
 2.Begin the second cut inside the branch crotch, staying well above the branch bark ridge, and cut through the stem above the notch.
3.Cut the remaining stub just inside the branch bark ridge through the stem parallel to the branch bark ridge
 .


D. Crown reduction    Fig. 6 D. Pruning cuts

To prevent the abundant growth of epicormic sprouts on the stem below the cut, or dieback of the stem to a lower lateral branch, make the cut at a lateral branch that is at least one-third of the diameter of the stem at their union.
 
Pruning Practices That Harm Trees


Topping
and tipping (Fig. 7A, 7B) 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 (Fig. 7A). Tipping is a practice of  cutting lateral branches between nodes (Fig. 7B) 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 (Fig. 7C). Flush cuts injure stem tissues and can result in decay (Fig. 7D).  Stub cuts delay wound closure and can provide  entry to canker fungi that kill the cambium, delaying or preventing woundwood formation     
(Fig. 7E).  

 
 


Figure 7 E. Practices that harm trees

Northeastern Area State and Private Forestry Offices - Headquarters - Northeastern Area State & Private Forestry,100 Matsonford Road , 5 Radnor Corporate Center, Suite 200 Radnor, PA 19087-4585 Durham Field Office. Northeastern Area State & Private Forestry - USDA Forest Service .Louis C. Wyman Forest Services Laboratory , P.O. Box 640 ,Durham, NH 03824-0640                                                                    Morgantown Field Office - Northeastern Area State & Private Forestry USDA Forest Service 180 Canfield Street Morgantown, WV 26505-3101 St. Paul Field Office Northeastern Area State & Private Forestry USDA Forest Service 1992 Folwell Avenue St. Paul, MN551081099 Authors                                                                                                                                                                                                                          Peter J. Bedker, Plant Pathologist, USDA Forest Service, Northeastern Area State and Private Forestry                                                            Joseph G. O'Brien, Plant Pathologist, USDA Forest Service, Northeastern Area State and Private Forestry                                                        Manfred E. Mielke, Forest Health Specialist, USDA Forest Service, Northeastern Area State and Private Forestry                                        Illustrations by: Julie Janke, Scientific Illustrator, Afton, Minnesota                                                                                                                        Project Coordinator: Gerard D. Hertel, Assistant Director, Forest Health and Management, USDA Forest Service, Northeastern Area State and Private Forestry.                                          

Contact: cell: 830.257.8871
                
email: jim.rediker@usa.net
                     Jim Rediker - Nurseryman -  Arborist  - TDA Certified
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