Natural Notes: Pruning

This post consists of my notes from a lecture on tree pruning as part of my training to become a certified arborist by the International Society of Arborculture.  The speaker was Luke Engel, a natural resources specialist with the Forest Preserves of Cook County and his lecture took place on February 18, 2015 at the Salt Creek Resource Management facility in Willow Springs, Illinois.  

 Pruning objectives
Pruning is the most common tree maintenance procedure.  It is used to optimize tree health and aesthetics.  One of the main objectives for pruning is to reduce potential branch failure (safety).    Thinning the canopy of a tree can also allow for better light and wind penetration, and it can also influence fruiting or flowering.  Views can be improved and the overall aesthetic quality of the tree can be enhanced.  Trees can also be pruned to a desired shape or size.  

 Pruning cuts
There are some main types of pruning cuts that arborists can perform on trees.  All types of pruning should incorporate the three cut method (see figure 1).  The cut should be made outside of the branch collar, and the first cut should be made from underneath the branch about 1/4 the way through.  This prevents the limb from peeling or tearing back.  
Figure 1:  Three cut method

Crown thinning is the selective removal of branches to increase light and air penetration.   Crown thinning reduces the weight of the tree and stimulates or maintains foliage. Generally, 1/4 of the inner branches are removed. 

 Improper thinning can result in the development of watersprouts, sunburned bark, and weakened branches.  

 Crown cleaning is a more refined selective branch removal method,  Crown cleaning is included with crown thinning, as it maintains the natural shape and structure of the tree.  Crown cleaning involves removal of dead and diseased branches, broken limbs from storm damage, and weakly attached branches.  

 Crown reduction is used to reduce the size of the tree while still retaining its natural form (see figure 2).  The lateral branches are cut back.  It should be noted that the lateral branches must be large enough to ensure apical dominance.  
Figure 2:  Crown reduction
Topping/heading is where the limbs are cut back to the bud.  This method of cutting does not allow for apical dominance and causes branch die-back, decay, and excessive sprout production.  Topping and heading is not recommended.  Liontail sprouts, think sprouts arising from a thick cut branch, are signs that topping was performed (see figure 3).
Figure 3:  Liontail sprout
  
Structural pruning is used when pruning young trees to promote a desired structure such as a certain canopy size or growth form.  It is preferred that trees are structured in a manner that avoids codominance and has an obvious dominant leader (see figure 4).  
Figure 4:  Structural pruning should ensure that there is a strong main leader (excurrent form, left) and decurrent forms with a strong main leader (right) should be avoided.  
Other pruning types
Vista pruning is a method of pruning used to provide a specific view.  Utility pruning is a method used to prevent damage to utility equipment.  Topiary pruning is used for specific shapes (e.g. animal shapes, neat squares, etc.).  

Wound dressings
Wound dressing were once used and thought to reduce decay and accelerate wound healing.  However, there is NO substantial advantage and is not recommended.  It is also considered a waste of money.  

Pruning tools
Tools should be of the appropriate size for the cuts being made.  They should be sharp with jagged edges, and they should be cleaned after each use with alcohol.  

There are two main types of pruning tools.  Anvil tools (where the two blades collide into each other) should be avoided.  Cross-cut tools (where the two blades cross each other) are the preferred tool type.  

The timing of when to prune depends upon the desired results.  Growth is maximized if pruning is done before the growing season (late winter/early spring). Growth reduction can be achieved by late spring/early summer, although some species vary from these rules.  

When pruning, the arborist should always choose the appropriate pruning method, tools, and timing.  

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