Tree Pruning

June 8, 2009

By Thembi Borras

Pruning involves removing the lower branches from live trees.

Why prune?
Pruning is an important practice to reduce fire hazard, but must be done properly so tree health is not compromised. Pruning may also improve safety, aesthetics, timber quality and tree vigor. Trees along roads are excellent candidates for pruning to create a fuel break or to facilitate quicker road surface drying.

How to prune? Following are guidelines for pruning:

• Make pruning cuts where two limbs intersect or where limbs intersect the main tree trunk.


• Make the final pruning cut just outside the branch collar. Avoid "flush cuts" which remove the branch collar or "stub cuts" which leave branch stubs protruding beyond the branch collar. The branch collar is the swollen area at the base of a branch where it connects to the trunk and contains special tissue that prevents decay from moving downward from the branch into the trunk.


• For large limbs, make an undercut so that when the branch separates it doesn't strip off bark from the bole of the tree. The following figure (copied from USFS publication NA-FR-01-95 entitled, How to Prune Trees by Peter J. Bedker et al and illustrations by Julie Martinez of Afton, MN) displays a cutting sequence to minimize pruning damage:


• If there is reason to remove a large number of live limbs, it may be prudent to prune the tree in stages increasing the distance from the ground to the lowest limb slowly, pruning a portion of limbs each year.

• Consider staggering the distance between the ground and the height of the lowest limbs to avoid the “lollipop look”.


• Hand saws can be used if the branch can be reached. Otherwise, use a telescoping pole saw or shear. Visit http://ext.nrs.wsu.edu/handtools/tools/pruning/index.htm to view power and hand tools for pruning.


• Be careful not to scar the bole as the scar may invite the introduction of stem rot fungi into the tree.


• There is no need to apply any kind of “wound dressing” to the prune scars.


• Use recommended safety equipment, such as eye protection, hard hat, gloves and sturdy footwear. Use common sense and be safe.
  

When to prune?
Pruning can be done anytime during the year. However, consider pruning during the dormant season when the sap is flowing less. Try to avoid pruning during the spring and early summer when the tree is actively growing and is easily damaged. Another reason to avoid pruning during the growing season is that beetles are most active and are attracted to the sap from the pruning wound.

The area out to 200 feet from a home is referred to as defensible space. Trees in the defensible space and along emergency vehicle access routes should be pruned first.

Which trees should be pruned?
Pruning takes time and energy so to optimize these precious resources pick the best trees to prune and thin out the worst. There is an excellent article containing characteristics of “leave” trees and characteristics of “cut” trees in the winter 2008 edition of SW Oregon Woodland News entitled Thin for Quality, Not Spacing and can be accessed at http://extension.oregonstate.edu/sorec/sites/default/files/documents/WoodlandNewsJan08newsletter2.pdf.

The following is excerpted from it:
GUIDELINES: So what makes a high quality tree?
Following are some guidelines for “leave” and “cut” trees that relate to tree quality. These are all characteristics you can “eyeball” out in the woods.

Characteristics of “leave” trees
• Good live crown ratio (30% or greater)
• Good height growth for species and age
• Symmetrical crown with “pointy top”
• Abundant foliage with good color
• Good form (straight, without sweeps, crooks, forks, etc.)
• Species is well suited to the site over the long term

Characteristics of “cut trees”
• Poor crown ratio (<30%)
• Poor height growth and crown form (flat or rounded top, lopsided)
• Foliage is sparse or off-color
• Poor form
• Species not well suited to the site

Additional considerations include exposure to sun or wind. Sunscald is a tree bole injury that can result from pruning. This injury is more likely to occur on trees on the southwest side of a low density stand. To avoid sunscald a buffer of unpruned trees can be left on the south and west side of the stand. Similarly, to avoid wind damage unpruned trees and trees that would otherwise be “cut” trees can be left on the side of the stand from which prevailing winds blow to serve as a buffer.

How much to prune? Live crown ratio is an important concept to understand in determining how much of the crown to prune. Live crown ratio is the percentage of the total height of the tree that is occupied by the live green crown. For example, if the crown length, measured from the leader tip to the base of the live crown, is 55 ft and the total tree height is 100 ft, then the live crown ratio is 55%. Post pruning, the live crown ratio should be no less than 40% and preferably 50%. Therefore in this example, you can safely prune the lower 5 feet of live crown and no more than 15 feet. Dead limbs below the live crown do not factor into this calculation and should be pruned.

Another important consideration, especially within the defensible space is reducing ladder fuels which is vegetation that connects ground vegetation to tree crowns facilitating a ground fire becoming a crown fire, which moves much more rapidly. Reducing ladder fuels may be accomplished by, in part, removing “cut trees” and pruning “leave” trees. The minimum distance between the shrub layer and the canopy should be, at least, 4 times the height of the shrub layer. For example, if the shrub layer is 3 feet tall then the lowest limbs of the overstory trees should be 15 feet above the ground.

A portion of this article was gleaned from Conifer Pruning Basics for Family Forest Landowners available at http://cru.cahe.wsu.edu/CEPublications/eb1984/EB1984.pdf, the chapter on pruning in the Oregon-Washington Master Gardener Handbook entitled Sustainable Gardening and the article entitled Thin for Quality, Not Spacing in the winter 2008 edition of the SW Oregon WOODLAND NEWS.