Identifying what limbs to cut can be one of the most difficult parts of pruning a tree. I discussed the three reason’s why to prune trees in volume 1 of my pruning series: for tree health, cosmetics and structural integrity. However, many times these different reasons would lead you determine that different limbs need to be pruned. Balancing these priorities is where experience and an artistic eye really comes into play. Different people will often come to different conclusions. While weighing your priorities for pruning there are a few general rules you can follow to help error on the side of benefit for the tree.
The first general rule is to always avoid removing large limbs when you can accomplish your goals by removing multiple smaller limbs. Trees do not have an efficient system for healing wounds larger than 3” or 4” in diameter. These larger pruning wounds can take several years to callous over and lead to more extensive decay in the tree. While this is usually not a short term problem for the tree, removing large limbs will chisel years off the tree’s total life expectancy.
Remove any dead limbs. Dead wood is the food source for mother nature’s natural decay fungi. Removing deadwood both removes decay food sources and facilitates wound healing. In the winter months, identifying dead limbs can be tricky. A good arborist can easily tell the difference between a limb with dormant buds and a dead limb. For the average Joe I recommend you do your pruning during the spring and summer months when it is easy to tell which tree branches need to be pruned out.
Pruning soft wooded trees such as ash or pecan to thin the canopy will help improve the tree’s structural integrity. This reduces the overall weight load of the canopy and reduces the wind-sail effect since there will be fewer leaves catching the wind. For an otherwise healthy tree, canopy thinning should remove about 5-10% of the trees total canopy. This should be done by removing many ½” to 1 ½” limbs symmetrically throughout the extremities of the canopy.
These are some basic guidelines. Unhealthy trees, trees with mistletoe and trees in Oak Wilt sensitive areas are complicated even further.