In this post I’ll set out to explore a Challenge Question posed at the end of Chapter One (Introduction to Pruning) in Ed Gilman’s book An Illustrated Guide to Pruning: List five faults or defects in mature trees that are at least partially correctable with pruning.

I’ll use the mature Black Walnut, Juglans nigra, featured in the above photo, to help illustrate my discussion and offer some insight into my thought process, and maybe to yours as well.

Five faults that I can think of in mature trees that are at least partially correctable with pruning include co-dominant stems (stems with large aspect ratios); large over-extended limbs, especially limbs lower in the canopy that extend horizontally and swoop downward; clearance issues from a tree’s location next to unmovable structures like houses, outbuildings or above ground utilities; unwanted or excessive fruit/nut production; and deadwood accumulation or storm damaged limbs (broken/hanging branches) from the past that need to be retrieved, corrected or restructured.

It should be mentioned that some general challenges will be preexisting when pruning mature trees. First, their large stature requires an extensive examination of the existing architecture in order to perform a thorough tree evaluation. Second, if the only access is by climbing, and the tree is found to be safe to climb after a thorough inspection, then the skills necessary to safely and efficiently reach the locations in the periphery of the crown to make proper cuts poses another difficulty. Third, if the tree can’t be climbed, either because it is not safe or economical, then access with some kind of mechanical platform is in order. Fourth, if you must work over unmovable targets within the dripline of the tree, then rigging down branches with safety and control offers another dilemma in mature tree pruning. All of these things, and surely some others that I haven’t thought of, will need to be considered before attempting to correct the faults present in the mature trees we are working with.

In the above photo, I can identify 1 major co-dominant stem in the Walnut tree, and there are perhaps one or two more above that main union on the left side of the crown over the parking space that require some mitigation as well. The largest co-dominant stems are near the base of the tree, maybe at 10 or 15 feet, and there is a visible seam running below that union to the ground that may indicate some inclusion or in-rolling, possibly reaction wood that is building in response to the force on those stems. At this union we see a large aspect ratio (1:1). Through the process of addressing this co-dominant union and the leverage forces that exist there, we can address other faults as well in our journey through the crown. Therefore, many of our pruning cuts can serve a dual purpose by addressing the root issue of a large aspect ratio at branch unions low in the crown.

As per Gilman, when pruning mature trees, it is important that we specify keeping the pruning dosage small (less than 10%). Excluding the option of not pruning at all, which in some cases may be required, this option of small dosage will maintain maximum live tissue on the tree, avoiding the potential for extensive decay and maximizing photosynthetic capacity, which is critical for a mature tree’s health and well being. A small pruning dosage translates to making small cuts out on the periphery of the crown- the ends of branches-maybe in the 3 to 4 inch range, or possibly smaller. By working at the branch ends, we are removing surface area in the tree that is suspect to wind loading, and snow and ice loading (especially on long horizontal and drooping limbs) in temperate climates. These cuts will most likely be reduction cuts (maybe some removal cuts are warranted), acting as growth regulation cuts; they will also be removing some of the trees’ fruit/nut production platform, even if on a small scale. By climbing through the crown and outward to the periphery, we can also be actively removing deadwood, and correcting any branch failures from the past. Furthermore, working at the ends of the branches and making good reduction cuts will reduce the overall risk of large branches failing, particularly at the large aspect ratio unions where branch attachment may be weaker.

On the right side of the Walnut tree over the roof of the house, I can observe some issues with clearance on those scaffold and secondary limbs. Gilman asks us to imagine what the tree will look like in 5 to 30 years from now. This is a helpful consideration in order to guide our decisions. With that in mind, I can see these limbs encroaching further on the roof, so then some reduction cuts in this area of the crown can provide clearance over the roof of the house. Here is another area of the crown where less nut production will be beneficial, reducing some of the potential for nuts consistently dropping onto the roof of the house. These horizontal limbs, although I wouldn’t necessarily describe them as ‘over-extended’, may be susceptible to snow and ice loading, so reduction pruning here will reduce the risk of an untimely failure.

Pruning mature trees can be a delicate balance between tree physiology and desired architecture. It should be noted that sometimes it is difficult to completely mitigate the problems that exist in mature trees with pruning alone. It requires a thorough thought process and extensive knowledge of tree biology, and as a climber it requires a high level of skill and athleticism. Mature trees offer challenges that we do not find in young and medium age trees. Pruning faults out of mature trees can offer us many pruning cycles of exciting adventure and profitable experience.

“Proper pruning is one of the best things that can be done for a tree; improper pruning i sone of the worst things that can be done to a tree.” Dr. Alex Shigo