While the majority of my work is as a Consulting Arborist, I have been pruning trees professionally since I started working for a tree service in 1992. I enjoy many things about pruning, including being outdoors, the climbing, the workout, and the challenge of achieving my goals for the pruning while maintaining/improving the trees’ beauty. On my best days, it seems as though I am on a retreat in someone’s back yard! When it’s 100 degrees, I’ve grossly underbid the job and nothing seems to be going well; it can be an arduous chore, however.

The most important objective of my pruning is usually structural improvement of the tree (in some cases I may also be trying to remove diseased wood, or the objective may be aesthetic improvement only). Much of what I am attempting to achieve with the pruning is to reduce the likelihood of trunk or limb failure for those parts which I deem to have any or all of the following: a poor attachment, defect and/or excessive length, weight or exposure.

For trunks or limbs I wish to prune for structural improvement, I either use reduction cuts (reducing limb length by removing the terminal portion back to a lateral branch of at least ½ the diameter of the cut stem) or I selectively remove limbs (thinning cuts) near their ends (Figure 1). In this way, I reduce the lever arm, sail and stress on the trunk or limb and its attachment as well as suppress its growth by reducing leaf area and photosynthetic capacity. This suppression is especially effective if limbs above the pruned limb grow over it and shade it out. Unless I have trained the tree from a young age, the process of trunk or limb suppression takes a few to several prunings.

The following images illustrate how I have suppressed a large limb with a defect (Figures 2-4) and one of two codominant (similar sized) trunks over several prunings.

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Figure 1. Illustration of reduction cuts from “An Illustrated Guide to Pruning,” by Ed Gilman (the best book on pruning I have read). A reduction cut shortens a stem or branch by removing the terminal to a lateral of at least 1/3 to ½ the diameter of the cut stem.
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Figure 2. View of Chinese pistache before pruning. I used reduction cuts to reduce stress on this heavy limb which was weakened from a large wound resulting from the loss of a limb a few years earlier.
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Figure 3. Close-up of pruned limb with previous limb failure visible.
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Figure 4. Pistache post-pruning. Much weight and sail has been removed. The pruning has significantly reduced the likelihood of this limb breaking. Due to the increased exposure of the limb to sunlight, I anticipate a moderate amount of watersprouts (vigorous upright shoot) will grow on this limb. I recommended pruning the limb again within three years.
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Figure 6. Close-up of attachment of trunks, 2002. Note the nearly equal size of the trunks. There was evidence of possible internal cracking at this point (exudate). I decided to bolt (brace) the trunks together with threaded rod, washers and nuts in addition to the pruning.
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Figure 6. Close-up of attachment of trunks, 2002. Note the nearly equal size of the trunks. There was evidence of possible internal cracking at this point (exudate). I decided to bolt (brace) the trunks together with threaded rod, washers and nuts in addition to the pruning.
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Figure 7. View of the tree in fall 2011. Note the difference in size and height of the two trunks. I have pruned this tree almost every year since 2002. The pruning involved reduction cuts on the north (street side) trunk and maintaining its height to allow limbs above it originating on the south trunk to grow over it. It looked strange in the early years of its pruning, but I think it is looking pretty nice at this point.
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Figure 8. Close-up of trunk attachment, November 2011. Note the difference in size of the trunks due to the effects of regular pruning to suppress the trunk on the right.
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Figure 9. View of valley oak looking northwest. I find that in situations like these, suppressing the north facing trunk is normally best. This prevents sunburn and promotes more rapid shading of the suppressed trunk as trees tend to grow more rapidly on their south (sun exposed) side.

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