Properly Thinning Your Tree Part 3: Structure – February 2019

Structural pruning is needed for all trees. Pruning cuts will be small in nature leaving meaningless wounds when pruning is conducted on a regular basis. The goal is to prevent all branches and stems from growing larger than about half the diameter of the trunk. Younger trees respond better to structural pruning. Changes in branch diameter relative to trunk diameter brought about by structural pruning are slower to manifest on medium-aged and mature trees than on younger trees because the core trunk and branch structure has already been established. Prune often and begin at planting.

There are a few simple steps to maintaining a dominant trunk leader. The first step is to spot the stem that will make the best dominant trunk. It should be in the center of the crown, and crack free, openings, mechanical damage, large pruning wounds, cankers or other defects that could compromise its strength. The next step is to spot the stems and branches that might be competing with this stem. The last step is to remove those competing stems and branches back to the trunk, or subordinate by shortening them with a reduction cut. Be sure to remove branches that are clustered together and growing from the same point on the trunk. Ideally, only one large branch grows from one position on the trunk.

In contrast to younger trees, structure changes in older trees happen less because parts are much larger and growth has slowed. But, structural pruning is still recommended making two- to four-inch-diameter cuts on stems and other weak limbs. Similar to small trees, the focus is on reducing the length of stems and branches that compete with the leader, and those with defects. Removing additional branches behind the reduction cut can suppress growth rate on the pruned stem even further. Unlike reduction cuts, removing only secondary lateral branches from stems and branches that compete with the leader on mature trees causes less dysfunctional wood behind pruning cuts. However, secondary branch removal does less to relieve mechanical stress along the pruned branch than reduction. Moreover, the branch may be forced to grow too long unless it is reduced in length with a reduction cut.

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