What is training pruning?

Training is a type of pruning on young trees to help develop proper branching patterns and structure. Negative impacts of poor structure are not prevalent on young trees.  However, as trees grow and limbs become larger, structural weaknesses can become a point of failure.

Why is training important?

Unfortunately, proper care for young trees is often overlooked.  Most people are aware that proper watering, mulching, and fertilizing will help young trees establish.  When it comes to future performance of young trees, training pruning needs to be added to this list of important maintenance practices.  Here are a few reasons why.

1. Training will develop trees that are sound structurally minimizing the negative affects of co-dominant stems, weakly attached limbs, and redundant limbs.  Trying to improve structural weaknesses when trees are larger is not always possible.  This can create large holes in the crown as well as upsetting the tree’s energy budget by removing a large percentage of live wood.
2. Fewer failures, potential hazards, and defects in trees will make for a sound future urban forest.  Mature trees are not replaceable.
3. Well-spaced limbs will equate to fewer limbs to be pruned in the future.  This will result in less expensive pruning.
4. Well developed tree crowns will result in trees that are more aesthetically pleasing.  Nursery practices create trees (to make trees more attractive?) that have a low, dense branching structure that must be corrected.

The Biology of Training Pruning

Key Terms
Co-dominant limbs; Limbs of equal size and importance (or dominance)
Included bark; Point of branch juncture where bark is pushed in, instead of out
Redundant limbs; limbs of equal size growing in the same general area.

• Included bark often occurs where there is a sharp angle of attachment or between co-dominant leaders.  Branch attachments with included bark are inherently weak.  Strength of branch attachment comes from the trunk growing around the branch.  This is not possible with included bark.

• Young trees will tolerate having as much as 1/3 of its live wood being removed.  As trees get older and mature, the amount of live wood that can be removed decreases considerably.  Training must be done while trees are young and can handle the stress of removing live wood.

• Proper pruning cuts must be made to ensure that the tree is not wounded.  Leaving flush cuts or stubs can lead to decay.

• Proper timing for training is during the dormant season (November – March) while stored energy is at its highest level.  Without leaves, the overall structure is much easier to see.

1 Select the most dominant leader and remove others.  Trees having co-dominant leaders should have one of them removed or subordinated.

Ash tree with Co-Dominant leaders

If co-dominant limbs are too large to be removed with one cut, subordination may be possible (see subordination example in the right column).

2 Remove any dead, dying, diseased, weak, rubbing or crossing limbs.

3 Select the lowest limb that will be a permanent limb.  Limbs lower than this may be kept to help develop trunk taper.  They are temporary and can be removed later.

4 Permanent main limbs for future structure (scaffolding limbs) should be selected. Always look for good branch angles and attachment when deciding which limbs to keep or remove.  Any redundant or competing limbs should be removed during this step.

5 Suckers and waters sprouts should be removed since they are an energy sink.

6 Limbs growing towards wires, buildings, or other trees should either be removed or pruned in such a manner so they do not interfere with the obstruction. Hopefully, a proper planting site will allow for no obstructions.

Subordination Pruning

Subordination pruning is a type of training pruning that minimizes the impact of co-dominant limbs, usually done over several years.  It is used primarily in cases in which the leaders are too large to remove at one time.

A Subordination Example

The picture above shows an ash with co-dominant leaders.  If you feel the limbs are too large to be removed at one time, subordination would be a good choice. The leader on the left is straighter at the point of attachment, but splits into three limbs.  The leader on the right remains as one limb and is a slightly taller. This is why the left limb was chosen for subordination.

The picture above shows three red–dotted lines where pruning cuts were made to start the subordination of the left leader.  The remaining leader on the right will become more dominant over the next few seasons.  The limb on the left will be pruned back further in a future seasons.  Eventually the left limb can either be kept on as a subordinate limb or may be removed.