According to the Texas A&M Agriculture Site, there are several reasons for pruning:
• To train the plant
• To maintain plant health
• To improve the quality of flowers, fruit, foliage or stems
• To restrict growth
The main objective of proper pruning is to strengthen the integrity of the plum tree to support fruit production and eliminate or reduce limb breakage due to the weight of bearing the fruit. Properly pruned plum trees also insure that the right amount of sunlight and airflow are able to reach deep with the tree and allow all of the fruit to flourish. The health of your plum tree depends upon pruning to keep it stimulated.
Knowing when, how and what to use to prune your plum tree will help you to properly maximize your pruning efforts. Take a few minutes to watch this (three-part) YouTube video produced by the Alabama Cooperative Extension:
As you can see, there are various times of the year where pruning your trees is more beneficial than others. Young plum trees can be pruned during the summer months to remove the sucker limbs that sprout from the trunk but are unproductive in contributing to fruit production. The winter months, especially around February, are an excellent time to prune plum trees to insure their fruitfulness in the approaching growing season. The forty-five degree angle is the technique guideline for the perfectly shaped plum tree. You will need to begin this process early on in the growing years to maintain the shape as the tree ages without any detriment from the fruit load on the tree limbs. This forty-five degree angle pruning technique also insures open air flow through the center of the plum tree and should resemble a Christmas tree shape.
Proper pruning while the trees are young will certainly curtail the amount of pruning necessary in the second and third growing seasons. The fourth and fifth seasons should be a time to revisit heavy pruning to insure an abundance of fruit by thinning out the tree.
In the end, a properly trained and pruned pear tree will bear fruits for many years. For instance, during the winter months, a large portion of the tree can be safely removed while the tree is dormant. Summer pruning should be restricted to removing the upright and current season’s growth using only the thinning method of pruning and never after July to prevent winter injury in young trees.