Training plum trees will not only increase fruit yields, but the vigor of the overall tree health improves as well. Plum tree pruning is often neglected by growers. According to North Carolina University, plum training is performed over a period of years to achieve an open center tree. The idea is to train the plum tree so the tree shape resembles an open bowl. This allows for full air circulation throughout the limb system, which reduces mold and mildew problems to the developing fruit.
Plant the plum tree according to the nursery instructions. In most all cases, the plum tree will be placed in soil that is well drained with full exposure to sunlight. The graft point, for dwarf and semi-dwarf trees, must be 2 inches above the soil line.
Prune the central leader of the newly planted plum tree 24 inches to 36 inches above the ground with the pruning shears. Below this point the tree will then begin to emerge the lateral tree limbs (scaffolding limbs).
Choose three or four of the lateral shoots for the main scaffolding limbs during the first winter pruning. The scaffolding limbs must be 8 inches apart from each other in height. In the best-case scenario, the limbs should be at a 90-degree angle from each other in relation to the trunk. In other words, looking down the tree from the top, the four limbs would be at the 12 o'clock, 3 o'clock, 6 o'clock and 9 o'clock positions. Remove all other limbs from the plum tree.
Select one more scaffolding lateral branch during the second winter pruning. The plum tree should now have four main lateral branches that form a basic shape of an open bowl.
Install the 1-by-2-inch boards between the lateral scaffolding branches to increase the bowl shape. The boards will be cut to varying lengths as the spread of the branches will be different. The goal is to increase the distance between the four limbs of the open bowl shape.
Remove all sucker tree limb growth during the third winter pruning. The plum tree should now be showing a good bowl shape. You may wish to remove or install various 1-by-2-inch boards to increase the training of the bowl shape. Each tree will have to be evaluated on an individual basis. Continue removing any sucker limb growth that exceeds the overall height you desire for the plum tree. All dead wood branches must also be cut from the tree during winter pruning.