Plum trees must be pruned periodically from planting onward in order to promote the development of balanced and strong scaffold branches for fruit production. Plum trees fruit on wood that grew the prior year, making management easiest if trees are pruned to an open centre, which resembles a vase shape. Although recommended months for pruning vary across the country because of differing climates and growing season lengths, training and dormant pruning still serve the same purposes universally. Check with a local extension office or guide to help determine most exactly when to perform dormant pruning.
At planting, the tree manager must consider whether he wants scaffold branches low enough to make tree care and harvesting easily accessible from ground level or high enough clearance for management practices like weed control. If at planting the tree has no branches 20 to 30 inches above the groundline, cut the tree off about 30 inches above the groundline. Major branches will develop at the level of the cut. If the tree has multiple healthy branches between 18 and 30 inches above the ground, choose three or four that are well-spaced to be main branches. Cut these lateral branches in half to a healthy, outside-facing bud and remove any branches less than 18 inches above the ground.
Summer Pruning (First Year)
During the summer, generally before August, prune to remove broken, diseased or low limbs. Remove any upright shoots that have grown up on the inside of the main scaffolds. Also, prune back the turned up tips of main scaffolds to the shoots that are growing outward.
First-Year Dormant Pruning
Dormant pruning occurs before the second growing season begins. The dormant season varies between locations. Southern climates call for a detailed pruning in mid-February, and to never prune from October through January. Remove any low hanging limbs or damaged limbs, as well as any vigorous upright shoots growing on the scaffolds.
Mature Tree Pruning
In general, observe the same pruning principles as for the first growing season. During the dormant season, maintain the tree's open vase shape. Japanese plum varieties have many more long, thin shoots than their European counterparts, so heading is much more important in Japanese plum varieties. Plum trees require little pruning, once trained, to maintain their shape. Damaged or diseased limbs or especially vigorous shoots can be removed during summer pruning -- the majority of shape work should be saved for dormant pruning.