Mature plum trees, as with most stone fruit trees, require light annual pruning to keep fruit production stable for as long as possible and to remove damaged and unproductive tissues. Plum trees should be pruned and trained into a growth form when young and before they begin bearing fruit, according to the University of Colorado Extension. Once mature and fruit bearing, this structure should be adhered to and maintained with pruning, while dormant, in late winter or early each spring before the flower buds swell and break open.
Cut away any broken, diseased, dead or otherwise compromised branching down to the point of healthy tissue or to the parent branch from which it emanates. Place the cut just outside of the slightly swollen branch collar.
Cut away any water sprouts that are growing inward toward the trunk of the tree or any that cross or abrade other branches. Many of these will not bear fruit but simply draw on the trees resources and congest the canopy. Cut each back to the parent branch from which they emanate and pull from the canopy before discarding.
Cut back the tips of 1-year-old wood to stimulate new branching and redirect resources to the lower buds below the cut. Remove 1 to 3 inches of branch tip placing the cut 1/2 inch above a healthy, larger bud or leaf axil.