Prune dwarf plum trees with proper timing and an eye for tree structure. Plan to prune in early spring, before blooming, while the tree is dormant. The most important pruning you will do for a dwarf plum or any fruit tree is in the first three years after planting. Make cuts that leave the tree with an open canopy that receives maximum light and air for fruit production. Your objective is to develop 5 to 7 horizontally-growing scaffold branches, spaced evenly around the tree's trunk.
Clip off shoots, called suckers, from the trunk from the ground up, with pruning shears. Suckers contain a lot of water, and will never produce fruit while they sap vigour from the tree. Repeat clipping off suckers in the summer, because they will re-sprout.
Cut the tree back, leaving only 5 to 7 horizontally growing scaffold branches in the first year, spaced 5 to 7 inches apart and evenly around the tree. Cut the other branches away.
Thin the canopy of your dwarf plum from the top down in the second, third and all subsequent years. Picture an open vase or goblet as you go, because that's the shape that will allow light and air to penetrate the dwarf plum's dense foliage. Prune off vigorously growing vertical branches that block sunlight and keep fruit from developing or ripening.
Prune off rubbing or crossing limbs in the canopy interior. Rubbing can cause bark injury and rot. Cut in order to leave the strongest branch intact. Angle the blades at 45 degrees, cutting down and away, leaving a 1/2-inch stub. The angle allows moisture to drip off pruning wounds instead of collecting and causing problems.
Test flexibility of the bottom third of the dwarf plum's canopy. Because foliage on a dwarf is so dense, the lowest branches are the most likely to die. Bend them with your fingers to make sure they're dead before you cut. Occasionally a branch can lose its leaves and still be able to re-sprout them later.
Always wash the blades of pruning tools in hot soapy water after use to prevent bacteria, fungus or insect eggs from being transmitted to other plants.