The Japanese plum tree can reach a height of 15 to 20 feet, and a spread of slightly over 20 feet. The fruit is not as sweet as European plums, although it is juicier. It blossoms in early to late spring and is hardy in zones 5 to 9. This is a high-maintenance tree, requiring pruning and fruit thinning. The reasons for pruning are to remove dead, broken or diseased branches, less-productive branches, shoots growing straight up the center of the tree and water sprouts and suckers.
Prune dead, damaged/broken or diseased branches as soon as possible; make your cut at the breaking point, or you may want to cut off the entire branch. Be sure that you do not leave any ragged or torn edges, as they can be an entry point for disease. This can be done at any time during the year.
Visually inspect your tree for branches that are growing up through the center of the tree, and branches that are growing inward (these are less-productive branches). These branches will need to be removed. You want the branching of the Japanese plum to be bowl-like, with three to five branches radiating out from the trunk of the tree.
Find the branch collar (this is on the underside of the branch where it connects to the trunk) and the branch bark ridge (this is on the top of the branch where it connects to the trunk) of the branches you are going to remove. Cut right in front of the branch bark ridge and the branch collar. The branch collar and branch bark ridge should remain in tact. Pruning can be done anytime before flower petals begin to fall.
Prune away water sprouts (shoots growing straight up into the tree) and suckers (stems that grow from the bottom of the trunk and from the roots). These just take much-needed nutrients away from the tree. This can be done at any time during the year.