Two varieties of plum grow well in California: the Japanese plum (Prunus salicina) and the European plum (Prunus domestica). Each is pruned to a different shape, but other pruning aspects remain the same. Prune either type lightly during the first five years and only to remove extra limbs and shape the tree. When the tree reaches fruiting age, increase pruning to thin excess fruiting spurs, reduce fruit load and control growth.
Prune European plums to the modified leader system. This means developing a strong central leader branch and removing other vigorous shoots during the first two years. Allow scaffold branches to grow 6 to 8 inches apart vertically.
Prune Japanese type plums to the open center system, meaning trim the tree into a vase shape, opening the center up to light and air circulation. Choose two to four branches to form the main scaffold branches during the first winter, removing remaining branches. Allow one or two more to grow during the second year. Aim for four primary scaffold branches evenly spaced around the tree.
Remove dead and diseased branches during the winter dormant season. Thin out remaining branches for the desired shape. Space branches to allow light and air to penetrate to the tree's interior.
Remove vigorous shoots from the interior of the tree during summer months. Keep interior shoots pruned back to improve air and light penetration.
Thin fruit to 4 to 6 inches apart when plums are marble-size. Thinning fruit will encourage larger plums and protect branches from breaking due to fruit overload.