There are many plum varieties, but they fall into two main groups: European and Japanese.
European plums need only occasional thinning and heading once the tree shape has been formed. Japanese plums overgrow and overbear. They are particularly prone to branch splitting when they are mature and bearing heavy crops. Cut back long whips and thin fruit when it reaches thumbnail size, leaving about 4 to 6 inches between the remaining fruits.
Remove one-third of the new wood on Japanese plums each year by thinning and heading. This heavy pruning is necessary to produce larger fruit. Keep long, thin branches headed to give the tree a stubby, wide shape. When the fruit spurs on a branch have borne for five to six years, select a new branch from lateral shoots on this branch. The next year, remove most of the old branch, cutting it off just above the selected lateral.
Apricots appear on the previous season’s shoots and on short-lived spurs on older wood. Pruning is essential to apricot production for several reasons: It stimulates a certain amount of new growth for next year’s crop; it keeps the tree open; and it prevents fruit from being borne only high in the tree.
Like plums, apricots bear on spurs that produce for two to four years and then need to be pruned out and replaced with younger wood. In pruning apricots you need to head back long new whips by one-half and remove the oldest fruiting wood. Fruit may form in the second year, but don’t expect a heavy crop until the third or fourth year.
Not as many heading cuts are required on a plum tree; instead concentrate on thinning small twigs and branches to increase fruit size on young trees.