Purple-leaf plum trees are easy to grow spring flowering trees, valuable for their purplish pink flowers that contrast with the dark bark of the branches. They aren't as picky about soil conditions as cherries and are less prone to disease, but they do grow a thicket of small branches that, if left in place, will obscure the graceful shape of the tree.
Choose a time to prune, preferably in spring, either before bloom or after leaves unfold. If you prune before blossoming, you'll lose some buds, but since purple leaf plums flower so prolifically, you may not notice the loss.
You can also prune in late summer or early fall, but you will also lose buds that would flower next spring.
Plan your cuts before reaching for the pruning shears or lopper. Is the size of the tree about right, but the main branches are obscured by extra sprouts? Or is it too tall or wide and you need to reshape it?
If the tree needs thinning, plan on starting at the center and working outwards, taking out unnecessary branches as you go. If it needs shaping, find the branches that stick out to the side or up to the top, follow them downward to their origin on the trunk or on a branch and plan on taking out the entire shoots. Leave shorter branches to hide the cuts.
Now, make your cuts, starting with the thickest branches. Use a pruning saw if these are more than 1 inch in diameter. Always cut flat against another branch; never leave stubs to sprout again.