@donkeypong Now is the time to do the pruning for the future shape of your tree. Plums are susceptible for weak crotches, especially the species with thorns. (Yes; I know it's a hybrid.) At this age, you should end up with an open vase shape. Select branches that go out at a right angle. Remove all cross branches. Also slip back the leader; forming a new one. This will help to squat out the tree. This technique exponentially strengthens the whole tree and sets the ability for a great framework of production. If you are unsure, write to me and I'll talk you through it.