Why have my three trees all gone the same way, two in the ground and one in a pot? The main trunk seems to split and you can see what I think is the core of the trunk. They are not watered only in hot weather. Any books I have been able to read never show any plants like I have. Your advice would be greatly appreciated. Margaret
From my research the stems of Plumeria splitting in some fashion is normal.
It can be caused by rapid growth, excess watering, sun scald or exposure to extreme temperatures. It also can be more common in certain varieties then others.
Unless you see soft spots on the trunk, there is no concern.