As many people have learned, dark oak trees can grow downwards, witch is bad for faction servers because bedrock donators, if there is any one your server, get griefed 100x more easily. If there is a fix for this, can you link me to it? and if there isnt, its a nice idea.
Easy to fix if someone want to do it just do : If (tree) is growing && If (tree) = (tree) just at the top (1-7 Block) of the sapling, then never grow