Minecraft wiki entry for Tree, associated text: Once a tree has passed a light check and chosen size to attempt, it checks if there is enough space for its chosen size. If it encounters an obstruction during this check, it fails to grow and must wait for the next pass before it can attempt to grow again. This means that a tree in an open field with enough light grows relatively quickly, but a tree in a cramped tree farm that stunts its size may make several attempts before finally growing.