I tried growing the dark oak sapling underground and found out that when i placed a layer blocking the sky the tree wont grow. I went in creative to test and place an obsidian layer at y 255 above the dark oak sapling, then almost 200 blocks below i tried to grow the tree and it didn't work. Later i tested it with other tree types and everything works except for all the 2x2 variants. I don't really know if this is a bug but there is absolutely no information to find about 2x2 saplings underground.

Kind regards,

Dimitri Lilov