I am building an underground orchard in Minecraft. What is the minimum height the ceiling needs to be in order for trees to grow?
Back when there was only one kind of sapling, it was easy: you needed 6 blocks of headroom. However now there are many types of tree, each with different requirements. Each sapling type requires this amount of space above the sapling:
As for the trees which require saplings be placed in a 2x2 square:
Source: Minecraft Wiki