Minecraft wiki entry for Nether_survival, associated text: Another catch: Trees need space to grow - and ghasts need space to spawn. Happily (and contrary to popular belief), ghasts do spawn on blocks, and they need a 5×5 space free of transparent blocks and slabs. You can scatter slabs, glass and glowstone around the floor of your tree farm. You can plant saplings with glowstone adjacent, and slabs on top of the diagonal blocks. You can do this in rows for max efficiency. If you're low on glowstone, jack o'lanterns and torches suffice. You also need at least 4 blocks of air above the sapling. Using this method of tree farming, you can have a large enclosure for mass wood farming without ghasts! If not too densely lit, your tree farm also serves for bone-meal assisted huge mushrooms, which is useful for Mushroom Stew. The most basic way to safely farm trees is to have a 5×5×7 room with a floor made out of glowstone or glass (if you are using glass, make sure to put torches next to the sapling) and have 1 dirt in the middle of the floor.