Minecraft wiki entry for Huge_mushroom, associated text: To grow, the lowest four blocks of the stem (i.e. the three blocks above the small mushroom being grown) must be air or leaves, and a 7×7×(height−3) region above that must similarly be clear of anything except air or leaves. The block under the huge mushrooms stem must be dirt, podzol, or mycelium. Light level does not matter if placed on podzol, or mycelium, if placed on dirt, coarse dirt or grass blocks both skylight and blocklight must be 12 or less.