Because mushrooms can't be grown in places with a brightness level greater than 12, growers must carefully consider how to plan for light sources. Mushrooms can be grown on dimly lit opaque squares without water, sand, or extra space, unlike crops, cacti, and saplings. But even at night, they can still only be grown in areas that are less than 13 bright during the day. The exceptions are mycelium and ashed soil, and if mushrooms are grown on mycelium or ashed soil, then there is no problem even if they are completely exposed to daylight. They will then slowly spread to other opaque squares, including areas where they cannot be grown. Mushrooms will only spread if there are less than 5 mushrooms in a 9x9x3 space.

Giant Mushrooms:

Using Bone Powder on a red or brown mushroom will spawn a Giant Mushroom if there is enough available space and the mushroom is being grown on dirt, grass, or mycelium. Lower the light so that your mushrooms grow closer to the surface. Then just fill in your tunnels with the dirt inside the mushroom when it grows.