Minecraft wiki entry for Mushroom_farming, associated text: The pistons need to be wired to an outside switch, the water stream shall be redirected to your favorite collection point. Then, the room needs to be sealed to be completely dark. The only opening should be 1x1 wide, for the water stream. This being the only opening, spawned monsters cannot leave the room. Eventually, the mushrooms will spread to the blocks in front of the pistons. Activating the pistons will then harvest the mushrooms and push them into the water stream. The system is reset by deactivating the pistons without any need for replanting.