Don't know how I missed this but yeah, any of the liginicolous (wood-eating) types of mushrooms can be successfully fruited on cardboard.

Well, that's pretty interesting. However, I would be concerned about the dyes, glues, insecticides and so forth that might be used in that cardboard.

Concerned yes, but don't let that stop you. Many non-toxic dyes and glues are used these days in lots of that stuff.

Ah, @thonotorose, you are definitely our angel of the organic, making sure we don't poison ourselves or anything around us. The way I see it, if people have been doing this for a long time, and they haven't died yet, there probably isn't much danger. Surely, there is no more danger than planting them on rotting logs, since tree roots spread far down into the water table, picking up all sorts of yukky stuff along the way, not to mention the toxins they gather from the air.