You can develop mycelium on cardboard, although some fungi won’t develop fruiting bodies on it. You have to have a wood loving fungi, like oyster mushrooms, to successfully grow mushrooms from cardboard. On the other hand, it can be used as an agar substitute for almost every fungus. Let me tell you how.
You have a mushroom you want to clone. First of all you’ll want to cut the cap off to make a spore print. You now have only the stem left to work with.
Cut the stem in smaller pieces. You can leave them intact, but cutting them in pieces will make later transplantations to other substrates easier. Use a sterile knife for this, preferably a sterile razor blade or scalpel. Wear latex gloves or wash your hands with alcohol and put on a mouth mask.
Try not to bruise the mushroom too much, as my experience has taught me that bruises take longer too develop mycelium, or don’t develop mycelium at all. So be careful. Try to touch the mushroom stem as little as possible.
Now you have to prepare your cardboard. We do this by providing it with the correct moisture content. Put the cardboard in boiling water for about 1 or 2 minutes. Take it out and let it drip. Shake off as much of the access water as you can. When held vertically, the cardboard shouldn’t be dripping any more water.
Place the stems on your piece of cardboard and put everything in a ziplock bag.
Store the bag in a dark place at the right temperature. For most edible and hallucinogenic mushrooms 22°C should be fine. After a week you should get this as a result (see picture on the left). You can then cut little pieces of the cardboard using your sterile knife and distribute them among your substrates. Or you can use the other agar techniques to isolate a strong monostrain.