Cardboard Mushroom Spawn

One of the great things about growing mushrooms is that growing materials are often free. Sometimes they even save you from throwing things away. Using cardboard as mushroom spawn is one such example. Cardboard is made of raw, unbleached paper, which is a good growing material for mushrooms and the corrugations are held together with a glue that is also very digestible by mushrooms. The channels in corrugated cardboard give the mycelium an easy channel to run down and travel quickly.

Personally, I find that mushrooms produced on cardboard tend to be kind of anemic. The open structure, while ideal for travelling mycelium, isn’t really dense enough to produce lots of mushrooms. However, the sheet form that cardboard comes in makes for easy transfer from one medium to another. If you are making a wood chip mushroom bed, you just lay down a layer of wood chips, then cover it with your cardboard spawn and then add another layer of wood chips, for a total of about 6” thick. You can use cardboard spawn for making mushroom logs as well. You take a chainsaw and cut a wedge in the log, line it with your cardboard spawn and hammer the wedge back in. When making a mushroom block out of wood chips or coffee grounds, you can just tear up the cardboard and mix it in. It will spread from the cardboard.

Making cardboard spawn is really easy. The first thing to do is find a good source of cardboard. Something that has already been through the mail is fine, as long as it isn’t covered in grease or other such toxic or unidentifiable chemicals. Cardboard spawn is best grown rolled up, so a container that will fit a roll of cardboard, like a glass jar or a bucket, works nicely. Then cut the cardboard to fit your container.

The next step is to clean the cardboard to remove potential contaminants. The nice thing about cardboard is that it is a pretty hostile environment, so you don’t need to worry about too many contaminants unless it has bee sitting outside for a long time. The biggest thing you need to worry about is mold spores. You will need to get rid of those. There are two basic ways to do that. The first is with boiling water. Put your cardboard in its container and fill it with boiling water and put on a lid if it has one. Let it sit about an hour. That will kill almost all mold and bacteria. The problem is, it also dissolves the glues, destroying the structure of the cardboard.  Fortunately, there is a second method involving hydrogen peroxide.

As many mushrooms grow, they naturally want to claim territory that they occupy as their own. One major source of possible competitors is spores of other fungi. Most mushrooms produce various peroxidase compounds as they grow. These compounds destroy the spores without harming the mycelium. Hydrogen peroxide is a very similar compound to what the mycelium produces and has much the same effect. So I will put some hydrogen peroxide in a spray bottle (or just transfer the spray nozzle to the peroxide bottle) and spray the cardboard down. I don’t dilute or anything. The hydrogen peroxide will break down into water pretty quickly and won’t harm the mycelium in small amounts.

Once the cardboard is treated, you can just layer your previous spawn on top. Wood chips from a spent mushroom block is a good medium of transfer. I also find stem butts particularly effective for transfer onto cardboard. Either way, you lay it on top and then roll it up as tightly as you can. Then put it somewhere and keep it moist. It is ready to transfer when the entire surface is covered with white, cottony mycelium and it no longer smells like wet cardboard.

Oh, and when you are finished transferring it to its new home, consider setting aside one sheet of cardboard. You can roll it in a fresh sheet of cardboard to make all new cardboard spawn.