Mushroom Cultivation: Cardboard Spawn

It has been quite a while since I last experimented with oyster mushrooms, and even longer since I decided to take note of my methods and record them all to then post here at your convenience as well as for my own record keeping. Indeed the pervasive nature and tenacity of the mycological world couldn’t keep me away for too long before the urges of wonder and discovery had me crawling back for more.

I have written about oyster mushrooms quite a few times already, but as should be expected my techniques are evolving with each failure and success which, for my best interests in the long run, there have been plenty of both. If you would like to catch up on my earlier adventures before reading along with what I am currently up to then I welcome you to check out my introductory post on Growing Mushrooms Using Straw.

I also wrote a post last year on how you can Growing Oyster Mushrooms From Wild Grasses, which was an extremely worthwhile endeavor that ended in success, proving my hypothesis correct that you do not necessarily need straw from agricultural grain crops (wheat, barley, rye, oats, rice etc.) but that the dried stalks from any grass species including wild species should work just the same, as long as they are processed properly.

This wild straw can be used as a material in which to generate spawn as well as a fruiting substrate. As of right now the biological efficiency (the weight ratio of fresh mushrooms produced compared with the myceliated substrate) as well as overall the yield that you would experience from a successful colonization of a wild grass-based substrate isn’t known. This knowledge does, however, allows anyone who lives anywhere tall grass can be found to have access to resources that have the potential to produce food. It’s a beautiful thing.


So now that you are as caught up as you would like to be, let’s move on. For starters, what is cardboard spawn? Well, it’s cardboard that has been soaked in water for an hour or so until it is completely saturated, drained until it is no longer dripping and then placed in layers inside an airtight container (such as a plastic container with a tight fitting lid in order to seal in moisture). Specifically you want the corrugated pieces of the cardboard, those being the rippled layers that are often sandwiched in between the thin, smooth layers of cardboard.

Cardboard soaking in cold water with a weight to keep everything submerged. I also had a lid on the pot to prevent dust and other unwanted particles from settling in with the cardboard.

Spawn is fancy mushroom growing terminology that refers to a material that is inoculated with mushroom mycelium with the intent of introducing that material to more substrate in order to initiate further colonization and eventually the fruiting of mushrooms. Spawn is often grain or sawdust based, which are materials that are easily digestible and allow for mycelium to quickly colonize it.

This spawn is then mixed with a more nutritionally dense material that takes longer for the mycelium to colonize but provides the growing culture with enough of what it needs to encourage the culture to producing fruiting bodies/mushrooms, which is usually the main reason that people are generally interested in growing mushrooms at all. Growing mushrooms is usually a two step process, generating spawn that is then mixed with a fruiting substrate/material that yields the mushrooms themselves.

The ripples in the corrugated cardboard are more desirable of a landscape for the mycelium to into. They seem to prefer the 3-dimensional aspect of the folds rather than smooth, uninterrupted/uniform surfaces of the smooth cardboard layers. In between the layers of soaked, corrugated cardboard you can place stem pieces from foraged or store-bought oyster mushrooms. These are the spongy, tough pieces that you cut off before you cook the mushrooms. They are chewy and generally not good eating, but these tissues contain live mycelium that can regenerate when placed into a suitable environment.

The corrugated inner lining of the cardboard sheets. As I separated out these rippled sections I left them on a dish for a few minutes to allow the excess water to drip off of them. This prevents access moisture from accumulating.

A suitable environment is generally one that is clean (limited competition from other micro-organisms), humid and has plenty of food, which in the case of oyster mushrooms is predominately lignin and cellulose. Conveniently, cardboard is essentially made up entirely of both of those compounds and has already been predigested during its processing and manufacturing, acting as the perfect nursery for your mycelium to grow.

If you notice that there is excess moisture accumulating at the bottom of your container (which you don’t want) you can briefly remove all the cardboard/mushroom layers together and sprinkle about a 1-1.5″ layer of moistened vermiculite on the bottom of the container and tamp this down before placing the cardboard/mushroom layers on top. This will absorb any excess moisture that may drip off your cardboard.

Assuming the oyster mushroom pieces are not too old or otherwise compromised and the cardboard wasn’t already infested with mold spores (not stored in a humid, filthy place or outside for long periods) the mycelium from the oyster mushrooms should grow into and throughout the cardboard in your container in about 2-3 weeks. This time line of course is dependent on factors including but not limited to the vigor of the mycelium, the humidity on the inside of the container and the temperature of the room.

A small plastic container that with layers of folded, corrugated cardboard in which I am wrapping cut-up pieces of oyster mushroom stem. I try to conceal and wrap them in as much cardboard as possible, limiting the amount of airspace between each layer of cardboard. It’s alright to fold or crumple the cardboard in order to make it fit.

As a general rule of thumb, if you are comfortable with the temperature in a room, so will your oyster mushroom mycelium. It’s fine if you need to have a sweater on, as the mycelium can handle temperatures down to about 8°C or so and still grow but of course it will be slower going than if they are situated in an environment that is somewhere in between 15-22°C. The temperature should never dip below 5°C or higher than 30°C because this can be detrimental (read deadly) to your developing mycelial culture.

It is also a good idea to clean the inside of your container with diluted bleach or rubbing alcohol and allow it to air dry before using it as an additional precaution to prevent contamination. You probably don’t have to do this if you clean things really well, but if your first attempt at generating spawn fails then I would recommend this extra step. Also, some people have issues with using cardboard for a fear that it may contain dioxins or other harmful chemical compounds. The assurances that I can offer are as follows:

The plastic container full of folded, lightly compacted cardboard with pieces of oyster mushroom stem nestled in between each layer. As the mycelium grows, this container will become heavier and denser than it currently is.

1) Cardboard that is manufactured and processed in Canada, the United States and Europe is expected to be free of dioxins thanks to strict environmental regulations. There is no 100% guarantee, as we can only trust that those manufacturers are following the rules (which they should be and if they are not then we should collectively hold them accountable for deliberately misleading us).

2) You are not growing the mycelium/mushrooms solely out of cardboard and then consuming them. The cardboard spawn that you produce can and should be mixed with another substrate which will provide the growing mycelium with enough nutrients so as to allow them to mature and produce mushrooms that you will then harvest. I don’t personally recommend growing mycelium indefinitely out of nothing but cardboard, although it can probably be done however yields likely won’t be anywhere near impressive or even decent.

Producing cardboard spawn is not difficult by any means. Cardboard is probably one of those things that you seem to accumulate without even really trying and if you do somehow have difficulty acquiring some on your own many local businesses with far too much will likely give you as much as you could possibly use for free. The spawn is ready when the cardboard has been almost but not quite colonized by white, threadlike networks of mycelium and the container has a fresh, slightly sweet smell to it.

Two different containers that both contain cardboard mixed with oyster mushroom stem pieces. The containers do not need to be full by any means, and a little bit of airspace is probably a good thing so that the mycelium can ‘breathe’ a little bit, although it needs less fresh air when it is colonizing than when it is time for it to produce mushrooms.

You don’t want the cardboard to be completely colonized/covered by mycelium because you want the mycelium to be actively growing when you transfer it into the container with which you plan on using to produce mushrooms. If the mycelium grows out too much then it may undergo cellular senescence as it runs out of space and nutrients, thereby losing vigor and it’s ability to successfully compete colonies of bacteria and mold.

If the mycelium is growing still actively growing out when you mix it into some new substrate material, and therefore additional food resources and space then it will ‘leap off’ onto the substrate with greater ease. This gives you a better chance of success, and so once you start your cardboard spawn it’s a good idea to already know what you are going to use for a substrate and a container when it comes time to make the move.

Cardboard spawn, besides being very cheap to produce is wonderfully versatile. It can be mixed together in a ratio of approximately 1:3 or 1:4 parts spawn to substrate in order to ‘grow out’ your mycelium for the purpose of producing mushrooms. Straw from grasses of all varieties, hardwood tree chips, mulch, sawdust or even coffee grounds, spent grain from breweries, old ripped up cotton clothing and many other seemingly useless materials can be used for the purpose of growing oyster mushrooms.

Just like Pleurotus mushrooms, the wine cap stropharia (Stropharia rugosoannulata) also takes well to being cultured in cardboard spawn using essentially the same process illustrated above, but they would need to be grown outdoors in a hardwood sawdust and wood chip bed, not indoors on straw or sawdust like oyster mushrooms. In my next post we will look at the next phase in this process (which you can check out right here) in which I took this finished cardboard spawn that I generated and mixed it with pasteurized wheat straw as my fruiting substrate.

Updated 02.17.17