Coffee Ground Mushroom Spawn

There is a fine line between a medium that is used for mushroom spawn (that is, to transfer to another medium) and one that is used as a growing medium. Some, like cardboard, are more structurally suited to transfer and may not have enough nutrition to take the mycelium all the way to a bountiful flush of mushrooms. Others, like straw, which is messy and difficult to fully sterilize, are more suited to fruiting and less good for production of spawn. Coffee grounds is one that is good for both. The granular nature of the grounds makes for quick and easily colonization by the mycelium. The process of making coffee out of the grounds conveniently sterilizes the growth media, limiting the opportunities for contaminants. The woody nature of the seed pod of the coffee bean also provides good woody material as well as abundant nutrition for the mycelium. Another nice feature is that most coffee filters are made of paper, which is also readily digestible by mycelium.

The first trick is to find a suitable mushroom to grow on coffee grounds. I have had great luck with both Pleurotus ostreatus (pearl oyster mushrooms) and Hypsizygus ulmarius (elm oyster mushrooms). I am currently attempting it with Agrocybe aegerita (pioppino/black poplar mushrooms), but my stem butt was small and it hasn’t made much progress yet. I suspect that there are other mushrooms that would do well in this medium as well, but not being a coffee drinker, I don’t have too much opportunity to try out new combinations. Also, I prefer to start my coffee ground cultures with stem butts from fresh mushrooms. If you are a one-pot-a-day household, this method works really well.

The method for growing mushrooms on coffee grounds is really easy. Wait until your coffee grounds are cool enough that they are no longer steaming, but not quite cold and put one pot worth of grounds, including the filter, in a bag or jar. Nestle the stem butt (or a little sawdust spawn, or whatever spawn you are using) into the center of the coffee grounds. In about two days, the spawn will recover from the transfer and will have visible signs of growth, in the form of a white, fuzzy coating. From there, you can add more coffee grounds at the rate of about one pot a day. Again, the coffee grounds should still be warm, but not warm enough to burn your hand. You add coffee grounds as the mushroom grows. If you start to get too far ahead of the mushrooms, as evidenced by a lot of uncolonized grounds in your container, stop adding for a few days until the mycelium catches up. The mycelium should more or less colonize the grounds after they have been in there just a day or two. If it takes much more than that, contamination can become a problem.

Moisture is another issue. Often coffee grounds have residual liquid in them. Mycelium can’t really colonize substrate that is under water. If I am using a gallon Ziploc bag, I will just pour the liquid out as it accumulates. In a glass jar, however, the liquid can be used for another purpose. Glass jars have less air flow than a bag that can be fully opened. When the jar is full, you can get fresh air down to the mushrooms if there is a little liquid in the bottom by just turning it upside down. As the liquid travels through, the pores in the material will be filled with air, which naturally has to be drawn from other areas. Just open the jar to get a little fresh air in the top, then turn it over and let it sit a few hours. Then turn it over again. Just don't do this before the jar is full as it will disturb the mushroom too much.

Overall, the container should be opened once a day to give the growing mushrooms a source of air. Usually this is accomplished when you open it to add the coffee grounds for the day. You can also give the mushrooms air flow by using a canning jar and replacing the sealing portion of the lid with a coffee filter (unused) or a piece of fabric.

Just keep the jar in a cool, dry location while it is growing. Once the jar is full and the mycelium has fully colonized it, as evidenced by the fact that it is all cottony-white and no longer smells like coffee, it can be used as spawn to transfer to another medium or it can be just fruited. To fruit it, give it another week or so to grow, and then open up the jar. Put it out in the light, but don’t put it in direct sun. Put a plastic bag over it as a tent, but punch a few holes in it for air flow. Then spray it a couple of times a day. Personally, I know a lot of people like to try to force the process, but I like to let the mushroom tell me when it is time. When, in the process of your daily airings, you see primordia, tiny baby mushrooms that look like pinheads, you will know it is time to fruit the mushrooms. You should get two or maybe three good fruitings out of a jar and maybe more out of a bucket.

Once the medium is done fruiting, you can still use it as spawn to start another kit. You can mix it with more coffee grounds, or just compost it again and start with another stem butt.