Home Mushroom Cultivation – Inoculating the Substrate

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I think it’s important to note at the beginning that these “how to” posts are in no way conclusive and I’m an absolute beginner figuring things out. Indoor mushroom cultivation is, to my great surprise, a relatively new subject and many home level cultivators are performing ongoing experimentation and tweaking different methods. As such, a deep search into some obscure growers forum might lead to incredibly creative ideas. I do think, however, that I might be the first to cultivate mushrooms in a van.

Reflecting on the experience up until now, I think I get quite a lot of buzz from the fact that nothing is written in stone yet and it’s a bit of a Wild West alchemy with plenty of scope for improvement. There are many ways to incrementally scale up or to keep it low tech too. If I ever teach again, I would most certainly do a mushroom growing lessons, perhaps using toilet rolls.

As noted above, I am in the process of Research and Development and I’m using this platform as more of a diary/documentation so I apologise in advance if it’s not rhythmically pleasing to read and if I dwell too much on certain areas where my current interests lie. Each step is so fascinating and diverse that I could write an individual chapter of a book for each step.

Inoculating the Substrate

Once you’ve multiplied your spawn following the procedures from my previous post, or have bought enough spawn, you’re ready to inoculate the growing medium from which you’ll harvest the mushroom. What we’re essentially trying to do is to create a woody environment where mushroom mycelium can feed and grow happily without too many competitors.

Step 1: Pasteurise the substrate

In his book ‘Growing Gourmet and Medicinal Mushrooms’, Stamets’ recommendation for a substrate with maximum yield is a mixture of 100 saw dust, 50 wood chips, 40 oat, wheat or rice bran, 5 to 7 gypsum. However, with the introduction of grains which are nutrient dense and more prone to contamination, it becomes necessary to sterilise it with a pressure cooker and uses more fuel. Heat pasteurisation – a standard process in which the substrate is submerged in water and kept at 74 degrees for 1 to 2 hours – may use slightly less fuel but still adds to the cost and complexity. And if you’re making any more than a cooking pot full, it becomes a lengthy cumbersome process. Imagine sterilising an entire straw bale using a small pressure cooker with each session taking 90 minutes!


The straw began to turn darker brown as the liquid seeped itself into the cells of straw. About 10 to 12 days in at autumn conditions, the brownish liquid began bubbling and frothing indicating fermentation had properly kicked in.

The alternative method called ‘cold pasteurisation technique‘ or ‘fermentation method’ was I believe, first proposed by Stamets and requires no heat source in the process. The substrate, in my case a load of chopped up straw (without any grain), is dumped in a plastic bin, submerged in rain water with a bit of weight, and then let alone to do its thing for a week or two. I’ve chosen a black bin to collect the sun’s energy a bit better as it’s starting to get a bit chilly here. After two weeks, (some say 3 days, others say a week but probably temperature dependant) the aerobic microorganisms die out due to lack of oxygen and anaerobic organisms take full charge, making the liquid concoction very smelly. This liquid is then dumped and straw is aerated, killing off the anaerobic organisms. Thus, the rapid changes in condition, requiring nothing but a bit of scheduling, rain water and patience, kills off most of the micro-organisms. Absolute genius.

Or, simply ask around your local cafe for some freshly spent coffee grounds. I’ve never tried this but if I was in a city, I’d definitely do it this way. It makes absolutely no sense that millions of tons of nutrient rich seeds are transported halfway across the world only to be washed for a bit of flavour and then discarded to fill up the land fills. Apparently, as long as they are fresh from the day, they are good to inoculate directly. More details can be found here. If you brew beer or live near a brewery, spent malt is also a good one to grow mushrooms on. Already heat treated and ready to inoculate. This is definitely my favourite idea for future utilisation. Get some beer and mushrooms out of it before composting and enriching vegetables.

Update: I’m now working on a second batch where I’ve started the cold fermentation but will finish off by adding wood ash. Some minor websites touch on using ash for pasteurisation but no concrete data and method could be found… until I found an obscure but highly interesting PDF presentation for Namibian growers! Any how, the idea behind this process is to increase the pH of the substrate rapidly to 11 – a highly alkaline state – killing off much of the contaminants while alkaline tolerant mycelium can get a head start on the substrate. The doubling up of cold fermentation and ash treatment that I’m currently doing came as a result of:

a) Dealing with smelly straw from previous batch that didn’t seem to stop smelling despite aeration. I am slightly worried that anaerobic organisms are still alive and competing with oyster mushroom mycelium in the buckets. So ways of killing off those organisms in addition to aeration was needed.

b) If you have plenty of unchlorinated water on demand to play with, wood ash alone will be sufficient and the whole process can be done in 18 hours. You could probably get away with gassing the chlorine out by leaving tap water for a while too. However, it seems a bit silly to use up clean drinking water when a timely placement of a black bin under a gutter will do. It does mean that the schedule is weather dependent and I try and offset that by collecting the rain water when it rains, chucking the straw in when I can, then finish off the brew by throwing in some ash when I know I have some time the next day. Theoretically, it should increase the level of pasteurisation while giving much more freedom for me to manoeuvre.

I plan to use lime for this purpose during summer months but right now I have access to plenty of ash from a wood burner. In the previously mentioned document for Namibian growers, 4.24kg of sifted ash brought 150 litres of water to a pH of 11. It took 1.25kg of lime to reach the same pH. The straw was then left for 16 hours, put into 1.8kg plastic bags then inoculated with 120g of oyster mushroom spawn. Interestingly, the average yield was highest with wood ash, perhaps reflecting its higher carbon content or maybe increased micro-nutrients.

I’m going to scale that down by a half to fit my 80L black bin so 2.4kg of ash should do the job. I’ll report on the progress in my next post.

Step 2: Prepare the containers

Many growers use disposable single use plastic bags for convenience but it can be anything that can hold the substrate and have a few holes in them. I’ve seen pictures of people growing oyster mushrooms out of laundry baskets and milk cartons. If you’re going for a slightly longer term approach though, with minimum waste and maximum yield, you will probably want to source 20 to 40cm diameter containers with a lid. Anything from buckets to ice cream boxes, to pvc tubes can be used. Where I’m currently at in rural France, I had to buy it online but if you live in a city, you could easily source these containers from local cafes/restaurants/shops who are usually desperately trying to get rid of them.

Once you have some containers, drill 12 or so holes (preferably 10mm but can be bigger or smaller) around the circumference for every 10cm in height. This seems to be the average consensus according to my research, but again, I’ve seen laundry baskets with massive holes all over fruiting like mad so I don’t think it’s an exact science. However, it does need to keep the humidity of the substrate, protect it from excessive light, and allow the fruiting bodies to grow. My bucket is about 20cm in height so I’ve drilled 24 holes in offset pattern. Clean and wipe the containers with alcohol just before loading it with the pasteurised substrate.

Update: I have since realised that having a see-through container is extremely useful in detecting contamination early. It also keeps you motivated when you can watch your mycelium grow on a daily basis. If you’re starting out, I would highly recommend going for something in the vicinity of 10L see-through, stackable and cuboid like pictured below.

Step 3: Inoculate the substrate

Spread the substrate on a large clean surface wiped with alcohol, add spawn, mix well, then place in containers. Or add the straw and spawn in alternating layers. The recommended percentage seems to differ from source to source but most recommend 10 to 20% spawn to substrate. Larger the percentage of spawn, quicker the colonisation and less likely for contamination. Basically, if you’re doing it in a DIY environment like me, without properly sterilised substrate, your best bet is to increase the amount of spawn. Another subject for a deeper investigation in the coming months.


Update 1: I have learnt a few very important lessons during this batch:

a) Draining water from a well drenched straw takes a lot longer than expected. I did a clench test to try and get to the optimum water content for fungal growth called the ‘field capacity’. I’m not sure whether it’s water that’s seeped inside the straw or the surface tension retaining the liquid but it kept draining excessive smelly liquid for days after. I have now ordered a compost sieve on which I could leave the substrate to drain properly over night.

b) French bin bags are not air tight. Thinking it’s well drained, aerated enough and bin bags leak proof, I placed these sweeties above my friend’s bedroom for maximum ambient temperature. Little did I realise that it was not drained enough, anaerobic bacteria was still highly active and French bin bags were absolute shite. As a consequence, my friend’s family had to evacuate in the middle of the night because the liquid in question was dripping through the floor onto their bed… They’ve been unbelievably patient with my rather peculiar endeavour. Merci!

Update 2: Sieve works brilliantly in cutting the water content.

However, I used some old straw left outside for months with added oak leaves thinking that cold water fermentation coupled with ash pasteurisation will kill off excess bacteria and fungi. Not so. Always stick with clean fresh straw. All buckets have struggled to colonise this time

Step 4: Incubate

Keep at the recommended temperature and humidity for your strain until the container is fully colonised and white mat engulfs the substrate. Usually two to four weeks. Most mushrooms prefer temperature ranges between 20 and 24C at close to 100% humidity. You can achieve this by placing it close to a heat source and loosely covering the container with a bin bag, spraying inside the bag with water from time to time. The bin bags allow localised higher humidity so if done right, the buckets can be stacked anywhere indoors without having to deal with excessive moisture – just make sure you’ve killed off all anaerobic microorganisms, it’s drained well and bin bags are properly water proof like it bloody well should be.

Update: I would try and get double the amount of buckets so that one set has holes and the other set has no holes. The ones with no holes can be used to contain the main bucket, reducing the need for bin bags.


My multifaceted van in a mushroom grow room set up. It works well as I have an excuse to warm my van with a heater and keep at a comfortable living temperature while not having to worry about humidity leaking into the air. My hygrometer measuring humidity in my van has remained at normal levels of 40 to 60%. The bucket below was a precaution and no longer used. The smell had altogether disappeared after a few days but I’m still keen to get the previously mentioned ash processing to make sure it’s pleasant off the bat.


If you’re getting serious about home cultivation, or hyper sensitive to disposable single use plastic, a small garden green house about the size of a closet, like the one pictured above left, will work best to create that high humidity grow room conditions within your house/room.

My next hypothetical move towards mycological world domination is to get the plastic greenhouse like the one on the top right where I can contain the humidity, then cover all sides with straw bales to super-insulate the structure. My tiny oil heater will be hooked up and thermostat set to keep the temperature inside constant. Some areas will be sealed off with second hand computer fans providing air circulation during fruiting. The roof of this structure will be covered with a tarp that collects rain water into black bins that rests on casters. These black bins filled with rain water and fermenting straw will be moved outside during the day to collect solar heat, increasing microbial activity, then rolled back into the green house to add thermal mass during the night, thus reducing energy expenditure in colder months

During the summer months, I will be switching to summer fruiting varieties such as pink and yellow oysters that prefers hotter climate, but will also most likely modify the incubation chamber to draw air through an underground tube through convection. It’s an idea stolen from a standard earthship design that I think could work well here in principle. See the image below of a cross section of an Earthship for visual reference. The heated air rises out through the top, creating lower pressure within the chamber and draws in air through a tube that runs underground where temperatures remain at 12C through out the year, cooling the grow chamber well below outside temperatures without using any energy. I do think that to create sufficient suction of cold air through convection would require making the chamber as air tight as possible. Another fascinating subject for further research.

*It is also worth noting that I suspect (since no definitive information could be found online) that at the end of this incubation stage, and prior to fruiting induction, might be the best time to pinch a little bit of the substrate and inoculate the next batch. Some extended tests need to be done in the near future on the viability and strength of the mycelium as you allow it to continue running.

Greenhouse at the South facing side heats the air and flows out of the top creating a draw on the underground cooled air from North side of the house.  

So a few weeks into this project, some of the buckets are fully colonised with mycelial mat clearly visible from the holes, some not so fast but developing. I’ve made some god awful mistakes and learnt some good lessons. Now that I feel a little more confident in the process, I’ve ordered shiitake and lion’s mane spawn again to slowly diversify the varieties that I’ll be growing. They require more specific substrates and conditions than oysters so I’m pretty sure there’ll be future blog posts as I tweak the formula to fit them. In the next post, I’ll be writing up the most exciting part of mushroom cultivation: the fruiting, harvesting, cloning and eating of the oyster mushrooms!!

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