Home Mushroom Cultivation – How to Multiply Spawn

I can’t pinpoint exactly when I began finding the fungi kingdom fascinating but I do vividly remember finding gigantic mushrooms as a child and being mind blown by the beauty and the inherent mystery of it. And like most Japanese kids, my childhood fantasies were littered with images from Ghibli’s Nausicaa where gigantic fungal species dominate the land to cleanse the pollution left by the previous civilisation. In fact this idea of myco-remediation was re-introduced to me through “Mycelium Running” by Paul Stamets and my subsequent viewing of his appearance on a Joe Rogan podcast really re-ignited my interest as an adult.

But being taken foraging by a friend and finding big tasty mushrooms that I can eat have raised the world of mushrooms to a whole new level for me. I’ve been going for walks at every opportunity and picking giant porcinis (or ‘penny buns’ in the UK), chanterelles, field blewits and shaggy ink caps amongst a whole host of other fungal bloom that I’ve been obsessively trying to ID for the last month or so. I’d highly recommend checking several different sources when identifying wild mushrooms until you are 100% sure. If you are a novice forager like me, stick to these that are easy to identify and very few lookalike species that could poison you. Read up and get to know them REALLY well.

From trying to identify beautiful mushrooms and eating chunky, flavour rich porcinis and succulent, meaty oysters comes an obvious question: can I grow these myself? How easy is it? What materials will I need? How much would it cost to get it going? What’s the best way to cultivate it? As a slightly obsessive research geek with an interest in learning how to live sustainably, this prospect of cultivating the world’s tastiest decomposers excited me. I’ve been deep diving and compiling information on everything to do with mushroom home cultivation for the last month. The following information that I’ve laid out is based on analysing hundreds of sources and gathering the best ideas that suit my context of home cultivation.  Now, it’s important to note that most of it is purely based on research and the actual tests to see the viability of it will follow in the next month or two. However, I thought if there are people interested in the subject, it might save you the research time.

Context: I’m staying at a friends house in South West France, in my van. It can get really cold in the winter, and very dry and hot in the summer. I really can’t spend much money and I definitely don’t want it to take up much space either. I have access to a hob and some straw bales. Cheap and easy are the key component to this experiment. There is a possibility that I may increase production for sales at a local market as a side income if all goes well. Hence, I’ve picked suitable processes that could be expanded in scale later if I wanted to.

I’ve tried to eliminate the use of single use plastics – generally favoured by growers, and chosen methods that uses the least amount of fossil fuel as possible.

Equipment needed: essential

  • Jars (jam jars work best)
  • Cotton wool
  • Tin foil
  • A large cooking pot with a lid
  • Nail/hammer
  • Some form of grain (I currently use wheat grain for animal feed as it’s cheap)

nice if you have it

  • Pressure cooker

Multiplying a small amount of spawn.

Usually a bag of spawn, grown in grains or other medium, is purchased online and delivered. You can purchase a larger bag of, say, a kilogram and inoculate ten times the amount of substrate (the material that you will eventually harvest the mushrooms from) directly. In my case, I opted to buy a tiny amount (25 grams) and tried increasing the amount of the spawn to cut costs. This adds a little more complexity to the process and higher rate of failure but if I was to gradually scale this up or continue long term, being able to multiply the spawn could bring on enormous differences in expenditure. However, if you’re not that into it, wait a week or two for the Part 2 on “Inoculating the Substrate”.

Note the cotton wool poking out of the jars. It’s amazing how you can process the spawn medium in a relatively unsanitary and amateurish environment such as this outdoor kitchen and still succeed with a vigorous strain such as pleurotus ostreatus.

Step 1: Soak the grains for 24 hours. Any grains will do but some people swear by rye. In my experiment with using brown rice, roasted buckwheat, wheat and straw, winter oysters grew particularly well on wheat. Drain well until surface is dry to touch and place 3/4 full in a jar. Pierce a hole in the lid with a big nail and stuff it with a bit of cotton wool. This will allow air to circulate but prevent contaminants from entering the jar. Cover the top with tin foil to avoid water going into it in the next step. I would also add from my recent experience that the exposed metal where you’ve pierced with a nail will be best painted to prevent any rust formation.

  • Update 1: Tin foil is not necessary. The cotton wool gets wet anyway and dries quickly.
  • Update 2: Fill dried grains to about half way in your jar. As it rehydrates, it expands by about 50 percent, leaving 25% of the jar still open to fill with spawn and be able to shake to distribute the grains.

Step 2: Because grains are so nutrient rich and prone to contamination by mould etc, the standard procedure for prepping it is to sterilise it in a pressure cooker at 15 psi for 90 minutes. The increased pressure allows higher temperatures of about 121C which kills most endospores present in the grains. Unfortunately, I don’t have a pressure cooker and ones that have a gauge cost quite a bit of money. But thankfully there is an age-old process called ‘fractional sterilisation’, which does use more fossil fuel but can be useful if you’re experimenting like me and don’t particularly want to invest in a pressure cooker yet.

1. Put the grain jars inside a large pot with some water. Put a lid on, bring it to a boil and let it simmer on low for an hour. This kills some of the germs present in the grains. The length of time allows the heat to penetrate to the centre of the jar.

2. Leave for 24 hours at room temperature. This sprouts other endospores that are harder to eliminate.

3. Boil for another hour. This process kills the ones that have sprouted and in the process became more vulnerable to heat.

4. These two boils might have sufficiently sterilised the grains or you could add 1 or 2 times more boil to be sure. I did 4 boils while I waited for the spawn to arrive which was later than I expected.

Update: Fractional sterilisation dries the grains significantly. The extended exposure to heat evaporates the water absorbed into the grains, eventually pushing it out of the jars which does not get replenished when the jars cool down. I suspect that lack of moisture was the reason why the shiitake mycelium pictured below stalled, leaving open an opportunity for a yellow contaminant (slime mould?) to grow. I have now figured out that replenishing about 10ml of tap water before the final boil brings up the moisture content back to optimum.

Note the winter oyster mushroom spawn on the left is completely colonised with mycelium, having spent nearly two weeks in transit. The lion’s mane spawn to the right, however, has only a few grains here and there that are colonised and looks rather weak at the time of delivery.

Step 3: Once the sterilisation is complete, allow the jars to cool to room temperature. Wash yourself, put clean clothes on, and find a clean environment to move the bought spawn (or spore/mycelium syringe) into the jars. Make sure anything that comes into contact with the spawn is sterilised. Swiftly shift 1 to 2 desert spoon of spawn from the bag into the jars. Shake the jar vigorously to even out the distribution.

The straw on the left was the hardest to shake and even out the spawn, hence the gap at the bottom as it colonised from the top down. The larger gap between the strands of straw also meant that the mycelium had to reach further for the next source of food and possibly slowed the process. The ones munching on wheat in the middle and buckwheat to the right appears to be far happier.

Step 4: Place the jar on a shaded shelf at 20 to 24 degrees. The jars should be fully colonised with white strands of mycelium within a week. If you see bluish mould appearing, chuck it or compost. Once completed, you can use the contents to inoculate more jars or store in the fridge until needed. I don’t quite understand the mechanics of it but experts advises us to limit multiplication of spawn to three times as the strain will start to lose vigour.


All of the oyster mushroom jars were successfully colonised without any contamination using this process. I will try boiling the grains only twice next time to see if it will suffice. The lion’s mane jars have not changed at all in little over a week. No sign of contamination but no sign of life either. I suspect the spawn had suffered too much in high temperatures during transit and inside the post box under direct sun light. I was a bit disappointed as lion’s mane was the main mushroom I wanted to cultivate and oysters were a back up.

However, I am relieved to know that I don’t need a super sterile environment or specialist equipment to cultivate mycelium. I was half expecting all of the jars to have mould growth but every single one came out without a sign of contamination. It’s a process that can be done successfully on a shoe string budget, in your back yard with standard household items. Finally, I’d like to end with this link to a letter from a grower in war torn Syria. No food supply, stuck in a basement and malnourished, he started to grow mushrooms and spent two years cultivating and teaching 1000 men and women to grow their own. An amazing accomplishment and a reminder for us that we never know when these skills come in handy.

Check out the links below for follow ups

Home Mushroom Cultivation – Inoculating the Substrate

Home Mushroom Cultivation – Growth/Harvest

If you’re into mushrooms, I highly recommend this film featuring Paul Stamets.

2 Comments Add yours

  1. Like!! Really appreciate you sharing this blog post.Really thank you! Keep writing.


    1. cshimasaki says:

      Thanks, will do!


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