How I prepare bulk substrate and pack buckets for cultivating oyster mushrooms

Published on Apr 30, 2022

It takes about four to five hours for me to prepare my bulk substrate, which is made from straw and hardwood sawdust (I use oak) and I add a little bit of wheat bran for extra nutrition for the mushrooms.

bags of straw
Straw, before being chopped

You might ask why I have to shred the straw. The main reason why I do it is to reduce the size of the pieces in order for the mycelium to be able to colonise the substrate more effectively and to make an evenly mixed substrate. If I don't shred it, the wheat bran and sawdust won't mix well with the straw.

I use a cheap line trimmer (we call them Weed Eaters in South Africa) and a large refuse bin to shred the straw first, then afterwards I mix in the sawdust and bran.

A bag with chunks of sawdust
Premium oak sawdust
shredded straw in a refuse bin
The first shred run

I add straw incrementally to the bin, in order to make sure I shred it all thoroughly. The trimmer's line breaks often, and I think thicker line might work better. It is quite time-consuming to have to open the thing up and pull the line out every time.

After filling the bin up halfway with shredded straw, I fill the rest of the space with the sawdust. It has to be hardwood, oyster mushrooms do not enjoy eating softwoods. I then add about 10% of the total volume wheat bran. Too much and you risk contamination.

a curious German Shepherd doh
Obligatory dog photo #1

At this stage, the substrate is dry and not heavy to carry around. I close the lid and roll the bin around to mix everything up. Last time I did this with my hands and I got itchy arms.

When it's all mixed up good, I move the dry substrate into my large while polypropylene bags, which have small cuts in their bottoms to allow water to penetrate and drain.

a white bag with bulk mushroom substrate
Mixed dry substrate in the bag

While doing all of this shredding and mixing, I started filling my 200L drum with water, add the appropriate amount of hydrated lime and some gypsum and stir. The bags then get tied with cable ties and dunked into the lime water.

a 200L drum filled with water and a weight to keep substrate bags submerged

To keep the bags submerged, I use a plastic crate with some bricks inside. It works and it's easy to handle for someone like me who doesn't have a lot of muscle. I close the lid and forget about it for 24 hours.

Substrate bags draining

I don't have a clean space to hang them to dry, so I leave them in the drum suspended above the bottom with some old planks and wire. Doesn't look too professional, but it works! I let the bags drain for about an hour and a half, then I lift them out.

german shepherd trying to catch hose pipe water
Obligatory dog photo #2

This is the part where I need to start wearing gloves and spray everything the substrate touches with 70% isopropyl alcohol (IPA). Don't want contamination to ruin all this hard work. I also wear a dust mask to prevent me from breathing my bacteria all over the substrate. I hate masks...

I dump the contents of the bags in a sanitised 110L tote and close the lid.

Mushroom grain spawn ready to be opened and added to substrate
My tools are ready
3M micropore tape
The bucket holes need to be taped with micropore tape to keep organisms out and allow the mycelium to breathe

I forgot to tape up the bucket holes before starting, so I ran to fetch my micropore tape and proceeded to tape up. It's not necessary to sanitise at this point just yet.

That bag of white stuff is the grain spawn I bought from a local supplier and it was starting to become a bit out of control in there, so the time for making buckets is ripe. I banged up the bag to break apart the chunks of myceliated grain. Don't worry, this hard-handedness won't damage it! I spray the top side of the bag with the IPA as well as the scissors. Everything has bacteria on it, invisible to the naked eye. I also spray my hands and make sure I wipe everything down.

Mask up, hands sanitised, I lift the lid off the tote and pour out the bag of spawn.

chunks of grain spawn on bulk substrate
Look at them gorgeous chunks of myceliated sorghum grains!

Those bing chunks needed to be broken up further, so I did that and proceeded to mix everything with my hands. I have to find a better way to do this...

I take one bucket at a time to minimise contamination, spray it down with IPA first, then filling it with substate. You can't really see the sorghum grains anymore, but they're definitely there. I also spray the lid before putting it on.

After about 15 minutes, I am done. These buckets get labelled with a date and the type of mycelium it contains and I move them to the incubation tent, which is basically just a mylar grow tent. I had to connect two PC fans to the tent, in order to have air intake and extraction. I previously had issues with fungus gnats in buckets in there. I cleaned the tent with bleach before making last week's buckets to reduce risk of contamination and more damned fungus gnats.

Fifteen buckets filled with inoculated substrate in an incubation tent

I have so many buckets now, but some of them I am going to sell at a market next weekend. You'll see some white stuff inside the buckets. That is not mould, it's the mycelium running through the substrate, eating it and getting ready to fruit.

Myceliated plastic bucket
Gorgeous fluffy mycelium
A bucket that is almost fully colonised with white mycelium
That discolouration is simply excess moisture

What I've done differently with these is the substrate recipe. I hope to see bigger and more mushrooms from each yield.

These buckets need to incubate in the dark for about 14 days, before I move them to the fruiting tent.

I had a bag of grain spawn I made myself in the incubation tent, but it picked up some mould somehow. It's a pity I don't have the necessary funds just yet to invest in a proper laminar flow hood to make my own spawn :(

Mushroom grain spawn overrun by mould
Damnit, I hate mould. To the garbage.

It's a shame I had to throw this bag away. This is why I rather buy grain spawn until I can afford a laminar flow hood!