Growing mushrooms indoors: The grow mix

The types of mushrooms we grow mostly decompose hardwood trees in the wilderness, such as oak, maple, and beech trees for example. We try to replicate that environment indoors with our grow mixes, known as substrate.

This is currently a manual process for us. Our indoor workspace is a homemade wooden hopper full of hardwood pellets. A scale sits below it where we constantly weigh the mix as we add the ingredients for the ideal mushroom’s meal. Buckets on buckets of soy hulls or wheat bran line up to the right. A cutting board holding clear bags sits atop the buckets not in use, depending on the mix we’re making. 

We have two types of growing mix for our mushrooms – the supplemented sawdust mix and the Masters mix. Both contain hardwood pellets for starters and at least 1.75 kilograms of water added at the end.

The supplemented sawdust mix has wheat bran and a smidge of gypsum powder. Wheat bran is an effective by-product when milling. The outer part of the wheat grain gives dietary fibre and carbon to the mushrooms. The gypsum powder provides calcium and helps the mycelium build a network throughout the block to potentially grow more mushrooms. 

Above: Each layer of dry ingredients stack up in the bag. The pellets absorb the water, and we shake the bag to mix the ingredients together.

The Masters mix contains soy hull pellets instead of wheat bran, but the idea is similar. They contain the right amount of nutrients to boost the first yields. Most of our Oyster mushrooms grow very quickly and need the high protein content from the soy hull pellets to keep up with them.

Regardless the type of mix, they both go into polypropylene bags. They are clear, tough plastic bags that mimic the bark of a tree. The bags withstand the weight of the block, all the shaking to mix the ingredients, and the heat from the next step.

Once a batch of blocks are made – about 84 bags and three to four hours later – they are ready to go into our sterilizers. The least bit of contamination can ruin even the best blocks, so we must disinfect the ingredients for 18 hours. Water heats up at the bottom of the barrels, which steams the blocks and kills any contaminants.

When we explain the spawn-adding process, it will become more important why these blocks must be as sterile as can be.