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Sawdust & Woodchip Substrates

In this section, you’ll learn how to mix up a batch of sawdust and woodchips for growing mushrooms. Sawdust alone can be used for many species, but I’ve found better overall fruiting and performance when woodchips are added. Don’t use large woodchips except in outdoor beds. Smaller chips however, will help to hold your substrate together, and give it more ‘staying power’ over several flushes. Sawdust alone in my experience is more of a one flush wonder.

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Getting the moisture content right with sawdust-woodchip substrate is extremely important, because we’ll be colonizing in sealed, filter patch bags. Any excess moisture will drain by gravity to the bottom of the bag, drowning any mycelium that tries to grow there. Such often results in bacteria or mold contamination. If you're going to err on water content, err on the side of too dry, rather than too wet. I recommend to soak the fully colonized substrate blocks prior to first flush anyway, so they can be 'pumped up' with moisture for fruiting at that time.

The recipe I prefer uses roughly twice as much sawdust by volume as woodchips. We’ll be adding gypsum, and wheat, rice or oat bran to the sawdust before hydrating with water. The woodchips will be soaked in a separate container overnight to soak up moisture, or they can be boiled for an hour to achieve the same results if you’re making a small batch.

The amount of water needed to hydrate the sawdust is directly related to how dry it is to start. When using fresh sawdust, you’ll use half or less the amount of water you’ll use if your sawdust has been aged for a year or more under very dry conditions. I prefer using aged sawdust, because it tends to hold water better. You’ll know when you have the moisture content right because when you take a handful and squeeze it very hard, no water drips out. However, if you added any water, it would drip when squeezed hard. Each batch will require some trial and error. A simple correction if you get it too wet, is to add dry sawdust until it’s just right. Always let the sawdust rest for at least fifteen minutes after mixing, and then check the moisture content again. Often, what seems too wet at first will be just right after sitting for a few minutes. Also, check the bottom of the container you mixed it in. If there's any standing water at all down there, it's too wet.

When the sawdust is at the right moisture content and the woodchips have been soaked and drained, it’s time to load your substrate bags. Since I use twice as much sawdust as woodchips, I simply mix it in the bag as I load-one handful of woodchips, followed by two handfuls of sawdust. Just eyeball it, don't waste time trying to get the ratio exactly right, because it isn't that critical.

When the bags are loaded, clean off the entire inside of the bag above the substrate level of any sawdust that is stuck there. Then, insert a tyvek sleeve into the top part of the bag, and fold it up inside. The tyvek allows the steam to escape the bag while sterilizing, but then as it cools and the substrate contracts, a vacuum forms, making the only path for air to enter through the tyvek, which filters it. The tyvek is then removed under sterile conditions at inoculation time.

Enjoy the sample video clip!

Marc R Keith