Mushroom Growing Substrate

Mushroom growing substrate

Make the Most of Growing Mushrooms with Substrates

Just judging by the propensity for mushrooms to spring up in the garden or the yard, one might suppose it is very easy to grow the fungi. Truth be told, conditions must be exactly right for this to happen. While it might seem as though a recent rain produces mushrooms, it really is more a matter of the combination of an existing substrate, shade, moisture and the perfect temperature that results in mushroom growth. Rather than take a chance on which randomly growing mushrooms are correct for feasting, it is better to create your own controlled conditions so you know you won’t end up in gastrointestinal distress, and it all starts with the right substrate.

A Substrate is the Base Upon Which an Organism Lives

I offer this advice to new growers: Growing mushrooms and growing plants are two very different things. While you need a substrate upon which your mushroom mycelium can grow, it is not the same as soil is to plants. You must create the right environment for a species of mushroom to fruit. Even at a hobby level, your effort is that of a cultivator.

There Are Varieties of Substrates

Let’s start with the different types of substrates you can use to grow mushrooms:

  • Straw – cheap but efficient, commonly used to grow oysters; tends to be messy though
  • Manure – composted manure is often used for growing button mushrooms, if it is properly prepared
  • Coco Coir – commercially comprised of a mix of ground coconut husks and shells and is reasonably nutritious. When mixed with vermiculite, it is quasi-hydroponic and very effective.
  • Soy Hulls – often mixed with the sawdust from hardwoods at varying proportions for effectively high yields
  • Hardwood Sawdust – certain hardwoods such as maple, oak or beech are excellent substrates especially when supplemented with bran
  • Coffee Grounds – often suggested as a common substrate, high in nitrogen, however susceptible to contamination
  • General Agricultural Waste – there are numerous agricultural waste products that will serve as a fruitful substrate

The more you experiment with, the more you will discover how you can make the most of materials that you might otherwise throw in the trash.

What Constitutes a Good Substrate?

Mushrooms must have a suitable surface upon which to grow and fruit. The flush is the harvest, which can repeat within the same substrate depending upon the type of mushrooms and substrate. A good substrate is the ecological environment you create that your mushroom mycelium can use to gain the energy from sufficient nutrition to grow.

All substrates need to be prepared using water, possibly adding nutrients and applying a process of either sterilization and pasteurization or both. Once the substrate is prepared, it is then inoculated, the step by which you implant pieces of mycelium. The implant consists of crumbled pieces of grain-coated mycelium, also referred to as spawn. For beginners, hobbyists and other growers, you can purchase spawn bags in grow kits. There are different steps to follow for these kits found on the packaging instructions.

The next step is to be patient. If all the conditions are right, the mycelium will start to take off as they are working on decomposing the organic material. This is called colonization. Colonization must be thorough before the mushrooms are ready to fruit. In other words, the substrate becomes an amalgamate of the mycelium devouring all the good stuff in the substrate. At this stage, the mycelium has encompassed the entire substrate.

Fungi Facts and Formation

The way fungi grow is the opposite of what happens when plants grow. Where plants thrive on carbon dioxide producing carbon and oxygen, mycelium goes through a process of reverse photosynthesis, taking in carbon and consuming oxygen. This is exactly why carbon-rich materials such as wood that slowly breaks down are a preferred material as a substrate. This is also why you often see mushrooms growing out of trees and why many growers use logs to inoculate and grow culinary mushrooms.

Crucial to their growth is the decomposition cycle. Similarly, the decomposition of organic materials is what makes compost so valuable to growing plants in the soil. Your exhausted substrates can be added to your compost as a powerful nutritional supplement to use in cultivating your soil for planting. The value of mushrooms is in their biological alliance for the health of the planet as well as for us when we eat them.

Moisture is a Must

In addition to nutritional value, the ideal substrate must be moist. Moisture, however, also attracts bacteria and mold, which grow at a faster rate than the mushroom mycelium. You must take another step to assist in allowing the mushrooms to gain an advantage in the race to completely colonize the substrate. This step is the process of cleaning or sterilizing your substrate material.


Sterilizing might compare to the autoclave used to clean surgical instruments. This is a process of heating the substrate material to temperatures in excess of 250 degrees under pressure. The goal is to completely eliminate all possible contaminates that would otherwise be dormant and just waiting to be activated unless they are killed off under the extreme heat.

More nutritious substrates that have a tendency to attract mold growth are those that are slow to break down such as hardwood sawdust. Without sterilization, the mushrooms rarely have a fighting chance to outpace the mold growth in these substrates.

The ideal treatment for supplemented hardwood sawdust fruiting blocks is to pressure sterilize them at 15 PSI for 2 ½ hours. In this process, the inoculation must also occur under sterile conditions remaining sterile until colonization has taken place.


Another method of using heat to reduce the contaminates is pasteurization. This is a process of heating the bulk materials with steam or hot water reaching between 150 to 185 degrees. This can be done in a steam bath or hot water bath, and it helps give the mushroom culture a head start.

Pasteurization is useful when the substrate material is sufficiently nutritious for the mushrooms to thrive but not so nutrient-rich that it would otherwise need to be fully sterilized. Non-supplemented straw is a good candidate for pasteurization.

watering soil

Maintaining Water in the Substrate

It is important to understand you do not “water” your substrates like you would water your plants. While you do not water your mushrooms during the growing cycle, they must maintain from 70 to 0 percent water, which is drawn up from the substrate.

An appropriate amount of water must be added to the mix before the sterilization process. In pasteurization, the straw can be soaked in hot water. This is aside from using humidity to keep the substrate from drying out.

If the mix is too dry, the growth is very slow jeopardizing the colonization process. If it is too wet, it can encourage bacterial growth despite your efforts to clean the substrate first. Over saturation will also thwart proper colonization.

Mushroom Species and Their Preferred Substrates

Yes, it is true that mushrooms have a preference for different substrates. This is where you begin to appreciate your role as a cultivator. You will discover the flexibility you have with these combinations as you gain experience with them.

Culinary mushrooms grow well on straw. This would include the varieties of Oysters such as Blue, Pearl, Yellow and Pink Oysters. While you can grow King Oysters on straw, plan to use hardwood sawdust for a much higher yield with a more impressive appearance.

Medicinal and gourmet mushrooms are typically grown on supplements hardwood sawdust, which includes Shiitake, Maitake, Reishi and Lions Mane.

All the species mentioned here will grow well with a standardized fruiting block recipe. While supplementation supercharges your yield, too much can create more contamination than you would otherwise experience. You will learn what is the good measure with practice to avoid diminishing returns.

It is further helpful to know that different substrates may have better results depending upon your geo-location, grow style and species of mushroom you choose to grow. Keep in mind that you often get an extra flush or two out of your substrate depending on conditions. If you compost your substrate, don’t be surprised to see a further flush as it is decomposing in the pile. You can also break up spent blocks, re-sterilize them and reuse the organic matter to grow more mushrooms.

Your best practice is to experiment with different substrates and preparation methods and different supplementation to see the results you get. Growing mushrooms is a fascinating process that provides you with the satisfaction of growing your own harvest that you would otherwise pay too much for at the market as well as experiencing the enjoyment of the great-tasting items you could not buy at any cost.

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