Understanding Mushroom Substrates

Finding the perfect mushroom substrate for your grow- and getting the right substrate recipe– is one of the most important aspects of mushroom cultivation.

New growers will quickly realize that growing mushrooms is significantly different than growing plants.

Although the concept of a mushroom substrate can be thought of as somewhat analogous to the soil for a plant, the way that mushrooms actually use a substrate is different, and needs to be fully understood by the cultivator in order to achieve the best results.

Common Mushroom Substrates

What is a Mushroom Substrate?

To put it simply, a substrate is a bulk material that the mushroom mycelium can use for energy and nutrition. A good substrate is required in order for the mushroom to grow and to fruit.

Common substrate recipes for growing mushrooms usually involve straw or hardwood sawdust, although there are many other good options to be considered.

The mushroom substrate needs to be properly prepared by adding water, potentially adding additional nutrition, and further processed by sterilization or pasteurization. When ready, it can be “Innoculated”, wherein broken up pieces of mycelium-covered grain, also known as grain spawn, are evenly mixed into the substrate.

Under the right conditions, the mycelium will start to grow, rapidly devouring and decomposing the organic material in a process known as “colonization.”

The mushrooms are ready to fruit once the substrate is fully colonized- meaning it has been completely consolidated and encompassed by the mycelium.

Pink Oysters Pinning on a Fully Colonized Straw Based Substrate.

Pasteurizing or Sterilizing the Substrate

Ideal mushroom substrates are moist and full of nutrition. Unfortunately, there are many other things that also thrive in these conditions, such as molds and bacteria. These contaminants can grow much faster than the mushroom mycelium, so something needs to be done in order to give the mushroom an advantage in the race to colonize the substrate.

This can be accomplished by the process of sterilization or pasteurization.

Pasteurization is the process of heating up a substrate between 150-180 deg F for a period of 1.5-2 hours. It doesn’t remove all the contaminants, but it will reduce the overall population of other microbes to a level that gives the mushroom species a head start.

Some microorganisms will survive pasteurization and remain in the substrate in some capacity, but can actually helpful for the growth of the mushroom in the long run.

PASTEURIZATION:
The process of heating a bulk substrate between 150 – 185 deg F with steam or in a hot water bath in order to reduce the overall level of contaminates and give the mushroom culture a head start.

STERILIZATION;
The process of heating the substrate to extreme temperatures exceeding 250 deg F, under pressure, in an attempt to completely eliminate any living or dormant contaminants within the substrate.

So what kind of substrates are pasteurized?

One perfect examples is non-supplemented straw, a mushroom substrate which is nutritious enough for the mushrooms to grow, but not so rich in nutrients that it needs to be fully sterilized.

Pasteurization can be achieved either by subjecting the straw to steam injection or by soaking the straw in a hot water bath. Both methods serve to pasteurize and moisten the straw. After it is cooled, the straw can be inoculated with the mushroom culture.

Substrates which are more nutritious, such as supplemented hardwood sawdust, need to be sterilized rather than pasteurized. If not sterilized, the substrate will almost always contaminate with molds, long before the mushrooms have a chance to fully colonize.

This is usually due to the addition of overly nutritious bran, or other form of supplementation, which allows the molds to quickly claim victory over the substrate.

Sterilization is the attempt to completely eliminate all forms of contamination. This is done by heating the substrate under pressure for an extended period of time. For supplemented sawdust fruiting blocks, typical sterilization protocol is to pressure sterilize at 15 PSI for 2.5 hours.

This brings the temperature of the substrate up to 250 deg F, effectively killing off any potential contaminants.

The substrate then needs to be inoculated under completely sterile conditions, and remain sterile until the mushroom is able fully take hold.

Supplemented hardwood sawdust fruiting block after sterilization and before innoculation.

Water in the Substrate

Mushroom fruiting bodies are 70-90% water, and all of that water is drawn up from the substrate. Unlike plants, mushrooms are not “watered” during the growing cycle.

Of course, humidity needs to be added to the air so that the substrate doesn’t dry out, but all of the water that mushrooms require to grow is added to the substrate before inoculation.

For this reason, the substrate needs to be properly hydrated during preparation. For straw, this typically occurs during the pasteurization process, when the straw is soaked in hot water. For supplemented sawdust fruiting blocks, an appropriate amount of water should be mixed into the sawdust before the sterilization process.

If your substrate is too dry, growth will be slow, and the mushrooms may fail to overtake the substrate at all. On the other hand, if the substrate is too wet, it may encourage bacterial growth and inhibit full colonization. Getting just the right amount of water in your substrates definitely takes some practice.

Different Substrates for Different Mushrooms

Different species of mushrooms have different preferences of substrates, although there is definitely some flexibility in what you can use successfully.

Most varieties of oyster mushrooms grow well on straw. This includes Blue Oysters, Pearl Oysters, Yellow Oysters and Pink Oysters. Even King Oysters will grow well on straw, although you can expect a higher yield, and better looking mushrooms if you grow Kings on sawdust instead.

Most other gourmet and medicinal mushrooms grow best on supplemented hardwood sawdust. This includes Reishi, Lions Mane, Maitake and Shiitake.

Oysters will also grow well on hardwood sawdust. A standardized fruiting block recipe will work well for all the species listed here, but it is definitely worth playing around with moisture content and supplementation level to see if slight variations make a difference.

GROW ON STRAW:

• Blue Oysters
• Yellow Oysters
• Pink Oysters
• Pearl Oysters

GROW ON SAWDUST:

• Reishi
• Lions Mane
• Shiitake
• King Oysters

Using Straw as a Substrate

Straw is a cheap and extremely effective substrate that works well for many types of mushroom species. Straw is the stem portion of grains such as wheat, barley or oats – it’s basically what is left over after the grain itself has been removed.

In many ways, straw is an agricultural waste product.

If you live near a farming community, you should have no problem finding straw. A 40lb (dry weight) square bail should cost no more than $2-4. Keep in mind that straw is messy to work with – it needs to be chopped, cleaned and pasteurized before being used as a substrate. It works best for medium to large grow operations which have a separate prep area and a large drum for pasteurization.

A 40lb square bail of straw, compressed.

That being said, you can still use straw for hobby scale indoor grows. A smaller amount of straw can be chopped with garden shears and pasteurized by pouring hot water into a tote- instead of using a large drum and a gas burner.

If you are planning to do small scale grows using straw, you might want to consider using EZ Straw. This is especially convenient if you don’t live anywhere near a farming community. They sell coarsely cut straw in a convenient bag, already pre-cut and cleaned. Using something like EZ Straw will prevent a lot of the mess and hassle of dealing with large bales of straw.

Using Hardwood Sawdust and Chips

Hardwood sawdust makes for an excellent mushroom substrate, and is commonly used in industry. It is a waste product of the lumber industry, and might be cheap and easy to find, depending on your location.

The fine sawdust is typically mixed with wood chips, which seems to allow for faster colonization of the substrate and provide a better structure for the mycelium to take hold.

Commonly used hardwoods are oak, maple, beech, and hickory- or a mix of multiple species. Softwoods such as spruce, pine and fir are not suitable for mushroom cultivation.

If you are in an area where you don’t have easy access to hardwood sawdust, you can always use hardwood pellets. These are typically used for wood stoves- but will also work well for growing mushrooms! The pellets need to be soaked in water before being broken up into fine sawdust\. Soaking in water will also serve to hydrate the substrate.

You can usually find the pellets inexpensively, although they may only available seasonally in most retail stores. There is a large selection of them available here.

They usually come in large 40lb bags, which should be enough sawdust for most small scale hobby grows. Again, make sure the pellets are hardwood and not softwood. You also want to make sure that the pellets don’t contain a lot of glues, paints or other additives.

If you are growing mushrooms using sawdust pellets, the following substrate recipe works well for many types of gourmet mushrooms:

For every 5 lb fruiting block:

• 5 cups of hardwood pellets
• 1.4 liters water
• 1-1/4 cups wheat bran

Check out our article on growing mushrooms on supplemented sawdust fruiting blocks for more information.

Supplementation

Mushroom substrates can supercharged by adding an easily consumed and highly nutritious supplement that the mushroom mycelium can use to grow faster and stronger.

Supplementation will also produce larger yields! Be careful though, as too much supplementation can cause higher rates of contamination.

Typically, supplementation is achieved by adding bran (oat bran or wheat bran) at a ratio of 5-10% dry weight. Substrates that are supplemented absolutely need to be fully sterilized or else contaminants will quickly take over.

Even after full sterilization, the chance of contamination goes up linearly with the amount of supplementation- and eventually you reach a point of diminishing returns.

Interestingly, Shiitake mushrooms require lower levels of supplementation in order to produce the best results. Over-supplemented shiitake mushrooms can produce strange and unsellable mutated looking fruiting bodies.

Shiitake Naturally Growing on Hardwood Logs.

The Masters Mix

If you’re looking for an extremely effective substrate for growing oysters, you might want to try what’s know as the Master’s Mix – a 50/50 mix of sawdust and soyhulls, sterilized at 15 PSI for 2.5 hours.

The original “master” behind this mix is T.R. Davis from Earth Angel Mushrooms. You will not likely find any another substrate that will yield as much on the first flush as this mix.

There are two downsides to using soy hulls as far as I can tell. Firstly, the second flush will not be anywhere nearly as strong as the first flush, even though the huge first flush definitely makes up for it.

Secondly, the mix doesn’t seem to colonize as fast as the more traditional supplemented sawdust formula. It’s possible that this is only in my experience, so your miles may vary.

Growing Mushrooms on Manure

Most people think that mushrooms grow on manure- which is indeed true. Although most gourmet species prefer hardwood sawdust or straw, there are many common species that grow best on manure.

Agaricus species, such as the common button mushroom and the portobella mushroom, as well as many Psilocybin species, are good examples.

In commercial Agaricus bisporus cultivation, the manure substrate first goes through a multiday composting phase, where large piles of compost (manure and straw) naturally heat up to 160 deg F, and beneficial organisms proliferate.

Once this process is complete, the compost is properly pasteurized to remove all unwanted contaminants. This secondary pasteurization process also removes ammonia that naturally builds up during the first phase compost. Check out this neat video below that sums up the process of preparing compost for button mushroom cultivation:

Growing mushrooms on compost in this manner is usually not pursued by the home grower, though it is certainly possible.

Growing Mushrooms on Coco-Coir

Another common material used to grow mushrooms at home is coco-coir. Coco-coir is a natural material made from the husk and the shell of coconuts.

It is commonly used for quasi-hydroponic plant growing systems. It does a good job of retaining water, and although it doesn’t provide many nutrients for plants, it is reasonably nutritious enough for growing mushrooms, although not so much so that contaminates easily proliferate.

Coco coir is often mixed in a 1:1 ratio with vermiculite to form a suitable mushroom substrate. Vermiculite is an expanded mineral that provides structure to the substrate and retains water, though it is nutritionally inert. The coco-coir vermiculite mix still needs to be properly pasteurized before being inoculated with mushroom grain spawn.

You can buy compressed blocks of coco-coir on Amazon, and can also usually find it at your local garden center. The coco-coir will expand many times it’s size once it is broken apart and soaked with water.

Growing Mushrooms on Coffee

Although it is possible to use nothing but spent coffee grounds as a mushroom growing substrate, it is usually not the best choice. Coffee grounds are high in nitrogen, which provides plenty of energy for the mycelium and helps produce higher yields. This richness of the straight coffee substrates however, make for higher chances of contamination.

Coffee grounds are better used as a supplement added to other substrate materials, such as hardwood sawdust or coco-coir. Coffee grounds should be fully sterilized before being added to the substrate, but you can often get away with just pasteurization.

Uses for Spent Mushroom Substrate

If you start to grow a lot of mushrooms, you might soon end up with large piles of spent substrate that you don’t know what to do with. The shear volume of substrate can add up really quick- so it’s good to have some idea of what to do with it all.

The most obvious choice, if you have a reasonably small amount of spent substrate, is to compost it. Just leave your substrate in a pile outside and let the natural process of decomposition take hold. Chances are good that you might even get an extra flush or two of mushrooms in your compost pile! It doesn’t take long before your spent substrate will decompose into a rich loamy compost that can be added to your vegetable garden.

It is also possible that substrate can be reused to grow more mushrooms. For example, if you have a bunch of spent shiitake blocks, you might be able to break them up, re-sterilize them and reuse the organic material to grow more shiitake! Keep in mind though, you will likely need to add supplementation, and you are likely to have yields with diminishing returns.

If you have more spent substrate then you know what to do with, see if your city has a free composting program, or if a nearby farmer is willing to let you start a large compost pile on their land.

Many large commercial mushroom farming operations actually process and sell their spent mushroom substrate as branded compost for gardens.

Learn What Works for You

At the end of the day, what substrate is best for you will depend on your location, style of growing and type of mushrooms you want to grow.

The best way to learn is to experiment with different types of substrates, different methods of preparation and different types of supplementation.

Good luck!