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Ultimate Guide to Mushroom Substrates

Urban Farm-It
Updated

Choosing the right mushroom substrate can make or break any mushroom growing project – whether it is your first time growing using a mushroom growing kit or your 10th year in business.

Growing mushrooms on a substrate is somewhat like growing plants in soil. It is here that your mushroom mycelium will obtain all of the nutrients it needs to develop and produce beautiful mushrooms.

Much like how plants prefer different types of soil and growing conditions, different species of mushrooms prefer different substrates.

Despite the few similarities between plants and mushrooms, the way in which mushrooms utilise a substrate is very different to how a plant grows within soil. It’s important to understand how mushroom mycelium develops in order to achieve the best results.

By the end of this guide, you’ll know exactly what a mushroom substrate is, how to select the best substrate for your chosen species of mushroom, and how to prepare it for successful mushroom growth!

What is a mushroom substrate?

A mushroom substrate is a medium that allows mushroom mycelium – the vegetative part of a fungus consisting of a mass of branching threads called hyphae – to develop and establish itself. The substrate offers the nutrition, moisture, and energy that mushrooms require to grow and fruit.

To put it simply, a mushroom substrate is a material that the mycelium uses for energy, nutrition and structure.

Urban growers work with a range of various substrates. Different mushroom species have different preferences so it’s important to pair your mushrooms with the right substrate for optimal results.

What makes a good mushroom substrate?

There are several factors that make up a “good” substrate. In reality, a substrate is only as good as its match to a specific species’ requirements.

Water content

The amount of water in the substrate is key for almost any type of mushroom. Most mushroom bodies are made up of 70-90% water which is entirely sourced from the substrate. Whilst humidity in the growing environment prevents the fruiting mushrooms from drying out, ensuring your substrate has both the ability to retain moisture and has the optimal water content is key!

Aside from water content – and to get a little more technical – a suitable substrate often contains lignin, cellulose, and hemicellulose, which are woody, fibrous components. These are high in carbon, which is your mycelium’s primary food supply. Straw or hardwood sawdust are common substrates for growing mushrooms, but there are a variety of other good options to consider.

When selecting a substrate, keep the following factors in mind:

  • A modest amount of magnesium, potassium, calcium, sulphur, and phosphorus should be present in your substrate. These minerals are present in most raw substrates, but this varies depending on the origin of the material. You’ll probably have to play around with this to see whether you need to supplement with more minerals.
  • To allow for air exchange, your substrate must have a suitable structure. This is required for the mycelium to colonise effectively.
  • Nitrogen content of 1 to 2% is required in your substrate. To reach this barrier, most substrates (such as sawdust or straw) require additional ingredients.
  • Your substrate should be slightly acidic, with a PH between 5 and 6.5. (Some mushrooms, such as oyster mushrooms, can withstand a PH of up to 8.)
  • A minimum moisture content of 50-70% is essential for your substrate.

Last but certainly not least, there must be no competing organisms on your substrate. This gives your mushroom mycelium a clean slate on which to grow.

How to choose a mushroom substrate?

Selecting the best substrate comes down to a few factors. 

The first thing to consider is the availability and ease of working with a particular substrate. For example, a straw-based substrate may be far more accessible than a hardwood substrate – and can be prepared using modest home utensils. 

We would suggest that you select a substrate that is readily available in your area. If straw isn’t readily available where you live, you could consider sawdust or pre-inoculated pellets instead.

You should also choose your substrate to match the species of mushrooms you are growing. Wood-based substrates are optimal for mushrooms like reishi, lion’s mane, and maitake, while oysters grow on nearly any substrate. 

The time it takes for the mycelium to colonise the spawning material is lowered if it is already familiar with it.

Popular mushroom substrates

A suitable substrate is not only nutritious for one or more edible mushroom species, but it is also economical and straightforward to work with. As most mushrooms can grow on various substrates, urban farmers will sometimes experiment with substrates that aren’t exactly conventional for a particular species to record the findings.

This is because different types of substrates are best for various mushroom species. Some mushroom species, such as oyster mushrooms, are extremely resilient and may colonise a wide range of materials. But truffles, for example, seemingly love to grow on the roots of living trees, making them infamously tricky to cultivate in a commercial setting.

You can use a variety of materials as a substrate for mushroom cultivation. Some are more traditional, while others are more avant-garde. Here are some of the most commonly used substrates by mushroom growers worldwide.

Straw

Straw is an inexpensive and effective substrate choice. It’s available at farm shops, pet stores and other locations that sell animal feed or bedding. Similar agricultural products, such as corn stalks, can also be used. You have the option of growing in a 100% straw substrate or adding additives to provide additional nutrients. Pro tip: be cautious to avoid straw products containing eucalyptus as this is a fungicide sometimes used in animal bedding and can prevent growth.

Benefits of straw as a mushroom substrate include:

  • Cheap, efficient and easy to get hold of
  • Mimics natural wood-like conditions many mushrooms favour
  • Requires little preparation

Mushrooms that can be grown on a straw-based substrate:

How to prepare a straw-based substrate:

The preparation of straw substrate can be done in a variety of ways. The most common approach is to pasteurise the substrate mix. Some people like to use other methods, such as cold water lime pasteurisation or even substrate fermentation.

Our recommended preparation technique for hobbyists and commercial growers alike is to use heat pasteurisation.

Cut your straw into three or four-inch sections to begin. When working with large amounts of straw, it’s sometimes easier to shred the material with a garden strimmer.

Place your substrate mix into a heat resistant bag and fill it with boiling water – the amount will depend on how much straw you’re using, but you’ll want to submerge the mix. Next, carefully seal the heat-resistant bag and allow it to sit for up to 8 hours or until completely cooled. Once cooled, simply drain the water by cutting the bottom corners of the bag and allowing the bag to drain.  

Sawdust or wood-chips

Sawdust or wood chips are another great substrate choice for many types of mushrooms. The species of tree from which the material is sourced matters, much as it does with logs, but many hardwoods are appropriate, and material from several species can be blended. 

Benefits of sawdust as a mushroom substrate include:

  • It may be available for free, otherwise relatively cheap
  • Sawdust is rich in nitrogen
  • Quick to prepare

Mushrooms that can be grown on a sawdust-based substrate:

How to prepare a sawdust-based substrate:

Using your hands, combine around 450 grams of coarse oak sawdust, 225 grams of oak wood chips, and about 115 grams of millet and rice bran in a 25-litre bucket. Add small amounts of water until the ideal consistency is achieved, with all ingredients moist but not sopping wet with water collected in the bucket’s bottom. The final substrate will be entirely damp and will not stick to each other.

Fill a heat-resistant plastic bag with the mixture. Fold the tops of the bag over so the filter patch can allow gases to escape when heated. Preheat your pressure cooker to 120°C. Place the steamer basket in the pot with the recommended amount of water for your cooker. In the steamer basket, place the bag of substrate mix so that it does not come into contact with the water. The cooking time is 2 hours.

Put on your oven mitts. Remove the pressure cooker from the oven. Remove the bag from the cooker and store it in a room free of germs that could cause contamination. Before inoculating the block with spawn, let it cool to at least 30°C.

Coffee grounds

Growing mushrooms from coffee grounds is an easy to work with and low tech mushroom substrate choice. Coffee grounds do not require pasteurisation or sterilisation as long as they are put to use quickly and handled with proper hygiene and care.

This is a fun and alternative substrate choice for hobbyist growers who are looking to try something a little different.

One of the main advantages to using coffee grounds as a substrate is that the brewing process pasteurises the grounds, allowing you to skip the additional steps needed for other substrates. 

Local coffee shops will often store their wasted coffee grounds and provide them to you for free if you contact and ask them.

Benefits of straw as a coffee grounds substrate include:

  • 99% of the biomass of the coffee bean is wasted, so this is a great way to recycle the materials
  • Simple to get started
  • Easy to source
  • No need to sterilise

Mushrooms that can be grown on a coffee ground-based substrate:

How to prepare a coffee ground-based substrate:

Growing with coffee grounds is also one of the most straightforward methods. 

Combine a kilogram of coffee grounds – filter or cafetière coffee is typically overly wet, therefore espresso grounds are the ideal option – and 100 grams of mushroom spawn in a blender. For improved air exchange and faster colonisation, you can add 20% straw to your mix.

To easily get coffee grounds, drop off a pail or bin at your neighbourhood café and pick it up the next day. Use it when it’s still fresh – within 24 hours of brewing – to cultivate your mushrooms.

Cardboard

Benefits of cardboard as a mushroom substrate include:

  • Cheap and plentiful, can often get for free
  • Environmentally friendly
  • Recycling a product and adding another purpose to its lifecycle
  • Lightweight and easy to move around

Mushrooms that can be grown on a cardboard-based substrate:

How to prepare a cardboard-based substrate:

There’s no need for a recipe when it comes to growing on cardboard. Simply gather as much cardboard as you need and soak it in boiling water.

After allowing your cardboard to cool and squeezing out any leftover water, you’re ready to colonise it with mushroom spawn.

Logs

Many varieties of fungi feed on dead wood, so cultivating them on cut logs is a perfect choice.

While many fungus species tolerate a variety of woods, others are more selective, and most have at least a few types of that they can’t or won’t eat. Most hardwood tree species, such as beech, poplar, maple, oak, birch, elm, and others, can be used.

Around 1 metre long and around 15cm in diameter are the ideal log size for growing mushrooms.

Benefits of logs as a mushroom substrate include:

  • Cost-effective
  • Can produce a wide range of mushrooms
  • Adds value to wood species that are otherwise undesirable
  • Long-term harvesting

Mushrooms that can be grown on a log-based substrate:

How to prepare a log-based substrate:

You should avoid using wood that has been dead or dying for a long time. Other fungi may have already colonised the wood, making it difficult for your species of mushroom to establish itself.

Freshly cut logs should also be avoided because trees have natural antifungal qualities while still alive. Before inoculating, keep freshly cut logs somewhere clean and dry for a few weeks.

In order to inoculate a log, holes are drilled in staggered rows across the log. For the best hole size, use an 8mm drill bit. Rows should be staggered with two or three inches between them.

The plug spawn is used to introduce mycelium into the holes. These are little pieces of mycelium-colonized wooden dowels that are hammered into the holes. To prevent contamination, the holes are then filled with wax.

How to prepare mushroom substrates

Many varieties of fungi feed on dead wood, so cultivating them on cut logs is a perfect choice.

While many fungus species tolerate a variety of woods, others are more selective, and most have at least a few types of that they can’t or won’t eat. Most hardwood tree species, such as beech, poplar, maple, oak, birch, elm, and others, can be used.

Around 1 metre long and around 15cm in diameter are the ideal log size for growing mushrooms.

Many varieties of fungi feed on dead wood, so cultivating them on cut logs is a perfect choice.

While many fungus species tolerate a variety of woods, others are more selective, and most have at least a few types of that they can’t or won’t eat. Most hardwood tree species, such as beech, poplar, maple, oak, birch, elm, and others, can be used.

Around 1 metre long and around 15cm in diameter are the ideal log size for growing mushrooms.

What is substrate pasteurisation and sterilization?

Pasteurization is the process of heating a substrate to temperatures between 65 and 85°C for 1.5 to 2 hours. It won’t get rid of all the impurities, but it will reduce the general population of other bacteria to a point where the mushroom species will have an advantage.

Sterilisation is the process of heating a substrate to temperatures above 120°C under pressure in order to kill any living or dormant contaminants.

What is the difference between pasteurisation and sterilisation?

The key distinction is that sterilisation aims to kill all bacteria and pathogens, whilst pasteurisation simply reduces the number of competing organisms within the substrate.

Why do you need to pasteurise or sterilise a substrate?

Mushroom substrates must be moist and nutrient-rich. Unfortunately, moulds and germs also thrive in these conditions, as do many other things we want to avoid. Because these contaminants develop far quicker than the mycelium of the mushroom, something must be done to give the mushroom an advantage in the race to colonise the substrate.

When pasteurisation and sterilization are not required

You don’t always need to sterilise or pasteurize the mushroom substrate. This simply minimises the risk of contamination-related growth issues or sickness. Some mushrooms cannot be cultivated on sterile media and must instead be grown outdoors or in nutrient-rich environments that do not allow for sterilisation. 

Discolouration or decay in mushrooms growing in unsterilised growth media are indicators that your mushrooms are contaminated. Contamination can deplete the nutrients available to mushrooms or cause the mushrooms to break down, causing them to decay. If you’re cultivating edible mushrooms, be aware that if you consume mushrooms from a contaminated medium, due to bacterial development, it can make you ill.

Substrate fermentation – an alternative approach

If you begin to cultivate a lot of mushrooms, you may find yourself with big piles of spent substrate that you are unsure how to dispose of. The sheer amount of substrate can quickly add up, so having a plan for what to do with it all is a good idea.

If you have a tiny amount of spent substrate, the most obvious option is to compost it. Simply pile your substrate outside and allow the natural decomposition process to take its course. There’s a chance you’ll end up with an additional flush or two of mushrooms in your compost pile! It won’t be long before your old substrate decomposes into rich loamy compost that you may use in your vegetable garden.

It’s also feasible to repurpose the substrate to grow new mushrooms. If you have a lot of spent shiitake blocks, for example, you might be able to break them up, re-sterilize them, and use the organic material to grow new shiitake! Keep in mind that you’ll almost certainly need to supplement, and your yields will likely drop over time.

If you have more spent substrate than you can handle, investigate if your city offers a free composting programme or if a local farmer will allow you to build a large compost pile on their property.

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How to Pasteurise mushroom substrate

Pasteurisation is used to make the substrate relatively contaminant-free and to provide any mushroom culture introduced a head start. 

Types of Pasteurisation

Pasteurisation can be accomplished in a number of ways:

Hot water bath pasteurisation

Submerging the substrate in hot water for at least one or two hours is one approach to pasteurise it. Pasteurisation is sufficient for most enterprises to grow mushrooms with minimum risk of contamination.

Pasteurization takes place at temperatures ranging from 70 to 80°C. If the temperature rises above these levels, the healthy bacteria will be eliminated and the harmful bacteria will thrive. In a water bath, pasteurise your substrate by soaking it for one hour in 70°C water.

Coldwater lime pasteurisation

While this isn’t technically pasteurisation, it is a method of modifying substrate to give mycelium more leverage. This method is based on the idea that mushroom mycelium is significantly better equipped to handle high pH levels that many competing organisms cannot.

To use this technique, immerse your substrate for 24 hours in a bath of hydrated lime-treated cold water. This raises the pH of the water, thus destroying pollutants.

Hydrogen Peroxide

Foreign germs and competitor spores can be killed by hydrogen peroxide without harming mycelium.

Soak the substrate for about an hour in water. Drain it, then thoroughly clean it in water before draining it again.

Allow the straw substrate to soak in a hydrogen peroxide water bath for a day. For every 4.5 litres of water, use 1 litre of hydrogen peroxide.

After you’ve finished, rinse and drain your substrate for a while. Incorporate the spawn and incubate as you normally would.

How to Sterilize a mushroom substrate

Sterilization consists of heating the substrate to temperatures above 120°C and applying pressure to the substrate. This is the process of totally removing all pollutants from the substrate, both living and dormant.

Because boiling a substrate doesn’t get it hot enough to sanitise it entirely. A pressure cooker or equivalent piece of equipment is required for complete sterilisation.

Is sterilization always necessary?

To avoid contamination, some mushroom substrate varieties require sterilisation.  The nutrient level of the substrate is the most crucial aspect in determining whether you need to sterilise it.

Manure is an excellent example of a substrate that should be sterilised at all times. It’s already teeming with germs and microbial life by its own nature. It may not appear to humans to be a particularly desirable ecosystem. However, because it’s formed of partially digested food, it’s a handy food supply for a wide range of organisms.

For the same reasons, all substrate materials that could be considered food must be sterilised. Rye grain, popcorn, brown rice, and wheatberries are examples.

These substrates are all high in nutrients, and they attract a wide variety of fungus and mould. Mould can start to grow on food in just a week or two, as you’ve undoubtedly seen in your own kitchen.

To give the mushrooms you’re growing a head start, sterilise high-nutrient substrates. Pasteurisation can be used instead of sterilisation for less nutritious substrates. Straw is a wonderful example of this. After all of the grain has been harvested, straw is the dried stalks of various grain plants.

If you’re not sure, sterilise any substrate just to be safe. It just requires more effort and equipment.

Sterilizing a mushroom substrate with a pressure cooker

Sterilising substrates in a pressure cooker works well, especially for small batches. The pressure cooker chamber reaches temperatures high enough to destroy bacteria and mould spores.

Fill your cooker with your substrates and enough water to cover them. Seal the pressure cooker and heat it on a burner according to the manufacturer’s instructions. To totally kill out any potential contaminants, most substrates require a pressure of 15 PSI. Steam reaches a temperature of 121°C at this pressure

Sterilizing a mushroom substrate without a pressure cooker

You can sterilise a substrate without using a pressure cooker in a few different methods.

Tyndallization, also known as fractional sterilisation, is another option. This is performed by repeatedly boiling jars or bags for a specified amount of time. The goal is to eliminate any microbial life that may already be present in the substrate, as well as any spores that may take a few days to germinate.

Anything that raises the temperature of your substrate above 120°C for a long amount of time can sterilise it. This involves using an oven or autoclave to sterilise your substrate. The problem with this is that it cooks your substrate and dries it out totally. Some substrates may start to burn as well. If you sterilise your substrate in this manner, you’ll need to rehydrate it with distilled water to keep it sterile. 

Supplementing your mushroom substrate

If you’re thinking about growing mushrooms commercially, you might want to consider supplementing your substrate to boost your mushroom harvest.

Supplement ingredients are most typically bran or seed derivatives. You can also buy them as high-protein animal feed pellets. The latter is preferred as this has already been pasteurised.

To figure out how much supplementation to add, you’ll have to experiment, and the amount will also depend on whether the material you’re adding is sterilised. Starting at 5% and working your way up is a good place to start.

Popular substrate supplements

A great mushroom substrate should be sufficiently moist and nutrient-dense. Mushroom substrates are given nutritional sustenance in the form of extra nutrient feeds, which help the mushrooms grow quicker and healthier.

The most popular supplement materials for mushrooms are bran or seed derivatives, with oat bran and wheat bran being utilised in a 5-10% dry weight ratio. A popular mix includes 18% bran and 2% gypsum, as well as the sterilised substrate. The once sterilised substrate may come into contact with minute pollutants when supplements are added.

Another popular substrate combination is MASTER MIX, which is a 50-50 mix of sawdust and soybean husk created by T.R. Davis of ‘Earth Angel Mushrooms’. This mixture yields more than a traditional one, although it still requires sterilisation.

Colonization is a growth process in which the mycelium slowly engulfs the substrate and decomposes the organic material.

How to inoculate mushroom substrates

Inoculation is the term for adding mushroom spawn to the substrate. Now your substrates are ready, it’s time to add your mushroom spawn. To avoid contamination, always thoroughly wash your hands and forearms with soap, and wipe your work area down with an antibacterial cleanser.

Inoculating loose substrates (straw, sawdust, cardboard)

Crumble the mushroom spawn into little bits from the package. Add the spawn to the substrate after opening the black bag at the top. Using your hands, evenly distribute the spawn throughout the substrate. For rapid growth, an even combination is essential.

Close the top of the bag with a double/triple fold and compress the substrate bag to eliminate trapped air, then fix with a peg or tape. Cut 5-6 X shaped cuts of approximately 3cm across on each face of the bag (or just one face if you plan to grow with the bag in the horizontal position) with a knife, making sure the cuts are evenly spaced. This is where your mushrooms will grow.

Place the black bag inside the mushroom box and liner.

To help preserve moisture, fold the box liner over. Close the box’s flaps and secure them with weights. Store for 17-22 days in a warm environment with a steady temperature of 18-22°C (It is critical for maximum productivity that incubation is fully complete before starting the next stage). 

During this time, the kit will not require any additional attention. When the developing thick white cotton wool-like mycelium has occupied ALL of the substrates, the incubation is complete.

When you’re ready, relocate the box to a bright, but not draughty place that gets lots of natural light but isn’t directly in the sun. 8-18°C is ideal. Allow the mushrooms to breathe by opening the box’s flaps. 

Pull the liner’s edge over the flaps, which should be pointing up. The mushrooms are stimulated to fruit by the drop in temperature and enhanced aeration. If there is no drought, you can insert the black bag in the centre of the box, vertically, to produce more naturally shaped oyster mushrooms.

It is vital that the mushrooms do not dry out after the box is opened. Water should be sprayed into the perforations twice a day, but not excessively (this may encourage unwanted mould growth). Spraying the growing environment provided by the white liner is also a wonderful idea. This keeps the air moist, which is ideal for mushrooms! Increase the watering to three times a day when the young mushrooms (known as pins) begin to push their way through the bag, but do not spray the mushroom directly.

You’ll have your first flush in 10 to 30 days! The rate of growth will vary based on the environment, but it is possible that they will double in size every 24 hours. They will curl up at the edges when fully mature, so harvest immediately before this stage.

Harvest the clump by twisting it one full rotation rather than pulling it. It’s best not to cut the mushroom because the stump will encourage decay.

Many pins will be produced, however, some will need to be discarded. Only take mushrooms that appear to be healthy and new. Aborted or aged mushrooms are inedible and should not be consumed.

Inoculating hardwood substrates (growing logs)

If you’re using plug spawn, you’ll need to prepare your growing log before applying them.

You’ll want to choose a drill bit that ensures a snug fit for the plug spawn, depending on the size of your plug spawn dowels.

There are no hard and fast rules for how you should mark out your drill holes, but we recommend using the full length of the log with holes every six inches to provide ample growing area.

When turning the log and drilling additional holes, we recommend providing 3-4 inches between rows — just be sure to stagger your rows for the room! After you’ve drilled your holes, you’ll need to insert your spawn dowels and hammer them flat with the wood.

After you’ve installed your plugs, apply a wax sealer to each of the dowels to help preserve moisture and keep unwanted visitors at bay.

Simply place your wax block or wax pellets in a saucepan or other heat source and slowly warm them. Once the drill holes are totally melted, start covering them with a clean sponge across the expanding log.

If you live in a warmer environment than we do in the UK, or are particularly exposed to windy circumstances, it’s a good idea to treat the ends of the log with sealing wax to maximise moisture retention.

How to recycle your spent mushroom substrate

Many cities now offer a free composting service. You might be able to dispose of the used substrate in weekly-emptied compost bins. Alternatively, you may need to drive the used substrate yourself to a recycling centre that accepts garden waste, compost, and other recyclables.

If your city does not have a recycling programme for your substrate, you can contact local farmers. They may be interested in adding your leftover substrate to their compost piles and will gladly accept it for free. There have also been experiments on using mushroom substrates as a component in chicken and calf feed blends.

Composting your substrate

If you want to compost your substrate, you’ll need to start with a hot compost process that lasts at least two weeks. The idea is to have your compost pile reach 70°C for at least a few days, since this will kill any weed seeds and kill any hazardous bacteria that may be present.

During the heated composting process, it is required to turn it every day or two. This allows air to enter the compost pile. It also guarantees that the less-composted material from the pile’s edges makes its way to the centre, where the heat accumulates. Spraying the pile with a hose on a regular basis will help to keep it moist.

It’s time to age and cure the compost after it’s been hot-composted for a couple of weeks. Create a secondary pile and let the stuff sit for a few weeks. This will progressively darken the compost and cause it to decompose even more.

Because you’re not trying to restart the hot composting process, you don’t need to turn the secondary pile. Just keep it damp and let it decompose. You now have compost after that process is completed.

Inoculating hardwood substrates (growing logs)

If you’re going to use pellet spawn, soak the pellets for 20-30 minutes in fresh, clean water. Because the pellets are pasteurised throughout the manufacturing process to eliminate competing bacteria or fungi, you won’t need to sterilise or pasteurise them; all you’ll need to do is rehydrate them.

You’ll want to choose a drill bit that ensures a snug fit for the plug spawn, depending on the size of your plug spawn dowels.

There are no hard and fast rules for how you should mark out your drill holes, but we recommend using the full length of the log with holes every six inches to provide ample growing area.

When turning the log and drilling additional holes, we recommend providing 3-4 inches between rows — just be sure to stagger your rows for the room! After you’ve drilled your holes, you’ll need to insert your spawn dowels and hammer them flat with the wood.

After you’ve installed your plugs, apply a wax sealer to each of the dowels to help preserve moisture and keep unwanted visitors at bay.

Simply place your wax block or wax pellets in a saucepan or other heat source and slowly warm them. Once the drill holes are totally melted, start covering them with a clean sponge across the expanding log.

If you live in a warmer environment than we do in the UK, or are particularly exposed to windy circumstances, it’s a good idea to treat the ends of the log with sealing wax to maximise moisture retention.

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