Bulk Substrate Stage

Bulk Substrate Stage

Entering the bulk substrate stage marks a significant shift in your mushroom cultivation journey. Following the full colonization of the spawn, this stage provides a nutrient-rich environment designed for optimal mycelial growth. Mastering the nuances of this crucial phase is key to maximizing both the quantity and quality of your mushroom harvest.

Types of Bulk Substrates

Choosing the right bulk substrate is essential for mushroom cultivation. Different mushrooms have varying substrate preferences, rooted in their natural habitats and nutritional requirements.

Compost-loving Mushrooms

Species that thrive on nutrient-rich substrates often found in decomposing organic matter include:

  • Button/Cremini/Portobello: The common grocery store mushrooms, including their mature form, Portobello.
  • Agaricus Blazei: Known for its medicinal properties, it appreciates rich composted substrates.
  • Psilocybe cubensis: Often referred to as "magic mushrooms" due to their psychoactive properties. They thrive on manure-based composts and grain-based substrates.


  • Coco Coir: Favored for its water-retentive yet aerated texture.
  • Straw: Suitable for larger mushroom varieties like Wine Cap. Requires thorough sterilization before use.
  • Compost: Highly nutrient-rich and ideal for aggressive species.

Wood-loving Mushrooms

Wood-loving species grow naturally on trees or woody debris. Species include:

  • Oyster: This variety grows on deciduous trees in nature, making them a perfect candidate for wood substrates.
  • Shiitake: Known for its delicious taste and health benefits, shiitake thrives on hardwood logs or sawdust substrates.
  • Lions Mane: This species prefers hardwoods like oak and beech.
  • King Oyster: Larger than the regular oyster, they prefer hardwood substrates.
  • Maitake: Also known as Hen of the Woods, they grow at the base of oak trees in nature.
  • Enoki: These slender mushrooms love oak or wood from fruiting trees.
  • Nameko: Prefers broadleaf trees, especially oak and beech.
  • Beech: As the name suggests, they're found on beech trees.
  • Chestnut: They thrive on hardwoods, particularly oak.
  • Turkey Tail: A medicinal mushroom that can be found on dead hardwood logs and stumps.
  • Reishi: Prefers hardwoods, especially oak and maple.


  • Hardwood Pellets: Easily available and are often used as a base for wood-loving mushroom substrates.
  • Wood Chips: Suitable for outdoor beds, especially for Wine Cap mushrooms.
  • Sawdust: A finer substrate often used with supplements for better yields.

General Supplements and Mix-ins

These elements are often mixed with primary substrates to modify or enhance the growth medium's properties. These supplements can enhance the nutritional content, water retention, and aeration of the substrate, making it more conducive to certain mushroom species' growth. Species like Oyster and King Oyster mushrooms can thrive on a variety of supplemented substrates.

  • Vermiculite: A mineral that is commonly used in mushroom cultivation to improve water retention and is often mixed with other substrates.
  • Peat Moss: A type of sphagnum moss that's naturally acidic. It's typically mixed with a pH adjuster, like lime, to make it more suitable for mushroom cultivation.
  • Wheat Bran: A nutrient-rich supplement often added to wood-based substrates to enhance mushroom growth.
  • Soy Hulls: Often used with hardwood pellets or sawdust in the "Master's Mix" formula favored for Oyster mushrooms. They enrich the substrate by providing essential nutrients.
  • Rice Hulls: Lightweight and airy, they provide good aeration to the substrate. They are typically used in mixes due to their limited nutrient content.

Using the right mix and ratio of primary substrate and supplements is key to optimizing the growth environment for your chosen mushroom species.

Methods of Cultivation


Mycobags offer convenience and a lower risk of contamination, making them excellent for beginners. Sterilizing the substrate and adding it to a mycobag is often easier and less labor-intensive than other methods.

To prepare a mycobag, the substrate must be sterilized and allowed to cool before the colonized spawn is added. The bag is then sealed and kept at optimal conditions for mycelium growth.


Monotubs are essentially large, clear plastic boxes that have been modified to allow for air exchange. Designing and setting up a monotub involves drilling holes for air exchange and adding layers of substrate and spawn.

Inoculating the monotub involves layering colonized spawn and your chosen substrate in a sterile environment. After inoculation, the monotub is sealed and placed in a warm, dark area until colonization is complete. Throughout this period, humidity and temperature must be carefully controlled.


Outdoor cultivation is best suited for hardier species of mushrooms like Oyster and Shiitake. These species are more resistant to the variable conditions encountered outdoors. Preparing an outdoor bed involves selecting a suitable location, usually a shady spot, preparing the ground, and inoculating the area with spawn.

The maintenance of an outdoor mushroom bed is comparatively lower than other methods, but the risk of contamination is higher. Environmental conditions can't be controlled as precisely, making it a bit of a 'wild card' method. Despite this, successful outdoor cultivation can yield a large and ongoing harvest.

Sterilization of Bulk Substrate

Sterilization is a fundamental step in mushroom cultivation, ensuring that your chosen substrate remains free of unwanted contaminants, such as bacteria and molds, that could outcompete the mushroom mycelium. Different substrates and mushroom cultivation goals will dictate the method of sterilization you choose. Below are some of the common methods:

Pressure Cooking

Pressure cooking is one of the most reliable methods for sterilizing substrates. The high pressure and temperature ensure that even the most heat-resistant contaminants are destroyed. It's especially recommended for substrates that are particularly prone to contamination, like grain-based ones.

  • Procedure: Fill the pressure cooker with a bit of water, place the substrate containers inside without fully sealing them, close the pressure cooker tightly, and then heat to a gauge pressure of 15 PSI for 2 hours and 30 minutes. Allow the pressure cooker to cool before removing the substrates.


Unlike sterilization, which aims to kill all organisms, pasteurization aims to kill only harmful ones, allowing beneficial microbes to survive. This can provide a protective effect against contaminants. It's often used for straw and dung-based substrates.

  • Procedure: Submerge the substrate in water and heat to 160-180°F (71-82°C) for 1-2 hours. After this, it should be drained and cooled before use.

After sterilizing or pasteurizing the substrate, it's important to work in a clean environment to prevent contaminants from being introduced post-sterilization. This is where techniques such as working in front of a laminar flow hood or within a still air box become crucial.

Mixing Spawn with Bulk Substrate

The process of introducing fully colonized spawn to the bulk substrate is a pivotal step in mushroom cultivation. This union promotes mycelial growth into the new substrate, paving the way for a larger colonization area to later support fruiting.

Choosing the Right Ratio: The proportion of spawn to substrate is a crucial factor determining colonization speed and overall mycelial health. A common ratio is between 1:1 and 1:4, meaning for every unit of spawn, you'd use one to four units of substrate. This range can vary based on the mushroom species and individual cultivator preferences. Opting for a higher spawn proportion can expedite colonization, but it's typically more resource-intensive.

Mixing Technique: Sterility remains of utmost importance at this stage to fend off contamination:

  • Start by fragmenting the colonized spawn into smaller chunks.
  • In a sterile setting, such as before a laminar flow hood or within a still air box, merge the spawn and substrate in your chosen ratio.
  • Utilizing clean hands or sterile gloves, mix the substrate and spawn thoroughly. Aim for a uniform distribution without compacting the mixture excessively.

Layering vs. Mixing: Some cultivators have a preference for layering spawn and substrate, especially in monotubs. This method involves placing a layer of substrate, then a layer of spawn, and so on. Even with layering, ensuring each layer is consistent is essential for uniform colonization.

Post mixing, it becomes imperative to foster an environment that's conducive for mycelial growth, typically by sustaining appropriate humidity, temperature, and gas exchange conditions. As the substrate becomes colonized, its color will transition to white, signaling successful mycelial growth.

Environmental Conditions

Each species of mushroom has its own preferred temperature and humidity range, but generally speaking, conditions of 75-80°F and 95-100% humidity are optimal for most. Fluctuations outside of these ranges can lead to poor yields or contamination.

Proper air exchange and lighting are also essential. While mushrooms don't require light to grow, they do need some light to trigger the fruiting stage. Simultaneously, adequate fresh air exchange is necessary to prevent the build-up of CO2 and other waste products.

Environmental Conditions

Mushroom cultivation hinges not just on the right substrate and spawn but also on cultivating the right environment. While each mushroom species has its unique needs, there are some general guidelines that apply to a broad range of fungi.

Temperature: The mycelial growth phase and the fruiting phase might require different temperature settings. Typically, a range of 75-80°F is suitable for most species during colonization. But when initiating the fruiting stage, a slight drop in temperature can often be beneficial. However, it's crucial to be aware of the specific temperature needs of the species you're cultivating. Temperatures that deviate significantly from the optimal range can stress the mycelium, slowing growth or even halting it altogether.

Humidity: High humidity is vital, especially during the fruiting phase, to prevent the substrate from drying out and to facilitate the growth of mushroom pins. Maintaining a relative humidity of 95-100% is often recommended. This can be achieved using various methods, from simple misting to more complex humidification systems.

Air Exchange: Mycelium produces carbon dioxide (CO2) as it grows. High levels of CO2 can suppress the fruiting process and encourage undesirable long-stemmed growth forms. Regular fresh air exchange removes accumulated CO2 and supplies oxygen, which is essential for mushroom development. This can be achieved using passive methods like holes in fruiting containers or more active methods like fans.

Lighting: Unlike plants, mushrooms don't rely on light for photosynthesis. However, light plays a crucial role in signaling the mushroom to initiate the fruiting process. Many species require only a few hours of indirect light a day. The light source doesn't necessarily have to be natural; LED or fluorescent lights can be used. The type and duration of light might vary based on the mushroom species, so it's beneficial to research the specific needs of the ones you're cultivating.

Monitoring and Troubleshooting

A successfully colonized substrate will be evenly covered with white, healthy-looking mycelium. Should you notice any discoloration or off-putting smells, these could be signs of contamination that need to be dealt with promptly.

Troubleshooting often involves removing the contaminated sections and possibly adjusting environmental conditions. If contamination is widespread, it may be necessary to discard the entire substrate and start anew.

Initiating Fruiting Conditions

Initiating the fruiting stage involves changes in environmental conditions. Typically, this means lowering the temperature and introducing more light into the environment. These changes signal the mycelium to shift from a growth phase to a fruiting stage.

Misting and fanning are common techniques used to maintain the required high humidity levels and air exchange. These activities simulate the natural conditions under which mushrooms typically fruit and are essential for a successful harvest.

Fruiting Stage Guide
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