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How to Make Grain Spawn Jars

If you're a mushroom enthusiast like me, you've probably considered making your own grain spawn. The first step in this exciting journey is crafting grain jar lids. These specialized lids are equipped with filters that allow the mushroom mycelium to breathe while keeping contaminants at bay, ensuring your fungi thrive.

Image featuring a pair of sterilized grain jars, ready for inoculation in the process of mushroom cultivation.
A couple of sterilized grain jars

In this comprehensive guide, we'll delve into:

1. Understanding grain spawn jars

2. The art of spawn jar lid design

3. The process of creating your grain spawn jars

Mycelium: The Foundation of Mushroom Growth

Before we begin, it’s important to address why we need a modified mason jar lid. As mushroom farmers we’re trying to grow mycelium. Mycelium is the vegetative part of a fungus, consisting of a network of fine white filaments (hyphae). It's essentially the root system of your mushrooms.

Just as plants need healthy roots to grow and thrive, mushrooms need robust mycelium. This mycelium serves as the foundation for mushroom growth, absorbing nutrients from the grain in your spawn jars and using them to grow and eventually produce mushrooms.

Image depicting mycelium, appearing as white, thread-like structures, actively growing and colonizing on a substrate of coffee grounds.
Mycelium growing on coffee grounds.

We must create a contamination free environment for our mycelium to grow and thrive. That’s why we will sterilized grain jars after we’ve created our airport lids.

Understanding Grain Spawn Jars

Grain spawn jars, also known as spawn jars, are integral to mushroom cultivation. They serve as the nurturing ground where the mycelium, the vegetative part of a fungus, grows before being transferred to a fruiting substrate. These jars are filled with sterilized grain, providing a rich food source for the mycelium.

A variety of grains can be used in spawn jars, including rye, wheat, millet, and corn. Rye grain is often a popular choice due to its nutrient-rich profile and the ease with which mycelium colonizes it. However, the choice of grain can depend on the specific mushroom species being cultivated, as some may have a preference for one grain over another. At Happy Little Fungi, we have a particular fondness for whole oats due to their resistance to contamination.

The process of using spawn jars begins with sterilizing the grain, typically through pressure cooking, to eliminate any potential contaminants. Once cooled, the grain is inoculated with a spore syringe or a piece of colonized mycelium. The jar is then sealed with a specially designed lid that facilitates gas exchange while preventing contamination. Over time, the mycelium colonizes the grain, readying it for transfer to a fruiting substrate.

Image showcasing a mason jar filled with sterilized grains, with visible white mycelium growth indicating successful colonization in the process of mushroom cultivation.
Mycelium growing on sterilized grains in a mason jar

One of the main advantages of using jars over bags for grain spawn is the durability and reusability of jars. They're also transparent, allowing for easy monitoring of the mycelium's growth and potential contamination. On the other hand, bags, while capable of holding a larger volume of grain (beneficial for large-scale cultivation), are prone to tearing and are typically single-use.

For beginners, it's crucial to note that maintaining sterility is key when making and using grain spawn jars. Any contamination can impede the growth of the mycelium and potentially ruin the entire batch.

It's also vital to provide the right conditions for mycelium growth, including the correct temperature and humidity levels. Lastly, patience is a virtue, as it can take several weeks for the mycelium to fully colonize the grain.

Mastering Spawn Jar Lid Design (AKA Airport Lid)

The design of your grain jar is instrumental in reducing contamination risks and boosting the success rate of mushroom cultivation. The secret to crafting an optimal spawn jar lid lies in minimizing the size of the holes for the filter and the injection port, if one is being used.

Furthermore, it's essential to choose regular-mouth sized lids for your jars. While wide-mouth jars may seem more convenient for adding and removing contents, they actually present a larger surface area for potential contaminants to infiltrate, thereby increasing the risk of contamination.

Image displaying a set of 12 airport lids, designed with self-healing injection ports and filters, ready for use in mushroom cultivation.
Set of 12 airport lids

Here are the key elements to consider for an effective spawn jar lid design:

  1. Regular-Mouth Jar Lid: These lids are smaller than their wide-mouth counterparts, reducing the area through which contaminants can enter. They fit snugly onto regular-mouth jars, ensuring a secure seal that helps maintain sterility inside the jar.

  2. 9mm Hole for Filter: This hole is designed to accommodate a filter, which allows for necessary gas exchange while preventing contaminants from entering the jar. The size of the hole is large enough to provide adequate airflow but small enough to minimize the risk of contamination.

  3. 5 mm Hole for a Self-Healing, 7 mm Injection Port: If you're using an injection port for inoculation, a 5 mm hole is sufficient. This small hole allows for the needle of the spore syringe to pass through while maintaining the overall integrity of the jar's sealed environment. Typically these rubber injection ports can be used at least a dozen times and are easily exchanged.

Remember, the goal of your spawn jar design should be to create a controlled, sterile environment that promotes the growth of mycelium while minimizing the risk of contamination. By carefully considering the design of your jar lids, you can greatly increase your chances of successfully cultivating mushrooms.

The Role of Air Filters

Air filters play a crucial role in the life of mycelium. Just like any other living organism, mycelium needs to breathe. It does this through a process called respiration, where it takes in oxygen and releases carbon dioxide. This is where the air filter comes into play.

The air filter on your spawn jar lid allows this crucial gas exchange to occur. It lets oxygen in and carbon dioxide out, all while keeping unwanted contaminants from entering the jar. Without this gas exchange, the mycelium would suffocate and die. However, if contaminants were to enter the jar, they could compete with the mycelium for resources or introduce diseases.

The Role of Self-Healing Injection Ports in Mushroom Cultivation

In the world of mushroom cultivation, maintaining a sterile environment is paramount. This is where self-healing injection ports come into play. These ingenious additions to your grain jar lids provide a safe and sterile method for introducing mycelium into your jars without exposing the contents to potential contaminants.

What is a Self-Healing Injection Port?

A self-healing injection port is a small piece of rubber or silicone that is inserted into a hole in the lid of your grain jar. It's designed to be punctured by a needle, typically from a spore syringe or liquid culture syringe, and then seal itself back up after the needle is removed. This "self-healing" property allows you to inoculate your jars multiple times without introducing contaminants.

Why Use Self-Healing Injection Ports?

The primary benefit of using self-healing injection ports is that they allow for a sterile transfer of mycelium into your grain jars. When you're ready to inoculate your jars, you simply wipe the injection port with an alcohol swab, insert the needle through the port, and inject the mycelium. Once you remove the needle, the injection port seals itself, maintaining the sterility of the jar's contents.

Another advantage of self-healing injection ports is their durability. Most ports can be used multiple times without losing their self-healing properties. This makes them a cost-effective and efficient option for those who frequently cultivate mushrooms.

Incorporating Self-Healing Injection Ports into Your Spawn Jar Lid Design

When designing your spawn jar lids, a 5 mm hole is typically sufficient for the self-healing injection port. This small hole allows for the needle of the spore syringe to pass through while maintaining the overall integrity of the jar's sealed environment.

The placement of the injection port is also important. It should be positioned in a location that allows for easy access with the syringe but doesn't interfere with the placement of the air filter or the ability to seal the jar.

In conclusion, self-healing injection ports are a small but vital component in the process of mushroom cultivation. They provide a safe and sterile method for introducing mycelium into your grain jars, helping to ensure the success of your mushroom cultivation efforts. However, if you’re using agar, you likely will not need a self-healing injection port. Rather, you would need a laminar flow hood or a still air box.

Creating Your Own Grain Spawn Jars

Here's what you'll need:

The most basic grains will work for most mushroom cultivation. The most common grains are rye, popcorn, birdseed, millet, and whole oats. At Happy Little Fungi, we use whole oats since they are contamination resistant.

Firstly, your grains will need to be hydrated. This usually involves soaking them and then draining the excess water. For instance, when using oats, our friends at Happy Little Fungi recommend soaking them in cold water for up to 12 hours before draining.

Once your grains are drained, they're ready to be placed into your mason jars. After filling the jars, secure them with your specially designed grain spawn jar lids, tighten them with the band, and wrap the lid in aluminum foil.

Now, it's time for the sterilization process. Sterilization is a critical step in the process of preparing your grain spawn jars. It's the process that ensures your grains are free from any unwanted microorganisms that could potentially compete with your mushroom mycelium for resources or, worse, inhibit its growth entirely. Let's delve into the details of this crucial process.


It's important to fill your grain jars to a maximum of 3/4ths of the way to the top. This will allow you to easily shake and empty your grain jars once they are covered with mycelium.

Why Sterilization Matters

Before we get into the how, it's important to understand the why. In the wild, mushrooms have to compete with a myriad of other organisms for the same resources. In the controlled environment of your spawn jars, you want to give your mycelium the best possible start by eliminating these competitors. Sterilization effectively kills off these potential threats, creating a clean slate for your mycelium to colonize.

How to Sterilize Your Grain Jars

The most common method of sterilizing grain for spawn jars is pressure cooking or autoclaving. These processes use high temperature and pressure to kill off any bacteria, fungi, or other potential contaminants present in the grain.

Here's a step-by-step guide:

  1. Preparation: Once your grains are soaked, drained, and loaded into your mason jars, secure them with your specially designed grain spawn jar lids. Tighten them with the band and wrap the lid in aluminum foil. The foil helps to keep additional contaminants from entering the jar during the sterilization process.

  2. Loading the Pressure Cooker or Autoclave: Place your prepared jars in the pressure cooker or autoclave. Be sure to follow the manufacturer's instructions for loading the jars. Typically, you'll want to ensure that the jars are not touching each other or the sides of the cooker to allow for even heat distribution.

  3. Sterilization: Bring the pressure up to 15 PSI and maintain it for about an hour. This is typically hot and long enough to kill any microorganisms present in the grain or on the jar.

  4. Cooling: Once the sterilization process is complete, allow the pressure cooker or autoclave to cool naturally. Do not attempt to speed up the process by releasing the pressure valve, as this can lead to cracked jars or, worse, a dangerous situation.

  5. Storage: Once the jars are completely cool, they can be removed from the pressure cooker or autoclave. Store them in a clean, dust-free environment until you're ready to inoculate them with your mycelium.

Note about Pressure Cooker and Autoclave

While autoclaves are effective for sterilizing grain spawn jars, they aren't the only option. If you're working within a budget, a low-cost pressure cooker can serve as a viable alternative. The key is to ensure that the pressure cooker can reach and sustain a pressure of 15 PSI for a minimum duration of one hour. This is crucial for effective sterilization.

Troubleshooting Sterilization Issues

If you encounter contamination issues after inoculating your jars with spores, liquid culture, or agar, it might be necessary to extend the sterilization time during your next attempt. Contamination can be a sign that some resilient microorganisms survived the sterilization process. Extending the sterilization time or increasing the pressure slightly can help ensure a more thorough sterilization.

Remember, sterilization is a crucial step in the process of preparing your grain spawn jars. By taking the time to do it properly, you're setting the stage for a successful mushroom cultivation experience.

It's crucial to follow the manufacturer's instructions at this stage, as these devices can generate a lot of steam and can be hazardous if mishandled.

Remember, creating your own grain spawn jars is a process that requires patience and precision, but with careful attention to detail, you'll be well on your way to successful mushroom cultivation. Keep going, you're doing great!


Making your own grain jar lids and spawn jars can seem daunting at first, but with the right guidance and a bit of practice, it can become an enjoyable and rewarding part of your mushroom cultivation journey. By understanding the importance of each component and step in the process, you can increase your chances of success and enjoy the fruits (or rather, fungi) of your labor. Happy cultivating!


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