Mushroom grow bags are an essential part of the mushroom cultivation process and an industry staple. While there are alternatives to them, they often pose challenges in implementation, especially at larger scales. As a result, grow bags remain standard practice for many novice and professional growers.
In this article, we’ll teach you what you should know about grow bags and help you navigate the many types available on the market.
What Are Mushroom Grow Bags?
Mushroom grow bags are specially designed plastic bags used in mushroom cultivation. Their purpose is to provide a protected environment for the growth of the fungal mycelium, isolated from potential contaminants. They can offer numerous features, including built-in filters for breathability, heat resistance for sterilization, injection ports for inoculation, and a compact shape that allows for easy storage.
While the concept of a grow bag is relatively straightforward, it’s essential to know that many types of grow bags are designed to meet different needs. Grow bags utilized for spawn won’t necessarily be the best for bulk substrates, nor will those for Oysters be the best for Shiitake. Some grow bags can handle sterilization temperatures, while others cannot.
In this article, we’ll discuss the basics of mushroom grow bags and how to choose the right ones to suit your needs.
Mushroom Grow Bag Differences
While there are many grow bag models, there are three main characteristics you should consider when choosing a model. These are their material, the filter type, and the bag size. Additionally, some bags may include injection ports for inoculation.
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Materials: There are two standard materials utilized for mushroom grow bags; their main difference is their resistance to high temperatures.
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Polypropylene (PP) has a high heat resistance and is best suited for sterilizing grains and supplemented substrates.
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Polyethylene (PE) has a lower heat resistance and is excellent for filling with pasteurized/sterilized substrates. It can also be steam-pasteurized/sterilized.
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Polypropylene (PP) has a high heat resistance and is best suited for sterilizing grains and supplemented substrates.
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Filter Type: Numerous filter sizes are available that allow either more or less air exchange during growth.
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0.2 Micron (low permeability): This is the smallest filter size available and best suited for grains and slow-growing mushroom species.
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0.5 Micron (intermediate permeability): This allows for intermediate airflow and is great for in-bag fruiting of CO2-tolerant species like Reishi or dung-lovers.
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5 Micron (most permeable): This larger size is best suited for fast-growing species which will out-compete any potential contamination.
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0.2 Micron (low permeability): This is the smallest filter size available and best suited for grains and slow-growing mushroom species.
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Bag Size: There are dozens of different sizes available, ranging from small or extra large. Smaller bags are often the best for producing grain spawn, while larger bags are for bulk substrates.
- Injection Port: Some bags may come equipped with an injection port to allow for easy inoculation of liquid culture.
Materials: Polypropylene (PP) vs Polyethylene (PE) Grow Bags
The most crucial difference between these two materials is that polypropylene can withstand high sterilization temperatures while polyethylene cannot. This makes polypropylene the go-to bag for sterilizing grains or supplemented substrates (like masters mix). Polyethylene is best suited for filling with pre-pasteurized or sterilized substrates.
| Feature | Polypropylene (PP) | Polyethylene (PE) |
|---|---|---|
| Appearance |
Clear This allows easy observation of mycelial growth and potential contamination. |
Opaque or Black This limits light exposure to reduce premature fruiting. |
| Heat Resistance |
High Is capable of withstanding autoclave sterilization temperatures of 15 PSI or 250°F (121°C) for extended periods. Polypropylene melts above 25 PSI and 350°F. |
Lower Has a melting usually around 250°F (121°C). It is not recommended for sterilization under pressure but may be suitable for steaming. |
| Durability |
Relatively Durable It can withstand rough handling. However, polypropylene is prone to pin holes or tears at fold points under excessive physical stress. |
Very Durable It is more mechanically resistant than polypropylene, reducing the risk of pinholes and tears under stress. |
| Typical Uses |
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Filter Type
Filters are an important feature of mushroom grow bags, as they allow for fresh air exchange (FAE) between the mycelium and the external environment. They do this while limiting the entrance of microscopic spores, bacteria, fungus gnats, and other potentially harmful organisms.
The difference in filter types is the pore size, which determines how easily air can pass through the filter. The most common sizes are 0.2 microns and 5 microns, although some manufacturers also offer 0.5 micron filters. While 5 microns are microscopic, about 10 to 20 times smaller than the diameter of human hair, it is 25 times bigger than 0.2 microns!
What's the Difference Between 0.2 Microns, 0.5 Micron, and 5 Micron Filters?
A small pore size (0.2 microns) allows for air exchange while preventing the entrance of contamination. A larger pore size (5 microns) provides for significantly more air exchange but may allow for the passage of contaminants. In some cases, the increased air flow outweighs the consequences and risks of allowing some potential contaminants through. Essentially, the choice between the type of filter depends on the balance between your sterility needs and airflow in your cultivation context.
Filter Pore Size Comparison ChartWhen To Use What Filter |
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| 0.2 Micron | 0.5 Micron | 5 Micron |
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For example:
- A small polypropylene bag with a 0.2-micron filter and an injection port would be ideal for making grain spawn using liquid culture, as it provides a sterile environment for inoculation and prevents contamination.
- A medium or large polypropylene or polyethylene bag with a 5-micron filter would be well-suited for inoculating bulk substrate of fast-growing species (Lion's Mane, Oysters, ect.), as the increased airflow supports vigorous mycelial growth and fruiting.
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Why does Mycelium like Fresh Air Exchange (FAE)? While mycelium can survive in environments with limited fresh air, most species grow faster when provided with improved air exchange. This is rooted in the respiratory process of mycelium, which consumes oxygen (O₂) and releases carbon dioxide (CO₂) as it metabolizes nutrients. In a confined space without ventilation, CO₂ levels can accumulate while O₂ levels become depleted, creating suboptimal conditions for growth. Introducing air exchange replenishes O₂ and reduces CO₂ concentrations, thereby creating an environment more conducive to mycelial development. |
Bag Size: What's the Best Size For You?
Grow bags come in dozens of different sizes, each with their own benefits. Your choice depends on your specific process, equipment, and infrastructure. Generally speaking, small grow bags are best suited for spawning, while large grow bags are best suited for bulk substrates. Medium grow bags are intermediate and can be utilized for either.
Bag Size For Fruiting Blocks
Fruiting Blocks can be made in any bag size without significantly affecting the yield. One 10 lb bag should yield approximately the same as five 2 lb bags filled with the same substrate. The most significant difference is that making, handling, and moving five bags is much more work than one. This is why many growers prefer making fewer large fruiting blocks than many small ones.
| Pros | Cons | |
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| Large Fruiting Blocks |
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| Small Fruiting Blocks |
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Interestingly, in many parts of Asia, where 90% of the world's mushrooms are produced, smaller bag sizes are most common for gourmet mushrooms. This is unlike the United States or Europe, where large bag sizes are standard for commercial cultivators.
The reason Chinese cultivators prefer smaller bag sizes is that it results in fewer losses when there is contamination. After all, losing a 10 lb bag is a more significant loss than only one or two 2 lb bags. The difference is that Chinese growers are much more industrialized and have automated equipment, meaning a significantly reduced labor cost compared to US growers, who manually conduct many of the processes.
While utilizing small grow bags is not often convenient for commercial growers in the US, it might be a good option for amateur and hobby growers. After all, they are easier to place in equipment like pressure cookers or still-air-boxes and might improve your success rate. Also, sterilization is quicker and often more secure.
Bag Size For Spawn Production
To produce spawn, you typically want to utilize smaller bags. This is especially true if you are inoculating with agar or liquid culture. Utilizing a large grow bag in these contexts would mean a long colonization time and a higher risk for contamination. If you are expanding spawn, such as a grain-to-grain transfer, then medium or large grow bags are more suitable.
Bag Size Comparison Chart
| Bag Size | Approximate Dimensions* (LxWxH) | Max Capacity* (Hydrated Substrate) | Best Use Cases |
|---|---|---|---|
| Small | 15" x 4" x 4" | 2 lbs |
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| Medium | 18" x 8" x 5" | 5 lbs |
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| Large | 19" x 12.8" x 5" | 7 lbs |
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| Extra Large | 24" x 10" x 5" | 10 lbs |
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| *Dimensions and capacity are approximate as these vary based on the producer. Please consult the indications of your manufacturer for more precise details. | |||
Injection Port
Injection ports are only necessary if you plan to inoculate utilizing liquid culture. This is most used for the production of spawn. Inoculating with liquid culture is not typically recommended for bulk substrates, as colonization can be extremely slow. This is because of a low inoculum rate and because liquid cultures don’t have the same vigor as grain spawn. While it’s not too familiar, some growers also make “all in one bags” that contain grains and bulk substrate, making it suitable for inoculation with grain spawn.
How To Use Mushroom Grow Bags
While utilizing mushroom grow bags is pretty straightforward, some best-practices can make the process much easier and with greater success.
- Do not overfill the bags. This ensures you’ll be able to fold and seal the bag properly. If sterilizing the substrate, only fill the bag about halfway so it can be properly folded.
- Be delicate when filling the bags and avoid getting substrate on the interior and exterior walls of the bag. Wipe the bags before sterilizing to ensure they are clean.
- Notice that grow bags naturally have a rectangular shape. You want to attempt to maintain this shape as you compact the substrate into the bag. Do this by flattening the sides with one hand as you compress with the other. This not only allows for better storage but also makes folding easier.
- Utilize the existing creases when closing the bag. You should close it evenly, closing the top of the bag evenly in the center before folding it onto the bottom.
- It’s not usually necessary to fully seal the bag before sterilizing, as you must open it for inoculation. You can keep it closed before/during/after sterilization with a piece of tape, a clamp, a rubber band, a paper clip, or wire. If your bag has a 0.2-micron filter, you should NOT seal the bag as it can burst under the pressure of sterilization.
- After inoculation, you want to seal the bag for good. The most recommended method is with an impulse sealer, but there are alternatives. Many growers fold the bag repeatedly, both horizontally and vertically, then utilize zip ties. Clamps, paper clips, or sturdy rubber bands are also an option.
Quick Word on Alternatives
While mushroom grow bags are handy and excellent at what they do, we also understand that they pose issues of their own. The environmental impacts associated with the plastic waste they produce are of particular concern to many. While this can be very difficult to avoid for larger growers, we do encourage anyone motivated to try alternatives. Almost any vessel, such as a five-gallon bucket, can be turned into a container for mushroom cultivation if appropriately utilized. In a future article, we’ll cover how to grow mushrooms in a five-gallon bucket.
Conclusion
Well, there you have it, folks: everything you need to know about mushroom grow bags. If you have any questions, feel free to contact our experts at Redwood Mushroom Supply for more information. Until then, happy growing up!
