How to Use Mushroom Grow Bags: Sizes, Sterilization & Fruiting

Mushroom grow bags are the unsung heroes of home and commercial mushroom cultivation. They're what make it possible to prepare sterile grain spawn and fruiting blocks without expensive laboratory equipment. If you've ever wondered how cultivators produce clean, contamination-free mushroom blocks, the answer usually involves a good grow bag, a pressure cooker, and a bit of technique.

In this guide, we'll cover everything you need to know about mushroom grow bags — from understanding filter patches to choosing the right size, sterilizing, inoculating, and fruiting directly from the bag.

What Are Mushroom Grow Bags?

Mushroom grow bags are specially designed, heat-resistant polypropylene bags that can withstand the temperatures and pressures of autoclave sterilization (typically 121°C / 250°F at 15 PSI). Unlike regular plastic bags, which would melt or off-gas toxic fumes under these conditions, autoclavable grow bags maintain their structural integrity through repeated sterilization cycles.

The bags are made from thick, puncture-resistant polypropylene — typically 2.5 to 3.5 mil thick. This durability is important because you'll be handling them when they're full of heavy, wet substrate, and any small tear or puncture is an open invitation for contamination.

The Filter Patch Explained

The defining feature of a mushroom grow bag is the filter patch — a small square of microporous material heat-sealed into one side of the bag. This patch serves two critical functions:

Gas Exchange: Mycelium needs oxygen to grow and produces carbon dioxide as it colonizes substrate. The filter patch allows this gas exchange to happen continuously without opening the bag.

Contamination Barrier: The filter patch (typically 0.5 micron pore size in our bags) blocks bacteria, mould spores, and other contaminants from entering. To put that in perspective, most airborne contaminant spores are 1–20 microns in size, so a 0.5-micron filter provides an effective barrier against the vast majority of threats.

This combination — gas exchange plus contamination protection — is what allows a sealed grow bag to function as a miniature clean room. Once your substrate is sterilized and the bag is sealed, the contents remain protected from environmental contaminants as long as the bag's integrity is maintained.

Our Three Sizes: Which Bag for Which Job?

We carry three sizes of Satrise grow bags, each designed for different applications.

Medium (5" x 5" x 20")

The Medium Grow Bag is our most popular size for grain spawn production. Its narrow profile is ideal for:

• Grain spawn jars alternative: Fill with 2–4 lbs of hydrated grain (rye, wheat, millet, or oats), sterilize, and inoculate with liquid culture or agar. The bag format is easier to shake for break-and-shake than jars, and the flat shape means faster, more even colonization.
• Small fruiting blocks: Great for personal-use fruiting blocks using supplemented sawdust. A single medium bag produces enough for a small but satisfying harvest.
• Experimental batches: When you're testing new substrate recipes or species, the smaller size means less wasted material if something doesn't work out.

Large (8" x 5" x 18")

The Large Grow Bag is the workhorse size for home cultivators making fruiting blocks. Its wider footprint accommodates:

• Standard fruiting blocks: Fill with 5–7 lbs of supplemented hardwood sawdust (typically mixed with wheat bran at a 10–20% ratio). This is the standard size used by most home growers for species like oysters, shiitake, and lion's mane.
• Larger grain spawn batches: When you need more spawn for a bigger project, the large bag holds 5–8 lbs of grain comfortably.
• Straw-based substrates: The wider opening makes it easier to stuff in bulkier materials like chopped straw for oyster mushroom cultivation.

XL (8" x 5.5" x 25")

The XL Grow Bag is designed for commercial-scale production and ambitious home growers:

• Commercial fruiting blocks: Holds 8–12 lbs of supplemented sawdust, producing larger blocks that yield more mushrooms per unit.
• Bulk substrate preparation: Ideal for preparing large quantities of pasteurized or sterilized substrate for species that benefit from bigger blocks.
• Masters mix blocks: The extra volume is perfect for the popular 50/50 soy hull and hardwood pellet recipe that many commercial growers use for king oyster, lion's mane, and other high-value species.

How to Fill and Seal Your Grow Bags

Proper filling and sealing technique is essential for preventing contamination during sterilization.

Step 1: Prepare Your Substrate

Your substrate should be properly hydrated before bagging. For grain spawn, soak and simmer grain until fully hydrated but not waterlogged. For sawdust fruiting blocks, mix hardwood pellets with water and supplements to 60–65% moisture content. Our Bulk Substrate Calculator gives you exact weights for any number of bags so every batch comes out right.

Step 2: Fill the Bag

Fill to approximately 60–70% capacity — you need room at the top for folding, sealing, and expansion during sterilization. Pack gently to eliminate air pockets without over-compressing.

Step 3: Fold and Seal

Impulse sealer (recommended): Fold the top flat, press out excess air, and run through an impulse sealer. Make two parallel seals for security.

Fold and clip method: Fold the top accordion-style and secure with a binder clip or zip tie. Less reliable but workable without a sealer.

Important: Ensure the filter patch has clear airspace above the substrate — it must not be buried.

Sterilization: Autoclave or Pressure Cooker

This is the most critical step. Inadequate sterilization is the number one cause of contamination in mushroom cultivation.

Pressure Cooker Method (Home Growers)

Load your filled, sealed bags into your pressure cooker. Place them on a trivet or rack so they're not sitting directly on the bottom. You can stack multiple bags, but make sure steam can circulate freely.

Bring to 15 PSI and maintain for:

• Grain spawn: 90 minutes minimum (some growers prefer 120 minutes for extra safety)
• Supplemented sawdust: 150–180 minutes (2.5–3 hours)

Allow the pressure cooker to cool naturally. Do not rapid-release the pressure — sudden pressure changes can cause bags to burst or the filter patch to blow out.

Autoclave Method (Commercial Scale)

Commercial autoclaves provide more consistent temperature and pressure control. Cycle times are similar: 90 minutes for grain, 150+ minutes for supplemented substrates, at 121°C / 15 PSI.

After sterilization, allow bags to cool completely to room temperature (8–12 hours) before inoculation. Inoculating warm substrate can kill your inoculant and favour heat-loving contaminants.

Inoculation

Once your bags are cool, it's time to introduce your mushroom culture. Work in the cleanest environment possible — ideally in front of a laminar flow hood, or at minimum in a still-air box.

From liquid culture: Wipe the injection area with isopropyl alcohol and inject 3–5 cc of liquid culture using a sterile syringe. If no injection port, cut a small corner, inject, and reseal.

From grain spawn: Open the bag in your clean workspace, add colonized grain spawn, and reseal. Faster colonization due to more inoculation points.

From agar: Transfer a wedge of colonized agar using sterile technique. Slowest method, but ideal when working from freshly isolated cultures.

Fruiting From the Bag

Many species can be fruited directly from the grow bag, eliminating the need to remove the block and exposing it to contamination risks.

Method 1: Cut and Fruit

Once the block is fully colonized (completely white with mycelium), cut an X or a 3–4 inch slit in the side of the bag. Mist the exposed area 2–3 times daily and maintain humidity above 80%. Mushroom pins will form at the cut site within 5–14 days depending on the species.

Method 2: Top Fruiting

Fold down the top of the bag to expose the top surface of the block. This works well for species like lion's mane that prefer to fruit from a single point, and for oyster mushrooms in humid environments.

Method 3: Remove From Bag

For some species — particularly shiitake — you may get better results removing the block from the bag entirely and fruiting it on a wire rack or shelf in a fruiting chamber. Shiitake blocks benefit from a "cold shock" (12–24 hours in a refrigerator) before fruiting.

Common Mistakes to Avoid

• Overfilling bags — leave room for expansion and sealing.
• Covering the filter patch with substrate.
• Insufficient sterilization — err on the side of longer.
• Inoculating while warm — always wait for room temperature.
• Rough handling — sharp impacts can create micro-tears.

Explore More

Ready to dive deeper into mushroom cultivation? Check out our complete substrate guide for detailed recipes, our beginner's complete guide for a start-to-finish overview, or our shiitake growing guide for species-specific tips.

All Nature Lion grow bags, spawn, and substrates are available from our CFIA-licensed facility in Brantford, Ontario. We're growers ourselves, and every product we sell is one we use in our own cultivation.