Mushroom grow bags have become one of the most popular methods for cultivating gourmet and medicinal mushrooms at home — and for good reason. They're affordable, scalable, and when used correctly, they dramatically reduce contamination risk. But if you're new to mushroom cultivation, it can be hard to know where to start.
This step-by-step guide walks you through the entire process, from preparing your substrate to harvesting your first flush.
What You'll Need
Before you begin, gather your supplies:
- Mushroom grow bags — spawn bags, all-in-one bags, or fruiting bags depending on your workflow
- Grain (rye berries, millet, oats, or wheat berries) or a pre-made substrate
- Pressure cooker (15 PSI capable — a standard canning pressure cooker works great)
- Mushroom culture (liquid culture syringe, spore syringe, or agar wedge)
- Alcohol wipes / isopropyl alcohol (70%)
- Impulse bag sealer or zip ties
- Still air box (SAB) or flow hood (recommended but not required for beginners)
- Injection ports (if your bags don't have them built in)
Step 1: Prepare Your Substrate or Grain
The most common substrate for spawn bags is sterilized grain. Rye berries and millet are the most popular choices because they colonize quickly and hold up well to pressure sterilization.
How to hydrate grain:
- Rinse your grain thoroughly under cold water.
- Soak it in water for 12–24 hours.
- Drain and simmer on the stovetop for 15–20 minutes until the kernels are fully hydrated but not splitting or mushy.
- Drain and let the grain surface-dry on a towel or baking rack until the outsides are no longer visibly wet (usually 1–2 hours).
Proper hydration is critical. Grain that's too wet becomes anaerobic and promotes bacterial contamination. Too dry, and the mycelium won't have enough moisture to colonize efficiently. Aim for "field capacity" — the grain should clump briefly when squeezed but not drip.
For all-in-one bags, the substrate may already be pre-mixed into the bag. In that case, skip to Step 2.
Step 2: Fill Your Bags (Do Not Seal Yet)
Fill your mushroom grow bags to about 1/3 full — this leaves enough headspace for mycelium growth and prevents the filter patch from being blocked.
Once filled, invert the gussets so the bag sides lay flat, then fold the top over loosely. Do not seal the bags before sterilizing. Steam pressure builds rapidly inside the pressure cooker, and a sealed bag has nowhere to vent — the filter patch can't offgas that volume fast enough, and the bag might blow out. Leaving the bag unsealed with inverted gussets creates a wide, open throat that lets steam escape freely throughout the run.
Pro tip: Label your bags with the species, grain type, and date before sterilizing. A permanent marker on medical tape works great.
Step 3: Pressure Sterilize
This is the most critical step. Sterilization kills all competing bacteria, mold spores, and organisms that would otherwise out-compete your mycelium.
Settings:
- Pressure: 15 PSI
- Time: 2hrs at sea level, add 30min for every 1,000ft of altitude.
- Temperature equivalent: ~250°F (121°C)
Place your bags in the pressure cooker on a rack or trivet — not directly on the bottom, which can cause bags to melt. After sterilization, let the pressure drop naturally, then allow the bags to cool completely — ideally overnight. Inoculating warm bags can damage your culture and accelerates contamination risk. Once fully cooled, seal the bags with an impulse sealer or tight zip ties before moving to inoculation. Sealing with inverted gussets also gives you a much better impulse seal, since you're pressing through two flat layers instead of four layers of folded gusset.
Step 4: Inoculate
Once your bags are fully cooled to room temperature, it's time to inoculate.
If your bags have injection ports:
- Wipe the port with an alcohol wipe and let it dry.
- Flame-sterilize the needle of your syringe until it glows red, then let it cool for 5–10 seconds. If you have a new syringe, it's not necessary to flame sterilize since it comes sterile in the package.
- Push the needle through the port and inject 1–3 ml of liquid culture per pound of grain.
- Withdraw the needle — the self-healing silicone port seals itself automatically.
If your bags don't have injection ports, work in a still air box or flow hood. Wipe the bag exterior with alcohol, make a small hole with a flame-sterilized scalpel, inject your culture, and seal the hole immediately with micropore tape.
Step 5: Colonization
Place your inoculated bags in a clean, dark area at the ideal temperature for your species:
- Oyster mushrooms: 70–75°F
- Shiitake: 70–80°F
- Lion's Mane: 65–75°F
- Reishi: 75–85°F
During colonization, you'll see white, fluffy mycelium spreading through the grain. Healthy mycelium is bright white and smells earthy or slightly mushroomy.
Signs of contamination to watch for:
- Green or black patches = mold (Trichoderma, Aspergillus) — discard immediately
- Pink or orange tones = sporulating bacterial contamination — discard
- Foul, sour, or rotten smell = bacterial wet rot — discard
- Caramel color with some liquid = rod bacterial infection — keep an eye on it and discard if liquid is more than 2 inches deep or the light caramel color darkens. The liquid is a collection of secondary antibacterial metabolites. Fungi are surprisingly hardy against rod bacteria, but fighting it does weaken the organism and diverts significant energy away from producing fruit.
Colonization typically takes 2–4 weeks depending on species, inoculation rate, and temperature.
Step 6: Fruiting
Once fully colonized, move your bags to your fruiting environment:
- Temperature: Generally 5–10°F lower than colonization temp
- Humidity: 80–95% relative humidity
- Fresh air exchange (FAE): CO₂ buildup causes long, leggy stems and poor yields
- Light: 12 hours of indirect light per day (mushrooms use light as a directional cue, not for photosynthesis)
For fruiting bags, you can open the top and fold it down to expose the colonized surface, cut an X or slits in the side to encourage side fruiting, or remove the substrate block from the bag entirely and place it on a fruiting shelf.
Pins should appear within 5–14 days under good conditions. Maintain humidity by misting the air around the block, not directly on the pins — direct misting can cause aborts.
Step 7: Harvest
Harvest mushrooms just before or as the caps begin to flatten. For oysters, harvest when the caps are still slightly cupped. For lion's mane, harvest when the white teeth are fully developed but before yellowing begins.
Twist and pull the cluster off cleanly, or cut it at the base with a clean knife. Remove any leftover stub material to prevent contamination between flushes. After harvesting, let the block rest for 5–7 days — a second, third, or even fourth flush will follow.
Common Beginner Mistakes to Avoid
- Sealing bags before sterilizing: Always sterilize unsealed with inverted gussets — a sealed bag will blow out under pressure.
- Inoculating warm bags: Always wait until bags are fully room temperature.
- Skipping sterilization: Pasteurization is not sterilization. Spawn bags require full pressure sterilization at 15 PSI.
- Overcrowding the pressure cooker: Leave space for steam to circulate.
- Ignoring contamination: Remove contaminated bags immediately to protect your other grows.
- Not enough FAE during fruiting: If pins elongate without forming caps, increase fresh air exchange.
Ready to start growing? Browse our full selection of mushroom grow bags — spawn bags, fruiting bags, and all-in-one bags in 1, 3, 5, and 10-pound sizes, all made in America from American resin.
Pegasus Bags is a Texas-based supplier of pre-filtered mushroom cultivation bags. We partner with Eden Reforestation Projects and theGoodAPI to plant trees and remove ocean plastic with every order.