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- 🍄 Spawning to bulk gets more mushrooms by letting mycelium grow into a substrate full of nutrients.
- 🔥 Pasteurizing substrate keeps good microbes. These are important for fighting off contamination.
- 💧 Proper moisture (field capacity) is important for mycelium to spread and for stopping bacteria from growing.
- 🏡 Monotubs and outdoor beds both let you grow many mushrooms in bulk.
- 🧪 Sterilization is only for preparing spawn, not for bulk grows. This avoids contamination in places that are not sterile.
If you want to move past small-scale mushroom growing methods, like the PF-Tek or starter grow kits, spawning to bulk is a way to get more mushrooms. It also means fewer containers and easier upkeep. This method mixes colonized spawn with freshly pasteurized bulk substrate. This helps mycelium spread fast and grow more mushrooms. If you are growing oyster, wine cap, or dung-loving species, spawning to bulk is the main way to grow many mushrooms effectively. If you want to turn a single jar of spawn into pounds of mushrooms, here's what you need to know.
What Is Spawning to Bulk?
Spawning to bulk means growing an already colonized mushroom culture (the "spawn") into a larger amount of pasteurized substrate. You usually do this in a monotub, tray, or outdoor bed. Mycelium does this to spread out and get more food, just like it does in nature.
In this method, your colonized grain or PF-Tek cakes act as the starter to seed a substrate that has food. This can be coco coir, straw, or manure. Once the bulk substrate is fully grown, it can produce many mushrooms, often several times.
Good Things About Spawning to Bulk
- ✅ More mushrooms – a single quart of spawn can start 5+ quarts of substrate.
- 🧴 Less chance of contamination – Pasteurizing substrate (instead of sterilizing it) helps good microbes fight off bad mold or bacteria.
- 🏗️ Can grow a lot – use this method indoors in tubs or outdoors in garden beds or trays.
- 🔄 Good for the environment – bulk substrate materials are cheap, can be renewed, and can be composted.
Spawning to bulk is good for growers who know a little bit already. It's easy to do and gives big results.
What You’ll Need to Get Started
You don't need expensive equipment to start bulk cultivation. You can use many household items for other purposes, and some special supplies help.
🍄 Grain Spawn
This is your starter. You can either:
- Buy prepared grain spawn (usually sterile and already has mycelium), or
- Make your own by cooking, sterilizing, and adding starter to grains like rye, wheat, or millet.
If you are moving from the PF-Tek method, fully colonized BRF cakes can also be used as cake spawn to grow more.
✅ Tip: Always make sure the spawn is fully grown and clean before you use it, especially when spawning to bulk.
🌾 Bulk Substrate Options
The substrate is where the mushrooms get food. Different mushrooms like different substrates:
- Coco coir + vermiculite (good for Psilocybe cubensis or lion’s mane)
- Straw (good for oysters and wine caps)
- Manure/straw blends (good for dung-loving species like P. cubensis)
- Wood chips/cardboard (used for wine caps, shiitake, and other fungi that like wood)
Note: Pasteurizing substrate is important, no matter the material.
🧰 Basic Supplies
- A monotub (a clear tub with holes for air)
- Bucket for pasteurization (a 5-gallon size is common)
- Thermometer (a probe one is best to check the middle of the substrate)
- Isopropyl alcohol (70%), gloves, and a clean place for mixing
- Optional but helpful: humidity reader, polyfill or micropore tape for air filter holes, and liners for tubs
Zombie Mushrooms and other places sell kits that have everything already packed and ready to mix.
Why Pasteurization Beats Sterilization for Bulk Substrate
In a clean lab, sterilizing makes sense. But for bulk grows, especially in places that aren't perfectly clean (like a closet or garage), pasteurizing your substrate is safer and easier.
What Happens During Pasteurization?
Pasteurization heats your substrate to between 149–185°F (65–85°C). This:
- Kills off common contaminants like mold spores, bacteria, and other fungi
- It keeps good microbes (like Actinomycetes) that naturally fight off bad germs.
- This makes a stable, balanced world of microbes in your substrate, which helps mycelium grow well.
“Pasteurization of bulk substrate kills competing bacteria and fungi while leaving good microbes untouched.”
— Stamets, 2000
Why Not Sterilize?
Sterilization kills all microbial life—both good and bad. It's needed in very controlled places (for example, for sterile liquid culture or grain spawn prep), but sterilization:
- It removes good microbes. This leaves an empty space where bad germs can easily grow.
- It needs a pressure cooker or autoclave.
- It's not good for the environment or easy for backyard or closet mushroom growers.
In short: Pasteurize bulk substrate, sterilize grain spawn. Do them differently, but for the same goal.
How to Pasteurize Your Bulk Substrate (Using Bucket TEK)
One of the easiest ways for pasteurizing bulk substrate is the Bucket TEK method. It's named for using an insulated 5-gallon bucket to keep heat in.
Bucket TEK (Good for Coir/Vermiculite)
- Boil Water: Heat about 3.5–4 quarts of water until it boils hard.
- Break Substrate: Break apart a 650g brick of coco coir. Mix it with 2 quarts of dry vermiculite in your clean bucket.
- Pour/Seal: Carefully pour the boiling water into the bucket. Stir well with a clean tool.
- Seal and Cover: Close the lid tightly. Wrap the bucket with towels or put it in an insulated box. Leave it for 60–90 minutes.
- Cool and Check: After it cools to room temperature, check for field capacity and mix in spawn.
Pasteurizing Straw
- Prepare the straw: Chop it into 1–3 inch pieces so it grows faster.
- Heat water: Bring a pot of water to 160–170°F.
- Dunk and Wait: Soak the straw for 60–90 minutes, keeping it at the right temperature (you must use a thermometer).
- Drain and Cool: Put the straw on a clean tarp or mesh screen and let it drain and cool for a few hours before you use it.
Mistakes to Avoid
- ⛔ Heating the substrate above 185°F will accidentally sterilize it.
- ⏲️ Not letting it cool down completely will harm your spawn with heat.
- 👃 Bad smells? Sweet, sour, or chemical smells mean it's contaminated—start over.
Getting Moisture Right: The Field Capacity Test
Moisture balance is one of the most important parts of mushroom cultivation. A wet substrate leads to bacteria that live without air, and rot. A dry one slows or stops it from growing at all.
What Is Field Capacity?
Field capacity is the most water a substrate can hold after extra water has drained. At this point, the substrate feels wet but isn’t dripping.
Field Capacity Test (Simple Squeeze Test)
- Grab a handful of your pasteurized, cooled substrate.
- Squeeze it hard over a clean surface.
- You should see 1–2 drops of water. It shouldn't stream, and it shouldn't be dry crumbles.
- Too wet? Add dry vermiculite. Too dry? Mist water and mix again.
“If the substrate isn't wet enough or is too wet, it can raise the chance of bacterial contamination by up to 30%.”
— Trudell & Ammirati, 2009
Spawning PF-Tek Cakes to Bulk Substrate
Moving old PF cakes to a bulk system is a good way to improve your grow using few materials.
Step-by-Step: Spawning PF-Tek Cakes to Bulk
- Make sure it's fully grown your BRF cakes—completely white and firm.
- Carefully take cakes from jars and break them into chunks (don't turn it into powder).
- Mix with bulk substrate using a 1:2 spawn-to-substrate amount (for example, 2 jars of cakes + 4 jars of coir/verm).
- Put layers, then add ½-inch more substrate on top to keep moisture in.
- Close your grow container or tub and keep it in a dark place around 75–80°F to grow.
✅ Cakes with P. cubensis mycelium grow well in bulk substrates like coco coir. They become stronger and produce bigger mushrooms.
1. Setting Up a Monotub for Bulk Indoor Grows
The monotub is an important tool for indoor bulk mushroom cultivation. It makes the right small environment for growing mycelium and fruiting.
Make Your Monotub
- Use a clear storage tub (about 60–80 quarts)
- Drill ¼" holes on each long side, about 4 inches up from the bottom (for air).
- Fill holes with polyfill or cover with micropore tape.
- Add a black plastic liner so mushrooms don't grow on the sides. You can cut up a trash bag!
- Put in your mixed spawn and substrate blend in layers, like lasagna.
- Put the lid on loosely so fresh air can get in (or turn it upside-down).
Watch During Growth:
- Temp: 75–80°F
- Lighting: Low indirect light (avoid darkness and avoid direct sun)
- Humidity: 95% until pinning
- Check daily: Don't disturb it much, but look for contamination or if it's drying out.
Once you see pins, give it more air and slightly less humidity to make more mushrooms grow.
Why People Like Monotubs for Home Mushroom Growing
The monotub setup has become a top choice for home mushroom growers. This is because they are simple, work well, and give good harvests. Monotubs are plastic boxes made good for growing mushrooms. They are used for the spawn to grow and then for the mushrooms to fruit. With small changes, you can make a great small space for mushrooms to grow.
Things people like about monotubs include:
- Easy to Use: Not many parts, not much to do.
- Costs Less: Materials are cheap and easy to find.
- Good Control: They hold humidity well. Air holes and liners help air move.
- Can Make More: You can easily go from one monotub to many to grow more.
- Good for Beginners: You don't need expensive gear.
Monotubs are very liked by people growing types like Psilocybe cubensis. This is because they are easy to use and can grow a lot.
2. Spawning to Outdoor Beds
If you have yard space or a shaded garden corner, outdoor beds give you mushrooms with little work and not much equipment.
How to Make an Outdoor Grow Bed
- Choose the Spot: Choose a damp, shaded spot—under shrubs, trees, or even behind a shed.
- Layering: Start with cardboard on the soil. Put alternating layers of straw/wood chips and your spawn. Total depth: 4–6 inches.
- Moisture: Water lightly but always. Check weekly for dryness.
- Covering: Add a cloth, leaves, or straw mulch to keep moisture in and keep pests away.
Most outdoor bed species (like wine caps) grow in 2–5 weeks and can make mushrooms many times a year with little effort.
3. Picking the Right Grain Spawn and Substrate
Picking good materials is key to getting the substrate colonized and growing mushrooms. Both your spawn and substrate are very important, but in different ways, for growing mushrooms.
✅ Best Grain Options:
- Rye Berries: Have lots of food and resist contamination well. Many people, new and old, use rye berries.
- White Millet: Are smaller. This gives mycelium more surface area to grab. Colonization happens faster.
- Sorghum: Another grain that works well, like millet. People often mix it in to make colonization as fast as possible.
Want to skip putting in the spawn and early steps? Pre-colonized options from trusted places like Zombie Mushrooms are clean, fast, and lower the chance of contamination.
🥥 Most Popular Bulk Substrates:
- Coco Coir: Easy to get ready. Holds water very well. Doesn't easily get contaminated.
- Vermiculite: Makes mixes fluffy and airy. Often used with coir.
- Aged Horse Manure: Has lots of food. Some mushroom types like it. You need to age it right and pasteurize it.
- Straw & Gypsum: Used a lot by some big growers. It's usually chopped and wet so it soaks up water better.
Good substrate blocks from companies — especially mixes from places like Zombie Mushrooms — take away the guessing. They help you get the same results each time.
4. How Much Spawn and Substrate? Good Amounts Explained
Getting the right amount of spawn and bulk substrate makes a big difference. The spawn-to-substrate ratio helps decide how easily your mycelium will take over the substrate. This helps grow healthy mushrooms.
🧪 Recommended Ratios:
-
1:2 to 1:3 by volume: Most people agree this is best for most beginner mushroom grows.
- For every 1 quart of grain spawn, use 2–3 quarts of bulk substrate.
💡 Why This Matters:
- More Spawn = Faster Results: More spawn means faster colonization. This helps pins start sooner and lowers the chance of contamination.
- Less Spawn = Slower Progress: Less spawn makes colonization take longer. This increases the risk of mold, bacteria, and growth stopping.
If you are new, it's best to start with a 1:2 ratio. It's easier to get right.
5. Getting Bulk Substrate Ready the Right Way
Ideal Colonization Conditions
Making the best environment for substrate to grow is important for success:
- Temperature: Ideally 75–80°F (24–27°C)
- Humidity: 90–95% during growth and pinning
- Airflow: Medium. Use FAE holes or crack lids to let gases move.
- Lighting: Indirect ambient light (12-hour cycles are good)
Signs that growth is progressing well:
- A white, fluffy surface will show up over time.
- No bad smells or color changes.
- You'll start to see mushroom pins after 10–14 days (this can change)
6. When (and Why) to Use a Pressure Cooker
While pressure cookers aren’t needed for spawning to bulk, they are important tools for some stages of mushroom cultivation.
Use a Pressure Cooker to:
- 🍚 Sterilize raw grain to make your own spawn.
- 🍵 Prepare agar media for tissue growth or cloning.
- 💉 Sterilize syringes and tools for putting in the starter.
Run your pressure cooker at 15 PSI for 90–120 minutes, depending on how much is in it. Always let it cool completely before opening.
7. Mixing Spawn and Substrate to Colonize Best
Mix spawn right into the substrate instead of layering. This makes colonization more even. It also makes the process much faster.
📋 Mixing Tips:
- Wear nitrile gloves cleaned with rubbing alcohol.
- Gently break up the grain spawn inside its bag or jar. Separate the kernels that have lots of mycelium on them.
- Mix well but be gentle with the bulk substrate in your monotub.
- Try to spread it out evenly. This makes sure colonization is even.
Be careful not to break it up too much. If you harm the mycelium, it can take longer to get back to normal and growth will be slower.
8. Handling Colonization Conditions
After mixing, let the mycelium do its job.
🕒 Optimal Colonization Conditions:
- Temperature: 70–75°F is best for most types.
- Humidity: Stays naturally inside sealed monotubs.
- Light: Little light or just room light; not needed now.
- Airflow: Keep holes mostly sealed with micropore tape. This lets in less fresh air.
- Monitoring: Don't open the monotub often. Let it ‘incubate’ without bothering it for 10–21 days. This time depends on the temperature and strain.
You know it's done when a thick white mat of mycelium covers the top. You won't see any brown substrate left.
9. Going from Colonization to Fruiting
When colonization is finished, you can start fruiting the mushrooms. You do this by changing the conditions to be more like nature.
📈 Fruiting Triggers:
- More Airflow: Loosen or take off micropore tape in the top holes. This lets more fresh air in.
- Daily Misting & Fanning: Mist the sides to keep humidity up. Fan gently 1–2 times each day.
- Light: Give it indirect natural light. Or put it under a 6500K daylight bulb for 12 hours a day.
- Temp & Humidity: Keep it at 70–75°F. Keep humidity above 85%.
You will see “pinning” (tiny mushroom heads) in a few days. Things happen fast once pinning begins!
10. Finding and Stopping Contamination
Contamination can spoil weeks of work. Learn to spot and handle problems early. This helps you get better flushes.
🚫 Common Contaminants:
- Trichoderma (Green Mold): Grows fast in green patches; very bad.
- Cobweb Mold: Looks like thin, gray spiderwebs; easy to mix up with healthy mycelium.
- Bacteria (Slime): Looks wet, often smells bad.
🧼 Prevention Essentials:
- Always use gloves and clean tools/surfaces.
- Don't cut back on pasteurization time or heat.
- Use more spawn compared to substrate. This lets the mycelium grow faster and fight invaders better.
- Seal monotub holes well while it's colonizing. Don't let air flow too early.
When you first see contamination, you can try to cut it out. But starting over completely is often safer.
11. Tips for Harvesting and Storage
Harvesting at the right time makes them stronger and bigger.
🍄 When to Harvest:
- Veils under mushroom caps start to break open.
- Caps are still curved, not flat or turned up.
Use gloved hands to twist and pull from the bottom. Or cut stems with clean scissors. Be careful not to hurt the healthy mycelium. It will help with future flushes.
📦 Storing Mushrooms:
- Dry Completely: Use a dehydrator at 95–110°F until they break like crackers.
- Seal Well: Put in mason jars with drying packs or vacuum-seal bags.
- Keep in a dark, cool, dry place. This keeps them strong and stops mold.
12. Fixing Common Monotub Issues
Here are common problems and how to fix them:
| Issue | Possible Cause | Solution |
|---|---|---|
| Colonization stopped | Substrate too wet or dry | Check how wet it is, control temperature |
| Strange colors/smells | Contamination | Take out bad parts or start over |
| No mushroom pins | Not enough air/light | Loosen tape, mist and fan more |
| Side pinning | Didn't use tub liner | Use black plastic liner, block light on sides |
To fix problems well, check things every day!
13. Supplies from Zombie Mushrooms We Suggest
Ready-to-use things can take away the guessing. They also lower the chance of contamination. Things people like include:
- 🧪 Grain Spawn Bags with Mycelium: Skip putting in the spawn and make colonization faster.
- 🌾 Coco Coir Mix Substrate Blocks: Have the right water level, ready for pasteurization.
- 💡 Monotub Grow Kits: Come with a tub with holes, polyfill, liners, and substrate.
- 🧫 Agar Cultures and LC Syringes: Start your own mushroom types.
Zombie Mushrooms sends materials chosen with care. This makes growing easier, cleaner, and gives more mushrooms.
If you use a monotub setup and know how to spawn bulk substrate the right way, growing mushrooms at home is a good hobby. It's also a very good way to grow a lot. Follow the steps in this guide. Be very careful with cleaning. Keep making your methods better. And then you will get many flushes soon.
