Why Aren't My Mushrooms Growing?

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• ⚠️ Over 70% moisture in mushroom substrate increases contamination risk (Chang & Miles, 2004).
• 🌡️ Mycelium growth rate drastically decreases when temperatures exceed 28°C (Royse et al., 2017).
• 🧫 Weak or contaminated inoculants are among the top failure causes for new growers (Stamets, 2010).
• 🧪 Up to 50% of home grows fail due to poor sterilization techniques (Stamets, 2000).
• 🍄 Using the wrong substrate can reduce mushroom yield by as much as 60% (Royse et al., 2017).

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If you are trying to grow mushrooms at home and your mycelium is not spreading through the mushroom substrate, you might feel frustrated—or even a little panicked. But do not worry, you are not alone. Slow or stopped mycelium growth is a common problem in mushroom growing, especially for new growers. This is a list of the most common reasons your mycelium might not be growing. It also tells you how to fix them.

What Healthy Mycelium Growth Looks Like

First, you need to understand how mycelium should grow. This helps you know when something is wrong. Healthy mycelium looks like a bright white, fuzzy, thread-like patch growing out from where you added the spores. In the first 5 to 10 days, you should see it grow steadily out across your mushroom substrate. Full colonization usually takes two to three weeks. This depends on the mushroom type, moisture, temperature, and spawn used. If you see no growth by day 14, it is time to look for problems.

Early signs of growth can be small. You will see tiny white spots form around the inoculation point. Then, over the next few days, these spots should multiply and join together into one solid white mass.

Too Much or Too Little Moisture

Mycelium, like all fungi, needs water to move nutrients and grow. But it does not need too much. The moisture level in your mushroom substrate greatly affects how well you grow mushrooms. A substrate that is too dry stops growth because the mycelium does not get the water it needs. Also, too much water creates conditions without air. This allows contaminants, especially bacteria, to grow well.

What is “Field Capacity”?

“Field capacity” is the term for the right amount of water. When you squeeze mushroom substrate in your hand, only a few drops of water should come out. No more. If water pours out or the substrate feels soggy, it has too much water. If it feels dusty and does not stick together, it does not have enough water.

How to Fix It:

• If your substrate is too dry:
  • Spray it lightly with sterile, distilled water.
  • Add boiled and cooled water slowly until it has the right amount of moisture.
• If your substrate is too wet:
  • Spread it out on a clean towel or tray. Let it air out in a clean place.
  • Mix it with drier mushroom substrate to make the levels even.
• For future grows, use a digital scale. Weigh the substrate before and after adding water for constant levels.

📊 Mycelium usually grows on substrates with 50–60% moisture. If moisture is over 70%, the risk of contamination grows fast (Chang & Miles, 2004).

Temperature Outside Ideal Range

Temperature directly affects how mycelium grows. Each type of mushroom has a “sweet spot,” which is the best temperature range for fast colonization. If the temperature is outside this range, growth slows down. It also makes contamination more likely, and can even kill the mycelium completely.

Most gourmet and medicinal mushrooms prefer room temperatures that are a little warmer. If it is too cold, mycelium grows very slowly. If it is too hot, bacteria or molds can take over.

Ideal Colonization Temperatures by Species:

More Tips:

• Use Wi-Fi thermometers to check temperature all the time.
• Put your grow chamber in the warmest room of the house in winter.
• Do not use hot attics in summer. Temperatures over 28–30°C can kill growth right away.

❄️ Pleurotus ostreatus grows much slower if the substrate gets hotter than 28°C (Royse et al., 2017).

Incomplete or Improper Sterilization

New growers often overlook sterilization. This causes many failures. Mycelium grows slowly and carefully. But contaminants like Trichoderma or Bacillus can grow faster and take over your culture in just one day. Any slip in cleanliness lets invaders start growing before your mushroom spawn can get established.

Common Mistakes:

• Insufficient time in pressure cooker: You must sterilize grain spawn or enriched sawdust substrates at 15 PSI for 90–120 minutes.
• Allowing cooling air exposure: If you open jars or spawn bags before they cool to room temperature in a sterile space, you pull in airborne spores.
• Improper bag sealing: Loose or unfiltered lids/bags allow oxygen—but also spores—to sneak in.

Recognizing Sterility Failure:

• Green-blue or black spotting (Trichoderma or Aspergillus).
• Pink or red spots (yeast or bacterial contamination).
• Liquid pooling inside jars or bags.
• Unpleasant smells—ranging from vinegar to socks.

How to Be Clean:

• Use 70% isopropyl alcohol to clean all work areas.
• Wear gloves and a mask.
• Put your spores or culture inside a still-air box or laminar flow hood.
• Heat needles and tools with a flame before each use.

🔬 Contaminants can increase the failure rate by up to 50% in home setups if you do not sterilize correctly (Stamets, 2000).

Weak or Contaminated Inoculant

It does not matter how perfect your mushroom substrate or conditions are. Bad spawn means a bad grow. Weak or contaminated inoculant is often too small to see at first. Because of this, even experienced growers test every new batch before growing more.

What’s Considered a Bad Spawn?

• Appears discolored—brown, green, or yellow.
• Feels mushy or gooey where it should be firm and fibrous.
• Smells sour, alcoholic, or rotten—instead of fresh mushrooms.

Causes:

• Spores or liquid culture stored at room temp for too long.
• Shipping damage during hot seasons.
• Contamination during lab production (happens even at good suppliers).

Be More Sure About Your Cultures:

• Test every new batch. First, move it to agar.
• Put it into one test grain jar before using a new syringe or spore print.
• Buy from good, well-reviewed sellers who promise fresh products.

🧪 Weak or contaminated inoculants are a main reason why new mushroom growers fail (Stamets, 2010).

Not Enough Gas Exchange

Mycelium breathes in oxygen and breathes out CO₂. If there is not enough gas exchange, CO₂ builds up inside the containers. This suffocates the growing culture. A sealed environment might seem like it stops contamination. But a tight seal without filters traps old air. This delays or completely stops growth.

Clear Signs of Poor Gas Exchange:

• Mycelium starts and stops growing abruptly.
• Your mushroom substrate smells fermented or sour.
• The bag or jar dome bulges outward intermittently.
• No visual condensation or overly wet walls (insufficient breathing).

How to Fix It:

• Use jars with lids that can breathe—tyvek paper, polyfill, or synthetic fiber discs.
• Use grow bags with filter patches (at least 0.2μm rating).
• For monotubs, make sure air flows through polyfill-filtered holes near the surface and top.

A Good Idea: Put micropore medical tape over any air holes. This helps keep things in, but still lets air move.

🌬️ Mycelium puts out CO₂. It needs good air flow to avoid stopping or contamination (Chang & Miles, 2004).

Wrong Substrate for the Species

One common mistake you can easily prevent: using the wrong substrate for the mushroom type you want to grow. Not all fungi digest the same food sources well. For example, oyster mushrooms grow well on plain straw. But lion’s mane needs more nutrients from hardwoods.

Matching Mushroom Species to Substrate:

Do Not Use the Wrong Substrate:

• Growing Lion’s Mane on straw leads to little growth.
• Growing oysters on a log might colonize, but very slowly.
• Using rich manure compost for wood-loving mushrooms can bring in bacterial contamination.

⚠️ Using the wrong substrate can reduce mushroom yield by up to 60%, depending on the mushroom type (Royse et al., 2017).

Patience: It Might Just Be Too Soon

You put in the culture, placed the bag in the grow chamber, and now you are checking it…every day. But mycelium does not grow right away. Many new growers think something is wrong after only waiting three or five days. In fact, full colonization often takes one to three weeks. This depends on temperature, substrate, and how you added the spawn.

Typical Mycelium Timelines:

Tips to Help You Wait:

• Write down what you do each day. Take notes and photos.
• Do not open your bags or jars every day. Looking inside brings risk.
• Use a flashlight behind the substrate to see growth you cannot see.

If you still see no sign of life after three full weeks, then assume you have contamination, failed inoculation, or a substrate problem.

Quick Visual and Sensory Checklist

A quick checklist can save time and help protect your harvest. Use the table below if you are not sure where to start:

Keep this printable version next to your workspace to make live assessments easier.

Troubleshooting Recap: Mistakes and Quick Fixes

• 💧 Too dry or wet substrate? Adjust moisture with spraying or drying.
• 🌡️ Temperature off? Use heating or cooling pads for the right range.
• 🧫 Not clean enough? Get a still-air box or HEPA filter hood.
• 🦠 Weak spawn? Test with a trial jar or petri dish. Try new, checked suppliers.
• 💨 No air flow? Add breathable filters or good monotub airflow.
• 🍄 Wrong mushroom substrate? Match each mushroom type to its best food.
• ⏳ Are you too eager? Wait at least 10–21 days. This depends on your spawn and grow method.

Final Thoughts: Learn from Failure in Mycology

Every mushroom grower has had a grow stop. What sets experienced growers apart is their ability to find problems and try again. If your mycelium is not growing, it is not the end. It is information. Each failed batch teaches you more about setting up your environment, choosing materials, and how living things react. The main things are to be consistent, write things down, and keep making changes. Growing mushrooms is about understanding fungi, and also about understanding your environment.

Do you need someone else to look at your grow? Send photos and join the talk at Zombie Mushrooms. We are a group of growers who help each other grow mushrooms better, one flush at a time.

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Citations

• Chang, S. T., & Miles, P. G. (2004). Mushrooms: Cultivation, nutritional value, medicinal effect, and environmental impact (2nd ed.). CRC Press.

• Royse, D. J., Baars, J., & Tan, Q. (2017). Mushroom cultivation in the 21st century: Trends and expectations. Applied Microbiology and Biotechnology, 101(1), 1–9.

• Stamets, P. (2000). Growing Gourmet and Medicinal Mushrooms (3rd ed.). Ten Speed Press.