Mushroom Grow Bag Not Fruiting? 6 Beginner Mistakes (and How to Fix Them)

Growing mushrooms at home with a ready-to-fruit bag is one of the most satisfying ways to start your cultivation journey. But like any living system, it helps to understand what your fungi need to thrive. These are the top six beginner mistakes we see (and how to fix them), so your block stays healthy and productive.

Already started? Check out our original post on how to maintain your first fruiting block for more hands-on care tips.

1. Cutting in the Wrong Spot (or Not Cutting at All)

Many bags arrive sealed to keep the environment sterile until you're ready. A common mistake is slicing the bag too low, too wide, or not at all. This can suffocate your mycelium or expose too much substrate, inviting contamination. Only cut the bag once the entire block has colonized to prevent competing organisms on uncolonized surfaces.

Carbon Dioxide is required in lethal levels during the inoculation process, once Oxygen is introduced the mushroom will almost always product a fruit at the site. This has been the process for the 1.5 billion years.

Fix: Cut an 2 inch 'X' or small window in the plastic about 2/3 up the block, where white mycelium is most dense. This gives your mushrooms an easy exit point while maintaining protective humidity. 

Lion's Mane (top fruiting technique)

Pro Tip: Species like Lion's Mane, and Reishi benefit from "top-fruiting" where the top of the bag is cut off and the mushroom will produce a heavier first-flush. Maintain proper humidity for this approach.

2. Over-Misting or Under-Misting

Mushrooms need moisture to fruit, but more isn’t always better. Over-misting can lead to bacterial blotch or encourage mold. Under-misting can dry out your pinning mushrooms, stalling growth.

Fix: Mist the area around the cut 2–3 times a day (not the surface directly-here's why), or tent the bag with a clear dome to help retain moisture. Keep a close eye—your environment matters more than the calendar.

3. Contamination

Contamination is one of the most common frustrations for beginner mushroom growers. It can show up as green, black, or fuzzy patches—usually mold or bacteria—competing with your mushroom mycelium for nutrients and space. Contamination is also nearly impossible to prevent, in theory. Unless you have airlocks and a clean room it is almost always an issue. Here is a list of 5 common contaminants you'll likely encounter.

How It Happens:

• Growing medium was not entirely colonized before inducing fruiting
• Improper sterilization or pasteurization of the substrate
• Touching the block or bag with unwashed hands or tools
• High humidity with poor airflow, creating a breeding ground for bacteria

What to Do:

If the contamination is minor and far from the fruiting site, you may still get a flush—just monitor it closely. But if the block smells sour, shows widespread discoloration, or loses vigor, it’s best to discard it safely and start fresh.

Pro Tip: Always have 90% isopropyl alcohol and hydrogen peroxide available when working with your grow. Change into clean clothes if possible. If you end up dealing with a microbial breakout, remove your grow materials and you can mix a bucket of high-strength bleach water and let it fumigate the grow area. The fumes alone will denature DNA and destroy all microorganisms, even those resistant to peroxide based sprays, like Trichoderma (green mold). Use precaution and proper ventilation for personal safety. Do not re-enter the area for 12 hours. Make sure to clean anything that was removed during the process.

When in doubt, take a photo and ask a mushroom community, like r/ContamFam (or tag us!). Mycelium can look weird during colonization, and not all discoloration is contamination.

Stay tuned for a blog on mitigation practices to help prevent this.

4. Too Little Light

Contrary to popular belief, mushrooms do need light—just not direct sunlight. In fact, light helps the mycelium know when it's time to fruit.

Fix: Place your grow bag in a room with indirect natural light or use a low-watt grow light only when fully colonized. If you can read a book in the room, it's probably enough.

Pro Tip: If you can get access to full spectrum LED lighting those have been shown to provide slightly higher yields. See this Reddit thread for what others have done.

5. Poor Air Exchange

Fungi breathe too. If your grow area is too stagnant (especially in sealed totes or small rooms), CO₂ can build up, causing tall, leggy mushrooms with small caps.

Fix: Use an oscillating fan if growing in a tent. If using a few blocks just gently fan them a few times a day or open your humidity tent briefly to refresh the air. If you see fuzzy stems or sluggish growth, airflow is usually the issue.

Pro Tip: Use CO2 to your advantage when colonizing growing medium, fold the bag and place it upside down to allow the levels to rise. Once colonized, you can cut the bag and provide more light to induce a beautiful flush!

6. Giving Up Too Early

Mycelium runs on its own time. Some grow bags fruit in 7 days, others take 2–3 weeks depending on species and conditions. Impatience or fear of failure leads many to toss a perfectly viable block. Usually, you will see a recovery patch form on the cut site - this will eventually become a pin set.

Fix: Wait it out. If the block still looks white and dense, it’s alive. Give it time, and you might get multiple flushes. Even a spent block can be buried outside for surprise mushrooms later.

What Growers on Reddit Say: Real-World Lessons

A popular thread on r/MushroomGrowers asked: “What’s the hardest part about growing mushrooms at home?” The responses echoed many of the same themes we hear from our customers:

• Patience: Waiting is often harder than it seems.
• Contamination: Especially with poor pasteurization or unclean environments.
• Temperature Swings: Indoor growing during heatwaves proved tricky.
• Overthinking: Many new growers try to optimize before they understand the basics.
• Too Many Species at Once: One user noted better results focusing on a single species at a time.

If you’re nodding along, you’re not alone. Let this be a reminder: mushroom cultivation is a long game, and every grow teaches you something valuable.

Don’t Just Grow Mushrooms—Grow Your Know-How

Mushroom cultivation is part science, part observation. Pictured above, was our first time growing Cordyceps in mason jars on brown rice with nutrient broth. These common grow bag mistakes aren’t failures; they’re part of the learning curve. Whether you’re growing oysters on your windowsill or dreaming of a full backyard myco-bed, every bag teaches you something.

Ready to level up? Start by revisiting our last post on how to maintain your fruiting block—and let your next flush be even better.

Disclaimer: This blog is for educational purposes and not a substitute for professional cultivation advice. Always follow food safety guidelines when growing and harvesting mushrooms at home. Never consume mushrooms found in the wild.