How to Prevent Contamination in Mushroom Growing (Trichoderma & More)

Every mushroom grower—no matter how experienced—has opened a bag to find the dreaded green fuzz. Contamination is part of the journey, but that doesn’t mean it has to take over your grow room. Whether you’re using pasteurized straw or sterilized substrate bags, understanding how contamination happens (and how to prevent it) is the key to a clean and productive harvest.

What Causes Contamination?

Contaminants like Trichoderma (green mold), bacteria, or black pin mold are always floating around in the air. They’re opportunists, waiting for the perfect conditions to take hold: too much moisture, unclean tools, improper sealing, or uncolonized sections of substrate.

The most common culprits:

• Unsterile tools or hands

• Overly wet substrate

• Poorly sealed bags or containers

• Inadequate pasteurization or sterilization

• Dirty grow space or equipment

• Grain spawn that was already contaminated

How to Keep It Clean

1. Start with clean spawnIf your spawn is contaminated, everything else will be too. Buy from trusted sources or follow strict sterile practices when making your own.

2. Dial in your moistureContaminants thrive in soggy substrate. Do the squeeze test—if a few drops come out when squeezed hard, it’s just right. If it pours water, it’s too wet.

3. Improve air exchange (but not too much)Stale air can encourage mold, but wide-open bags invite contamination too. Use filter patches or micro-perforations to strike the balance.

4. Keep it clean and consistentWipe down surfaces with 70% alcohol. Use gloves. Work quickly. If using a still air box or flow hood, let it run for 10–15 minutes before starting your work.

5. Monitor temperature and humidityHigh temps and wet conditions can speed up mold growth. Most gourmet mushrooms do best in cooler, stable conditions—aim for consistency.

Low-Tech vs. Sterile Contamination

In low-tech growing, contamination usually happens after inoculation because pasteurization doesn’t kill everything. That’s okay—your spawn just needs to colonize faster than the mold.

In sterile growing, any contamination is usually from you, your tools, or your spawn. Since the substrate is completely sterilized, contaminants only enter through human error. The solution? Better technique, not better luck.

What to Do If You Spot It

See a patch of green? If it’s small and isolated, you can sometimes cut it out (outside, wearing a mask). But more often, it’s best to toss the whole bag. Never open contaminated bags indoors—it’ll spread spores and invite future issues.

Final Thoughts

Contamination is frustrating, but it’s not the end of the world. Each failed bag is a clue—an opportunity to tighten up your process and get better. The cleanest growers aren’t just lucky—they’re consistent, meticulous, and always learning.

Have you fought off Trichoderma and won? Share your best tip in the comments! Let’s learn from each other and keep those flushes clean and full.