- 🍄 Spawn run is the important stage where mycelium fully colonizes the substrate to help strong mushroom growth.
- 🌡️ Stable temps between 21–25°C and high CO₂ help strong colonization and delay early fruiting.
- 🧼 Contamination risk peaks during spawn run. This shows why sterile technique is needed.
- ⚠️ Early fruiting can reduce yield—minimize light, lower oxygen, and wait for full colonization.
- ⏱️ Oyster and shiitake show different growth timelines and care needs throughout the spawn run stage.
What Is a Spawn Run in Mushroom Growing?
The spawn run is one of the most critical stages in mushroom cultivation. This phase begins right after inoculation, when the mycelium—a dense network of threadlike hyphae—spreads from the grain spawn into the substrate. During this time, the mycelium consumes nutrients and forms a strong, connected colony. A successful spawn run ensures contamination-free growth, proper nutrient absorption, and a solid foundation for high-quality mushrooms. Mismanagement, on the other hand, can cause stunted growth, premature fruiting, or contamination. Using reliable tools like mushroom grow bags helps provide the sterile, nutrient-rich environment needed for a vigorous spawn run and bigger harvests.
Where the Spawn Run Fits in the Mushroom Lifecycle
Mushrooms you grow have several main stages. Each one depends on the one before it working out:
- Inoculation: Putting mushroom spawn (often grain or sawdust with mycelium) into new, clean substrate.
- Mycelium Colonization (Spawn Run): This is the main part of growing. During this stage, the mycelium grows, spreads, and takes over the substrate, eating its food.
- Fruiting Starting: This starts because of certain things in the environment (like light, fresh air, or more humidity). The mycelium then begins to grow mushrooms.
- Harvesting: Once grown, you pick mushrooms before they release spores or get too old, which makes them worse.
The spawn run is the shift from just a possibility to actually making something. When the mycelium completely takes over the substrate, it means it is strong enough to fight off bad germs. It also stops partial fruiting and helps you get good harvests every time.
Benefits of a Good Spawn Run
- Gets the most out of biological efficiency (BE)—this is how many mushrooms you get compared to how much substrate you use.
- Helps with strong pin formation and mushroom growth.
- Beats out molds or bacteria by taking over the substrate quickly and completely.
- Makes a solid block or log. This holds water better and handles being moved around.
How Mycelium Colonizes a Substrate
After you put healthy spawn into the substrate, mycelium starts to work as a decomposer. Mycelium releases strong enzymes that break down complex organic stuff—like cellulose and lignin—into simpler things it can take in. You'll then see a visible, snow-white mass slowly taking over the substrate.
Substrate Choices and Colonization Behavior
- Straw: Many use this for oyster mushrooms. It colonizes fast because of its loose structure and big surface area.
- Hardwood Sawdust or Chips: Shiitake and lion’s mane like this. It colonizes slower, but it gives more food and good harvests.
- Hay, Corn Cobs, Coffee Grounds: Other compostable materials. They colonize at different speeds and often need extra food or pasteurization.
Colonization Timelines By Species
How fast colonization happens can vary a lot. It depends on the mushroom type, how the substrate is, and the environment.
| Mushroom Species | Typical Colonization Time | Notes |
|---|---|---|
| Oyster (Pleurotus spp.) | 10–14 days | Grows fast; changes easily with the environment |
| Shiitake (Lentinula edodes) | 14–21 days | Slower but strong; does better with a rest after colonizing |
| Lion’s Mane (Hericium erinaceus) | 12–16 days | Makes thick mycelium; likes a lot of humidity |
| Reishi (Ganoderma spp.) | 3–6 weeks | Colonizes very slowly; needs clean substrate |
(Source: Stamets, 2000)
Ideal Conditions During the Spawn Run Stage
It's very important to make the right environment. This helps the mycelium grow quickly and completely. Every factor affects how well and how fast colonization happens. And it affects if your grow can fight off problems like bacteria, molds, or early pinning.
Optimal Environmental Parameters
-
Temperature:
Steady warmth makes mycelium grow faster.- Oyster mushrooms: 21–24°C (70–75°F)
- Shiitake mushrooms: 22–25°C (72–77°F)
-
Humidity:
Substrates need to stay wet so enzymes can work. But they must not get too wet.- Aim for internal substrate moisture: 60–65%
- Room humidity: 70–80% (this stops the surface from getting dry)
-
CO₂ Levels:
Mycelium grows well with high CO₂. Because of this, you purposely limit fresh air at this stage.- This helps by: stopping fruiting and making the mycelium grow more. (Source: Lelley, 2009)
-
Light Exposure:
Mycelium does not need light to colonize. For mushrooms that react to light easily, like oyster, light can accidentally start fruiting.- Keep it dark or almost dark.
- Use blackout bags/tubs or grow them in dark rooms.
Common Signs of a Healthy Spawn Run
Healthy mycelium usually grows strongly, looks white, and spreads evenly. Looking at it and seeing small hints will tell you if your spawn run is going well:
- 🧱 Full Substrate Colonization: Thick white mycelium covers all the substrate you can see.
- 🕸️ Hyphal Knotting or Roping: Strandy, branching rope-like growth is a sign of mycelium growing fast and strongly.
- 🧪 Pleasant, Earthy Aroma: Smells similar to fresh forest floor or damp mushrooms.
- 🏗️ Stays together well: Substrate should feel solid and bonded. Loose or crumbly substrate means it hasn't grown well.
Don't break the substrate apart just to check if it's colonized. This can make it unclean and stop colonization. The only exception is if you are doing a break test to make sure the center is fully colonized.
Early Signs of Trouble
Even when everything is just right, problems can happen during the spawn run. The sooner you find them, the better your chance to save the grow.
Visual and Sensory Threat Indicators:
- ❌ Patchy or Uneven Colonization: This shows uneven moisture or that you didn't inoculate deep enough.
- 🌈 Off-Color Growth: Any green (Trichoderma mold), gray (cobweb), black (Aspergillus), or pink (yeasts) growth is a big problem.
- 💦 Condensation Buildup: Too much water can reduce oxygen for the mycelium or cause bacterial "sour rot."
- 🦨 Smells: Sour, ammonia-like, or vinegary smells mean infection from bacteria or yeast.
How to Prevent Early Fruiting
One common but expensive problem at this stage is early pinning. This is when mushrooms start to grow before the mycelium has fully taken over the substrate. Early pinning not only means you get fewer mushrooms, but it also takes energy away from the mycelium. This can lead to oddly shaped mushrooms.
Best Practices to Prevent Premature Fruiting
- 🛑 No Fresh Air Yet: Keep bags/containers sealed or lightly filtered. This traps CO₂ and stops pinning.
- 🚫 Limit Light: This is very important for oyster mushrooms, which react fast to light signals.
- 🌡️ Keep Temperature Steady: Changing heat or cold can stress the mycelium. This can make it seem like the seasons are changing, starting fruiting.
- 📆 Wait for Full Colonization: Many beginners make the mistake of starting fruiting conditions too soon.
Species-Specific Tips for a Successful Spawn Run
Different types of mushrooms grow in different ways. Knowing their special needs makes managing the spawn run much easier.
Oyster Mushrooms (Pleurotus spp.)
- These colonize fast, so they are great for beginners.
- They colonize best in pasteurized straw or sawdust with extra food.
- Light or oxygen too early will almost always start pinheads.
- Colonization is done when the substrate makes a tight “brick” and smells like fresh mushrooms.
Shiitake Mushrooms (Lentinula edodes)
- They like clean hardwood sawdust or blocks with extra food.
- They colonize slower because they are dense.
- Unlike others, shiitake does better with a 1–2 week “resting” or curing phase after colonizing. This is called browning, and it lets the mycelium get stronger.
- They are less sensitive to light, but they still won't grow mushrooms well if conditions are bad.
Avoiding Contamination Like a Pro
Contamination is likely the biggest danger during the spawn run. Good methods and tools greatly cut down this risk.
Contamination Prevention Checklist:
- ✔️ Sterile Technique: Always clean hands, tools, and containers. Use a lot of 70% isopropyl alcohol.
- 🧪 Still Air Box or Flow Hood: This makes a clean, controlled air space for inoculating and moving spawn.
- 🧼 Pasteurize or Sterilize Substrate Completely: You need this especially when using substrates with many nutrients (like coffee grounds or soy hulls).
- 📦 Use Filter Patch Bags: These let gas out, but they block particles and spores.
(Source: Royse & Sánchez, 2008)
Recommended Tools and Gear
The tools and setup you use can make the spawn run much easier:
- 🛍️ Pre-Colonized Grain Spawn: Easiest way to get a clean, fast start.
- 💉 Liquid Culture or Spore Syringes: For experienced growers creating their own spawn.
- 🥤 Sterile Grow Bags with Filter Patches: Allow CO₂ to exit while keeping out contaminants.
- 🌡️ Environmental Monitors: Digital sensors for temperature, humidity, CO₂.
- 🔧 Sealants & Myco Tape: Fix small leaks that could cause contamination or airflow.
- 🧪 High-Purity Alcohol Spray: For keeping tools and gloves sterile.
Consider trusted suppliers like Zombie Mushrooms for good kits and high-quality ingredients.
How to Know When the Spawn Run Is Complete
Want to know exactly when to go to the next step? Use this 3-part checklist to see if it's done:
- 🧼 Look at it: 100% of substrate surface is covered well in healthy white mycelium—no gaps.
- 💪 Structural Test: Substrate holds form when handled. Feels denser and bonded.
- 👃 Smell it: Should have a fresh, earthy, mushroom scent. Anything sour or weird? Wait or discard.
Optional: You can gently break open a small part to see if the middle is colonized. Many people do this with blocks that have extra food and larger substrates.
What Happens After the Spawn Run?
After colonization is done, you start the fruiting phase. You do this by changing things in the environment:
- 🔄 Introduce Fresh Air Exchange: A big drop in CO₂ helps pinning.
- 💡 Add Light (12 hours/day): Copies a seasonal sign to fruit.
- 🌡️ Adjust Temperature: Lowering temps by 3–5°C helps copy outdoor signs, especially in cool-weather species like shiitake.
- 💧 Raise Relative Humidity to 85–95%: Helps pins grow and stops tissue drying.
In 4–10 days (it depends on the mushroom type and conditions), you’ll start to see baby mushrooms—known as pins—growing upward.
Quick Answers to Common Spawn Run Questions
Can I start fruiting before full colonization?
No. If colonization isn't complete, your grow is very likely to get mold and give you poor harvests.
What if the mycelium seems to stop growing?
Check the environmental factors again. Small changes to temperature or how wet the substrate is usually get growth going again.
Why is there foggy buildup in my grow bag?
Condensation often means changing temperatures. Don't let it get too hot.
What do I do if I see early pins?
Cut them off. Put the block back into spawn run conditions. Wait for complete colonization.
Can I put a colonized block in the fridge before fruiting?
Yes, some growers “hold” blocks at low temperatures (often with shiitake) until they have other space to grow.
Final Tips for a Smooth Spawn Run
- You can't just set it and forget it here. Check conditions every day.
- Keep a record of your progress with photos and notes. This is very helpful for learning and doing it again.
- Don’t worry if growth slows. Small changes to the environment usually fix it.
- Use trusted, clean supplies and good quality spawn to cut down on guesswork.
- Learn from each run. Small improvements build up to expert skills.
Learning the spawn run well means you will grow mushrooms well. Do this step right, and you can get a big harvest.
References
Stamets, P. (2000). Growing gourmet and medicinal mushrooms. Ten Speed Press.
Lelley, J. (2009). Mushroom Biology and Mushroom Products. World Society for Mushroom Biology and Mushroom Products.
Royse, D. J., & Sánchez, J. E. (2008). Current practices in the commercial production of mushrooms. Agricultural Engineering International: CIGR Journal.
