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- 🌫️ Oyster mushroom pins stop growing when humidity is lower than 85% or stale air causes CO₂ to build up.
- 💡 Indirect light (12 hours/day) helps oyster mushrooms grow past the pin stage.
- 🌡️ Temperatures between 60–75°F (15–24°C) are best for fruiting; very hot or cold conditions can stop oyster mushroom growth.
- 🦠 Contamination, such as Trichoderma or bacteria, can take over a block before or while pins form.
- ⚙️ Monitoring tools, like hygrometers, greatly increase success by reducing human error.
What To Do When Mushroom Pins Are Not Growing: A Guide for Oyster Growers
Your oyster mushroom pins have formed, but then stalled—sitting as tiny primordia that never mature, or even drying out completely. This is one of the most common frustrations for new growers. Pinning is a delicate stage in mushroom cultivation, requiring just the right balance of humidity, airflow, and light. If conditions slip, growth stops. Fortunately, with the right setup and reliable tools like mushroom grow bags, you can stabilize your environment and get those pins developing into healthy flushes. This guide explains the main reasons mushroom pins stop growing—and how to adjust your grow for a successful harvest.
What Are Mushroom Pins and Primordia?
To understand oyster mushroom pins, you should know their life cycle. First, mycelium completely takes over the substrate. Then, it changes from a growth phase (where mycelia grow and eat nutrients) to a fruiting phase. This change makes primordia. These are tiny white or grayish knots that show up on your grow medium.
These knots are the first signs of mushrooms. Growers call them pins. With the right conditions, these pins quickly grow into full-sized oyster mushrooms. But if conditions are bad, pin growth might stop completely. Also, the mushrooms might grow into odd shapes.
It's important to know if you have stalled pins or if primordia just aren't forming well. Good control over the environment during this key time helps decide if you get a good harvest or a slow, disappointing growth.
1. Humidity Levels Are Too Low or Too High
Humidity might be the most important thing for pin growth. Mushrooms are mostly water. So, they need a wet place to grow fruit bodies. Specifically, oyster mushrooms need 85–95% relative humidity (RH) for pins to form well. And then they need steady humidity to keep growing those pins into mushrooms (Stamets, 2000).
Signs Your Humidity Is Off:
- Pins get darker, turn brown, and dry out.
- The surface of the pins looks wrinkled.
- Mushrooms start to grow but then stop after a few millimeters.
How to Balance Humidity:
- Mist the air and chamber walls. Do not mist directly on the pins, this can drown them.
- Put a water container, like perlite trays, at the bottom of your chamber.
- Use covers like humidity tents, plastic bags, or changed totes to keep moisture in.
In grow tents or fruiting chambers, use digital hygrometers to watch humidity all the time. But do not fix it too much. Very high humidity with still air can also cause water to build up. This can drown or harm mushroom pins.
2. Poor Fresh Air Exchange (FAE)
A common oyster mushroom problem is bad air exchange. Mycelium makes CO₂ when it breathes. Too much CO₂, especially in a poorly aired space, stops pins from growing. Oyster mushrooms are very sensitive. They need a lot of oxygen to fruit. Without enough air exchange, you might see badly formed pins or no pins at all.
Problems from too much CO₂:
- Long, thin stems—called "legginess"—with small caps.
- Mushrooms growing up, away from places with more CO₂.
- Pins that start but then stop growing.
How to Get More Fresh Air:
- Poke more holes in your bag or tote. A 1/4" hole every few inches can greatly help with FAE.
- Use fans on timers or exhaust fans in grow tents to move out old air.
- If you don't have automatic fans, fan your chamber by hand 2–5 times each day.
The best setup lets air flow and keeps humidity at the right level. This can be tricky. But watching your pins and how your mushrooms grow will show you if you are doing things right.
3. Not Enough Light Exposure
Mushrooms are not like plants. They do not use light for energy. But they do react to light as a signal to start fruiting. Light helps oyster mushroom pins grow in a certain direction. It also helps all the mushrooms in a group grow at the same time.
Oyster mushrooms grow well with:
- About 12 hours of indirect or soft light each day.
- Cool-white LED or full-spectrum lights if growing inside.
- Light on one steady side to help mushrooms grow straight up and evenly.
If you grow in a dark basement, garage, or closet, get a grow light. A simple LED bar placed 12-18 inches away will work. Do not use strong or direct sunlight. This can make your grow area too hot. Or it can dry the surface, stopping pins from growing.
4. Temperature Is Not in the Best Range
Mushroom fruiting is very sensitive to temperature. If it's too hot or too cold, pins will stop growing. They might not get bigger, or they might make strange-looking mushrooms. Each type of oyster mushroom can be a little different. But most common kinds (like Pleurotus ostreatus) like these temperatures:
- 60–75°F (15–24°C) for fruiting.
- Fruiting might be slower below 50°F (10°C) or above 79°F (26°C).
Signs of Temperature Problems:
- Pins grow slowly or not at all.
- Mushrooms grow in bad shapes or are tiny, even with the right humidity and air.
- Pins show up but then quickly dry out or change shape.
In very hot weather, oyster mushroom pins often dry out or get bacterial spots. Colder temperatures might make the colonization phase longer and delay pinning. But these crops are often stronger. You can fix this by adding insulation, small heaters, or cooling mats, depending on your setup.
5. You Started Fruiting Too Early
Growers often get impatient, especially with new mushroom blocks. But if you start fruiting before the block is fully colonized, you will almost certainly get fewer mushrooms. And it can let contamination beat your mycelium.
Make Sure You Are Ready to Fruit:
- The substrate should be completely white with mycelium.
- Do not see any brown spots (this means the substrate is not yet colonized).
- The whole block should be colonized for at least 5–7 days after it looks fully covered.
Cutting holes or slits in bags too soon lets outside germs in. Substrate that is not fully colonized cannot fight off these germs well. If your mushroom pins are not growing, check again if the block was really ready to be open to the air.
6. Contamination Is Taking Over
Molds or bacteria can completely stop pins from growing. At first, oyster mushroom pins and contaminants might be together. But later, the mycelium will weaken, and fruiting will stop.
Look For These Signs:
- Green spots = Trichoderma.
- Gray or web-like stuff = Cobweb mold.
- Wet, slimy substrate or a sour smell = Bacterial contamination.
You might not always see contamination. If pins turn black, stop growing, or turn into goo, it is probably contamination. You can cut away some small infections. But big outbreaks usually mean you need to throw out the batch.
How to Prevent It:
- Clean tools very well before cutting, misting, or touching the grow bag.
- Do not fruit in dirty places (like garages or under sinks).
- Always clean old water bottles or misting sprayers before using them again.
7. Too Much Water Is Drowning Pins
If your fruiting chamber gets so foggy that water drips, it is too wet. Too much water in the air can cause pins to be covered in standing water. This can stop them from breathing. And sometimes it helps mold or bacteria grow on them.
How to Fix the Water Problem:
- Let the bag get air. Crack the top or add air holes.
- Use a drip shield or tilt the bag. This makes water run away from the pins.
- Make sure air changes help dry up extra moisture.
- Keep the room temperature steady to lessen water from temperature changes.
Good pins look soft or dull. They are not shiny or sticky. If your pins are shiny or like jelly, you need to lower the water levels quickly.
8. White Fuzzy Growth Instead of Pins (Overlay)
You might see mycelium growing widely over the surface but no pins. This means you likely have overlay. This happens when mycelium grows too much. It is often caused by too much wetness, still air, and not enough light.
What Causes Overlay:
- Not enough FAE.
- Too much humidity, with not enough drying.
- Not enough light to start growth.
How to Fix Overlay:
- Stop misting for a bit. Let the surface dry a little.
- Add new air slits or holes.
- Put the bag near a new light source for a 12-hour ON and 12-hour OFF cycle.
Sometimes, just moving the grow bag to a brighter or better-aired spot can make pins start again. Do not give up if overlay shows up early. It is a common small problem when you are learning.
9. Pins Are Drying Out
Oyster mushroom pins can dry up very quickly. This usually happens because the air is too dry. Or it can be from direct air movement slowly drying out those delicate growing points.
Look for These Signs:
- Pins turning brown or bronze on their edges.
- Caps curling up or in.
- Pin clusters that are smaller than usual and have stopped growing.
How to Add Moisture to Your Grow Space:
- Mist 3–4 times each day when pins are growing fastest. Make sure the mist is fine.
- Use a plastic tent or bag to make water dry slower.
- Do not let room fans blow right onto the mushroom block.
Adding trays of soaked perlite (like those used in monotub fruiting chambers) helps put moisture into the air all the time. This is very helpful in dry indoor areas.
10. Use Monitoring Tools to Stop Guessing
The best way to keep your oyster mushroom problems from getting worse is to use tools. These tools give you constant information about your growing area.
Important Tools for Growers:
- ✅ Digital hygrometer/thermometer combo: shows humidity and temperature right away.
- ✅ CO₂ monitors: you don't always need these, but they help in closed or winter-ready setups.
- ✅ Full-spectrum LED timer: controls light cycles so they are steady.
- ✅ Misting bottle with an atomizer head: makes small drops that are perfect for mushroom surfaces.
These tools often cost less than $30. And they make your crops grow more steadily because you do not have to guess.
Finding the Problem, Fixing It, and Being Patient
Growing mushrooms, especially oyster mushrooms, needs both skill and knowledge. If your mushroom pins are not growing, use what you see to help you fix things. Every pin that stops growing teaches you something. Sometimes it is about air, sometimes about timing, and often about waiting.
Steps to fix problems, from most important to least:
- Humidity and air exchange.
- Light and temperature.
- How much the block is colonized.
- How clean things are and if there is contamination.
As you grow more, it will get easier to set up your environment just right. Treat each grow as a learning experience. Keep notes, change one thing at a time, and you will have fewer oyster mushroom problems over time.
FAQ
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Why do my mushroom pins stall after forming?
Pins can stop maturing if conditions aren't stable—often due to low humidity, poor fresh air exchange (high CO₂), or light being too weak. -
Can wrong temperature stop pin growth?
Yes. If fruiting temperatures are too hot or too cold (outside 60-75°F / 15-24°C for oysters), pins may stall, deform, or dry out. -
How do I tell if there’s contamination preventing pin growth?
Look for discolored spots (green, grey), slimy texture, unusual odor, or pins turning dark and aborting. -
What environmental fixes help get pins growing again?
Increase airflow, adjust humidity to about 85-95%, provide soft indirect light, ensure the substrate is fully colonized, and avoid direct misting onto pin surfaces. -
Should I wait for full colonization before fruiting?
Yes. Fruiting too early can lead to pin aborts and gives contaminants a chance to invade. Make sure the block is fully white and stable before initiating fruiting.
✅ Want fewer problems and more mushrooms? Try a Zombie Mushrooms grow kit. It has good airflow, a design that holds humidity, and very clear instructions. This makes it the easiest way to avoid the usual troubles of pins stopping.
Citations
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Chang, S. T., & Miles, P. G. (2004). Mushrooms: Cultivation, Nutritional Value, Medicinal Effect, and Environmental Impact. CRC Press.
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Lelley, J. I. (2005). Mushroom Cultivation. Mushroom Office.
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Stamets, P. (2000). Growing Gourmet and Medicinal Mushrooms (3rd ed.). Berkeley, CA: Ten Speed Press.
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Department of Plant Pathology, Pennsylvania State University. (n.d.). Common Mushroom Diseases and Their Control.
