Mushroom Side Pins: Are They Ruining Your Monotub?
The first time I saw side pins in one of my own tubs, years back, I nearly dumped the whole thing. There was a little ring of pins tucked down in the gap between the substrate and the wall, nowhere near the top surface where I expected fruit, and I was sure something had gone sideways. It had not. Mushroom side pins are normal fruit bodies forming in a moist, sheltered gap between your substrate block and the tub wall. They are not contamination, and in almost every case your grow is fine.
The reason side pins appear there instead of on top is simple: the substrate shrinks as mycelium consumes it, pulling away from rigid tub walls and creating a narrow perimeter gap that is darker, more humid, and more protected than the exposed top surface. The mycelium goes where conditions are best for primordia survival. Once that clicked for me it made the fix obvious, and it also tells you the handful of times you really do need to worry.
Out-Grow Quick Answer: Mushroom Side Pins
- Side pins are normal fruit bodies responding to a more favorable microclimate at the tub walls. They are not contamination.
- The primary cause is substrate shrinkage creating a dark, humid gap at the tub wall. A liner eliminates that gap.
- You can harvest healthy side pins exactly like top pins. Check for species-typical color, firm texture, and a fresh earthy smell.
- Discard only if you see colored mold (green, black, or bright blue), slimy tissue in the gap, or a sour or rotten off-smell.
- Fix the root cause for your next grow: install a liner, keep the top surface glistening, and balance fresh air exchange so the top offers conditions as good as the gap.
"Side pinning refers to the phenomenon in mushroom cultivation where mushroom pins, the early stages of fruiting bodies, develop on the sides or near the bottom of the cultivation container, rather than on the surface of the substrate."
Why Do Mushrooms Pin on the Sides of a Monotub?
Run your thumb along the inside wall of a colonized tub and you can feel what the mushroom already figured out: the perimeter is where the moisture collects and the light does not reach. That is microclimate competition, and it is the whole story. Your mycelium is constantly reading its environment and forming primordia wherever conditions best support their early survival. When the top surface is drying out and the wall gap is staying moist and sheltered, the math is easy, and the gap wins.
How Substrate Shrinkage Creates the Wall Gap
When I load bulk substrate and watch it colonize over the next couple of weeks, the block quietly loses volume the whole time. The mycelium metabolizes the water and nutrients inside the block, so the block pulls inward away from the container walls. In a rigid tub without a liner, this shrinkage opens a narrow perimeter gap, often only a few millimeters wide, but that is enough. The gap traps evaporating water, restricts airflow, and stays darker than the lit top surface. That is exactly the microclimate primordia need in their most vulnerable stage, so the mycelium builds its first pins there.
A Drying Top Surface Sends Pins to the Sides
The gap is always the trigger, but I have learned that a drying top surface is what tips the balance decisively. If the top is losing moisture faster than you are replacing it through misting, the mycelium at the surface reads that as a hostile environment for pinning. It will commit its energy to the gap instead, because that is where primordia can complete their early development without desiccating. Keeping the top surface glistening and moist at near field capacity, meaning a firm squeeze of the substrate yields only one or two drops of water, closes that performance gap between the top and the sides.
What About Light as a Cause of Side Pins?
People send me this one all the time: they read that light triggers side pinning, or that blocking light from the tub walls stops it. Light does play a role as a directional cue, orienting mushrooms toward available light and away from zones of high carbon dioxide. But the research-backed consensus is clear, and it matches what I see in the grow room: the microclimate created by substrate shrinkage and surface moisture loss is the primary driver. Blocking light from the sides helps somewhat, especially in clear tubs or grow bags, but if you address only the light and leave the gap and moisture conditions unchanged, side pinning typically continues. The liner and the surface hydration fix are the more effective interventions.
Monotub Growth Timeline: When Side Pins Are Most Likely
When I am running a tub, I start watching the walls right around the time colonization wraps up, because that is when the block has shrunk the most and the gap is at its widest. Side pinning most often begins as the colonization phase nears completion, when the substrate block has had the most time to pull from the walls. Understanding the timeline helps you anticipate when to check conditions and when the gap microclimate becomes most influential.
Full colonization typically takes 14 to 21 days for most dung-loving species at a room temperature of 65 to 75 degrees Fahrenheit (18 to 24 degrees Celsius), which is the commonly recommended range for most edible species in monotubs. The timing from colonization to pinning initiation and from pinning to harvest varies by species, typically pinning within 5 to 14 days of fruiting initiation and reaching harvest size 3 to 7 days after pinning. The point I want you to take from this is that the substrate spends two to three weeks in close contact with the tub walls before fruiting begins, which is plenty of time for the block to shrink and a reliable gap microclimate to establish itself.
Are Mushroom Side Pins Bad for Your Yield?
I have pulled plenty of clean flushes that carried a fringe of side pins, and the harvest weight came in right where I expected it. Honestly, moderate side pinning rarely ruins a grow. The pins that form in the wall gap are real mushrooms and most of them are harvestable. Where side pinning does cost you is in inconvenience: you have to tilt or lift the substrate block to access them, and they are harder to harvest cleanly without disturbing developing top pins. In some cases, when side pins are extremely dense and the gap is holding free water, you can also see an increase in bacterial blotch risk, because compressed wet tissue with restricted airflow is an inviting environment for bacteria.
The bigger problem with heavy side pinning is that it often signals the top surface was under-maintained. If most of your pins went to the sides on a particular flush, the top surface conditions were not competitive. That means the top surface flush may be smaller than it should have been. Fixing the underlying conditions makes your next flush stronger, not just more convenient.
Approximate distribution based on cultivation community reports. Not a clinical measurement.
Can You Harvest Mushrooms That Grew as Side Pins?
I harvest side pins the same way I harvest anything else, I just have to work a little harder to reach them. Yes, in most cases they are good to go. Side pins are the same mushroom as top pins; they just formed in a different location. The question is not whether they grew sideways but whether they are healthy. Use the same criteria you would for any harvest: species-typical color, firm and resilient texture, no slimy patches, and a fresh earthy smell. If all of those check out, harvest them.
The practical challenge is access. You may need to tilt the substrate block carefully or remove it from the tub to get to side pins without tearing the tissue. Tearing causes wounds that invite bacterial infection on subsequent flushes, so I take my time here. Pinch the base of each mushroom and give a gentle twisting pull rather than cutting, which leaves a stub that can rot in place.
Side Pins vs. Contamination: How to Tell the Difference
When a grower sends me a panicked photo, the first thing I look at is color and the first thing I ask about is smell, not where the pins are sitting. That is where a lot of beginner panic comes from: side pins look unusual compared to a clean top flush, and anything unexpected in a grow tends to trigger alarm. Here is the complete picture of what to look for, covering both the normal and the genuinely concerning signs.
| What You See or Smell | Normal or Contamination | What It Means | What to Do |
|---|---|---|---|
| White or species-typical colored pins in wall gap | Normal | Side pinning from microclimate | Harvest when ready, liner next time |
| Condensation droplets on tub walls | Normal | Healthy transpiration from substrate | No action needed |
| Amber or yellow droplets on mycelium surface | Usually normal | Metabolic exudate, common in active colonization | Check smell. Earthy smell is fine. Sour smell is a warning. |
| Green patches on substrate or pins | Contamination | Trichoderma mold, often spreads fast | Isolate or discard, seal bag before disposal |
| Slimy, water-soaked tissue in gap | Contamination risk | Bacterial blotch, worsened by excess moisture in gap | Increase fresh air exchange. Discard if foul smell develops. |
| Black or dark sooty patches | Contamination | Black mold, possibly Aspergillus | Discard immediately, dispose in sealed bag |
| Sour, sweet-ferment, or garbage odor | Contamination | Bacterial or mold activity at problem stage | Do not consume. Discard the substrate. |
| Fresh earthy or mushroom smell | Normal | Healthy mycelium and fruit bodies | All good, continue monitoring |
The side gap is more humid and has less airflow than the bulk tub air, which is why contamination that starts there can feel alarming. But the color and smell test still applies. Healthy tissue in a gap looks and smells like healthy tissue anywhere else in the tub.
How to Prevent Mushroom Side Pins in Your Next Grow
After enough tubs, I stopped fighting side pins and started designing them out of the setup, and the single change that did the most was a liner. Once you understand that side pins are a microclimate response, the prevention steps become intuitive. You are not fighting the mushroom. You are making the top surface as attractive as the wall gap. Here is the complete prevention sequence I recommend, applied in order of impact.
The Liner: Your Single Best Intervention for Side Pins
If you take nothing else from this article, take the liner. I have set up tubs both ways more times than I can count, and an opaque, flexible liner installed at the base of your monotub and allowed to move with the substrate as it shrinks is the most reliable way to prevent side pins from forming. The liner eliminates the fixed wall gap by physically staying in contact with the substrate as it shrinks. Without a fixed gap, there is no trapped microclimate for primordia to colonize before the top surface gets there.
Cut the liner so it covers the bottom and lower walls, with the top edge sitting 3 to 4 inches above the substrate base. If it is too high, it traps moisture against the walls above the substrate level instead of under it. A trash bag that fits the tub works well. Press the liner snug against the substrate when you load and mix your bulk, then do not readjust it. The liner will settle with the substrate as colonization progresses.
Keeping the Top Surface Competitive Against Side Pins
The liner addresses the gap. Keeping the top surface hydrated addresses the other side of the equation, and it is the part most growers skip. A top surface that stays consistently moist and has good fresh air exchange gives mycelium no reason to prefer the wall over the top. Light misting of the tub interior walls during fruiting, every one to two days or as needed to keep a visible sheen on the casing, is usually enough. Avoid misting pins directly: mist the walls and let evaporation hold the humidity. Direct heavy misting onto developing pins invites bacterial spotting and aborts.
Common Bad Advice About Mushroom Side Pins
I have watched growers throw away perfectly healthy tubs over a few ideas that circulate widely in beginner communities, ideas that are either incomplete or flat wrong. Here is what actually happens when you follow them.
Myth 1: Side pins mean your monotub is contaminated or ruined. This is the most common reason growers throw away perfectly good grows. Side pins are fruit bodies, not mold. The color, texture, and smell of healthy mycelium and fruit bodies are your read, not their location. White or species-typical colored tissue, firm to the touch, with a fresh earthy smell, is healthy regardless of where it formed. Only colored mold growth, slimy bacterial blotch, or a sour or rotten odor indicates a genuine contamination problem.
Myth 2: Blocking light from the tub sides stops side pins. A common rule you will see is that strong light reaching the sidewalls is the main trigger and blocking it prevents side pinning. The reality is that the humidity and substrate shrinkage microclimate is the primary driver, and light is a secondary orientation cue. Blocking side light helps somewhat, especially with clear tubs or bags, but growers who block the light without addressing the liner and moisture situation typically still get side pins. Address the physical microclimate first.
Myth 3: Fan and mist heavily multiple times a day to prevent side pins. The logic is that more fresh air and more water keeps the top surface dominant. In practice, the opposite often happens: over-fanning dries the top surface faster than you can replace the moisture, which makes the sheltered gap even more appealing relative to the top. Classic monotub design is built around passive, self-regulating fresh air exchange. Heavy manual fanning disrupts that balance. Mist lightly and regularly, targeting the walls and casing, not the developing pins.
Relevant Out-Grow Supplies
The product I point people to for side pins is the one that removes the gap by design, a substrate container with flexible walls instead of rigid ones. All in one grow bags are built with flexible walls that move with the substrate as it colonizes, rather than the rigid tub walls that create a fixed gap. Because the bag material stays closer to the substrate surface throughout colonization and fruiting, the side gap microclimate forms much less readily than it does in a hard-sided tub.
- All In One Mushroom Grow Bag, 5 lb: grain spawn and bulk substrate in a single ready-to-inoculate bag. Flexible walls reduce side pin gaps. Well-suited for dung-loving species and for growers setting up their first consistent indoor grows.
- Manure Based All In One Mushroom Grow Bag: a manure-based substrate formulation in the same flexible-wall bag format, matched to species that perform best on manure-based bulk. Same flexible wall advantage for side pin reduction.
- Mushroom Supplies: browse the full supplies collection for grow bags, tubs, filters, and other equipment useful for setting up a clean, well-managed fruiting environment.
If you prefer a traditional rigid-tub monotub setup, the liner is not a supply item you need to buy from us; a standard trash bag works. The real supply that matters is a substrate that holds moisture evenly, which any of our all in one grow bags are designed to do.
The Bottom Line on Mushroom Side Pins
Side pins are not your grow going wrong. They are your mycelium going right, finding the best available conditions and fruiting there. The wall gap happens to be better than the top surface in most setups because the substrate shrinks away from rigid walls and the gap holds moisture the top surface loses. A liner is the cleanest fix. Keeping the top surface hydrated and well-ventilated closes the performance gap between the top and the walls. When you have side pins right now, check color, check texture, check smell. Healthy mushrooms in an unusual location are still healthy mushrooms. Harvest them and do better next flush.
About the author
Mike Wiberg is the founder of Out-Grow, a mushroom cultivation supply company he has run since 2009. He has more than 20 years of hands-on mycology experience and maintains a working culture collection of more than 300 species under a USDA permit. His cultivation work has been cited in peer reviewed research published in Frontiers in Cellular and Infection Microbiology, and he is one of only a handful of growers with a documented successful clone of Matsutake, one of the most difficult species in cultivation. He writes from the lab and the grow room, not from a keyword list.
Frequently Asked Questions About Mushroom Side Pins
The primary cause is substrate shrinkage as colonization progresses, pulling the block away from rigid tub walls and creating a narrow, dark, humid gap at the perimeter. That gap offers better microclimate conditions for early primordia survival than a drying top surface does, so the mycelium fruits there. A liner eliminates the gap. Keeping the top surface well-hydrated removes the conditions advantage the gap has over the top.
Yes, side pins that pass a basic health check are safe. Confirm they are species-typical in color, firm to the touch, and smell fresh and earthy, not sour, rotten, or chemical. If all of those check out, side pins are the same mushroom as top pins and you can treat them identically from harvest through cooking.
A liner is the most effective single prevention step for traditional rigid-tub monotubs. It works by staying in contact with the substrate as it shrinks, preventing the fixed gap microclimate from forming. Without it, the gap almost always forms on grows of 3 to 5 inch substrate depth in a rigid container. If you are using a flexible-wall grow bag">all in one grow bag, the bag material itself acts as a liner analog by conforming to the shrinking substrate.
Tilt or gently remove the substrate block to get access if the pins are behind it. Grasp the base of each mushroom and apply a gentle twist-and-pull to remove it cleanly at the base. Avoid cutting with scissors or a blade if you can, since leaving stubs invites rot on that spot for subsequent flushes. Some growers use a thin flexible tool to reach behind the liner and dislodge pins from the back wall gap without disturbing top pins.
This is normal shrinkage from colonization. As mycelium metabolizes the water and nutrients in the substrate block, the block loses volume and contracts. All bulk substrates in rigid containers do this to some degree. The shrinkage is not a problem; it is the wall gap that forms afterward that promotes side pinning. A liner manages that gap.
Target roughly 80 to 90 percent relative humidity during fruiting, with conditions closer to 90 percent during active pinning. The more important thing is keeping that humidity consistent at the top surface through light regular misting rather than trying to hit a precise number. An over-dry top surface promotes side pinning no matter what the ambient relative humidity reads.
Not unless you see colored mold growth (green, black, or bright blue), slimy bacterial rot that does not clear up with improved airflow, or a persistent foul off-smell. Side pins alone, even heavy ones on every wall, are not a reason to discard. Harvest them like any other fruit, address the liner and moisture conditions for your next flush, and the grow will continue.
Less often than rigid tubs. The flexible bag walls conform to the substrate as it shrinks, which reduces the formation of a fixed humid wall gap. You can still get some side pinning, especially at the bottom corners, but it is generally less severe than in a hard-sided monotub without a liner. The flexible-wall design also makes side pins easier to harvest since you can flex the bag outward to access them.