Why are My Mushroom Pins Not Growing?

Why are My Mushroom Pins Not Growing?

A grower emailed me a photo last spring of a bag covered in tiny pins that had sat frozen for six days, not one of them any bigger than a grain of rice. He was sure the block was dead. It was not. His room had drifted dry, and once he tented the bag the pins picked right back up. That is the short answer to the whole question: mushroom pins stop growing when their fruiting environment stops meeting their developmental needs. In more than twenty years of running Out-Grow, the culprit almost always turns out to be one of nine things: low humidity, too much CO2, the wrong temperature for your species, a block that was opened before full colonization, direct water on developing pins, not enough light, contamination, an exhausted substrate, or a missing species-specific fruiting trigger.

Here is the part that keeps me answering these emails instead of writing them off: most of these are diagnosable in under an hour once you know what to look for, and most are correctable before the pins die off completely. The trick is to check one variable at a time and to know your species, because oyster mushrooms, shiitake, lion's mane, reishi, and king oyster each want different things and respond to different fixes.

The 9 Reasons Mushroom Pins Stop Growing

When I walk a new grower back through a stalled grow on the phone, I do it in a rough order, because some causes come up far more often than others. The chart below is that order, built from the grows I have troubleshot and from extension guidance, not from a survey. Your bag may have two of these going at once, which is exactly why working down the list beats jumping straight to the most dramatic fix.

1. Low Humidity or a Drying Surface

Early pins are the thirstiest thing in the grow room, and they show it first. The Cornell Small Farms Program notes that humidity is most critical in the first four days after pin initiation, and should be kept above 85 percent relative humidity during that window. After about three days of visible pinning, it can drop as low as 60 percent without causing immediate damage, with the 80 percent range being a reasonable ongoing target for most specialty mushrooms.

When humidity drops too early, pins dry at the surface, and the look of it is unmistakable once you have seen it a few times. In lion's mane grows, Field and Forest Products describes dry fruit bodies that turn yellow, stop growing, and abort. In oyster grows, you will often see the edges of the small caps begin to curl and brown before growth halts. The fix is to restore humidity through chamber conditions or a humidity tent, not by spraying directly on the pins.

2. High CO2 from Poor Fresh Air Exchange

Mushrooms take in oxygen and exhale CO2 as they grow, and in a sealed or under-ventilated space that exhaled CO2 has nowhere to go. The fruit bodies respond by growing longer stems in a search for fresh air. The result is the familiar long-stemmed, small-capped oyster mushroom that growers sometimes mistake for a failed grow. It is not a failed grow; it is a ventilation problem, and opening the space up almost always corrects it once you balance the additional airflow against your humidity needs.

3. Temperature Outside the Species Range

This is the cause that surprises most beginners, because the species requirements vary more than most guides admit. King oyster mushrooms need the temperature below 62 degrees Fahrenheit to initiate a pinset. Shiitake on logs fruits between 55 and 65 degrees Fahrenheit. Lion's mane pins at 55 to 65 degrees Fahrenheit as well. Reishi is completely different, preferring 70 to 80 degrees Fahrenheit for fruiting. Running a single room at 70 degrees Fahrenheit is a good compromise for many gourmet species, but it will prevent king oyster from pinning and may cause reishi to stall if the room is too cold.

4. Under-Colonized Substrate: Opened Too Early

The mistake I see more than any other, especially with all in one grow bags, is a grower opening or cutting the bag before colonization is complete. The impatience is understandable, but it costs the grow. Cornell Small Farms states clearly that fruiting initiation follows full colonization. For oyster mushroom bags, the University of Florida Extension says the bag is ready to fruit only when the whole bag has turned white, which typically takes two to three weeks. Shiitake on sawdust blocks takes six to seven weeks minimum, and Cornell notes the blocks should be allowed to fully popcorn and brown before fruiting is initiated. Opening early does not speed things up; it exposes a vulnerable substrate to contamination and stress before the mycelium has the energy reserves it needs to fruit.

5. Direct Water on Pins

Heavy misting directly onto developing pins creates the conditions for bacterial blotch, and it is one of those problems that looks like bad luck until you trace it back to the spray bottle. Penn State Extension identifies Pseudomonas fluorescens as the pathogen and describes the problem clearly: mushroom caps that remain wet for four to six hours or longer after watering develop pale yellow lesions that deepen to golden yellow or chocolate brown. The lesions are superficial but disfiguring, and the affected pins often stop growing. The solution is to mist the chamber walls and the air, not the fruit bodies directly, and to allow the surface to dry between wetting events.

6. Not Enough Light for Light-Responsive Species

Many gourmet mushrooms use light as a developmental signal. Cornell Small Farms states that specialty mushrooms need light for proper morphology, even though mushrooms do not photosynthesize the way plants do. Oyster mushrooms, shiitake, lion's mane, king oyster, and reishi all benefit from regular light exposure, and the Ohio State University Extension specifically says that shiitake fruiting requires light. Field and Forest Products gives indirect sun or supplemental light for at least 8 to 12 hours per day for lion's mane and 10 or more hours per day for king oyster and reishi. Running your fruiting chamber in a dark closet with no ambient light can slow or halt pin development for most of these species.

Button mushrooms are the main exception. A 2020 peer-reviewed review by Sonnenberg, van Peer, Baars, and Scholtmeijer in Molecules confirms that Agaricus bisporus does not require light for fructification, which is why button mushroom production works in fully dark growing environments that would slow other species.

7. Contamination

Green, blue, black, or yellow patches are the visual tell, and when I see one in a photo I already know how the conversation is going to go. The University of Florida Extension states these colors indicate unwanted fungi or bacteria, and highly contaminated bags are best removed and discarded. Penn State Extension describes Trichoderma green mold specifically as dark green patches, or gray growth along edges that turns green as it sporulates. Cobweb mold, by contrast, is a weft-cotton mycelium that can cover pins in white or gray threads and is often treatable if caught early. When you see true green mold or spreading wet spots with foul odor, the grow is unlikely to pin and the contamination risk to your other grows is significant. Seal and remove it.

8. Exhausted Block or Rest Period Between Flushes

Not every stalled grow is a problem, and I have talked more than one grower off the ledge over a block that was simply resting. A block that pinned successfully and produced its first flush needs a rest period before the next flush. Field and Forest Products describes lion's mane as typically reflushable after about two to three weeks of rest. If you soaked and waited and nothing is happening, check whether you are in a normal rest window. A block that has produced multiple flushes and shows no sign of pinning despite good conditions and fresh water-soaking may simply be depleted and ready for the compost pile.

9. Missing the Species-Specific Fruiting Trigger

Some species need a specific cue to initiate fruiting that a generic bag-and-wait approach will not provide. Cornell Small Farms points out that shiitake responds very well to water soaking and cold shocking, while other specialty mushrooms like oyster initiate primarily from bag cuts that expose oxygen-rich conditions at the surface. King oyster in Field and Forest's block format fruits from the intact bag top with minimal disturbance. If you are treating every species the same way and getting inconsistent results, the initiating method may simply be wrong for what you are growing.

What Humidity Do Mushroom Pins Need?

Years ago I set up a king oyster grow and ran the room at 90 percent humidity because that is what I did for oysters, and I lost the whole batch to contamination while the pins never took off. That failure taught me the lesson I now give every grower who asks about humidity: it is not one universal number. The right humidity depends on your species, your grow format, and even the stage of fruiting. Here is what the research actually says.

For oyster mushrooms on straw or in bags, the University of Florida Extension gives incubation relative humidity at 80 to 90 percent and fruiting humidity around 90 percent. Cornell Small Farms says most specialty mushrooms need humidity above 85 percent in the first four days after initiation, which can then ease to around 80 percent. Lion's mane blocks run differently: Field and Forest Products gives 70 to 80 percent humidity, considerably lower than what many guides recommend. King oyster is the most striking example. Field and Forest Products specifically gives room relative humidity at or below 65 percent for king oyster blocks, while noting that the intact bag top creates a protected high-humidity microclimate directly at the substrate surface where pins form. Running your room at 95 percent for king oyster is actually wrong and creates conditions that favor contamination, which is exactly the wall I ran into.

Oyster Mushroom Pins Growing Long and Thin? The CO2 Problem

The first time a customer sent me a photo of what he called "alien fingers," I laughed, because I knew exactly what he was looking at before I read a word: oyster mushrooms with long, thin stems and tiny caps, reaching straight up out of the bag. That shape means the room has too much CO2. It is the clearest and most visually diagnostic indicator of a fresh air exchange problem there is, and it is worth knowing exactly what the targets are.

Getting CO2 below 800 parts per million for oyster mushrooms in a sealed fruiting chamber or tent means actively exchanging air, not just leaving a vent cracked. In practice, brief fan-assisted exchanges several times per day are more controllable than running a fan continuously, which can dry out forming pins if the incoming air is not humidified. Balance the two by watching the pins: if the stems start lengthening, increase air exchanges. If the pin edges start browning or drying, dial the airflow back and check your humidity source.

Note that CO2 thresholds are not universal. Penn State Extension gives 1,000 to 2,500 parts per million as the appropriate range during Agaricus pinning and cropping, which is significantly higher than what oyster mushrooms tolerate. The lesson is that CO2 guidance that works for button mushroom production will harm oyster mushroom grows, and vice versa. Know your species.

Temperature and Mushroom Pin Development by Species

One January a grower drove himself half crazy trying to pin king oyster in a spare bedroom he kept at a comfortable 70 degrees Fahrenheit. Nothing happened for weeks. He moved the blocks to an unheated basement corner, and pins appeared within days. Temperature is the variable that catches the most beginners off guard, because the range that works for one species actively prevents another from pinning. Here is how this plays out in practice.

King oyster mushrooms need the temperature below 62 degrees Fahrenheit for pin set, according to Field and Forest Products. That means a standard room held at 70 degrees Fahrenheit will produce no king oyster pins at all, not slow ones. Many home growers run these in a refrigerator-temperature space, a wine fridge, or a basement in winter specifically to hit that sub-62 degree window. Field and Forest gives 50 to 62 degrees Fahrenheit as the ideal fruiting range.

Shiitake on logs performs best at 55 to 65 degrees Fahrenheit for pinning, according to Ohio State University Extension. The same source notes that temperature and moisture changes trigger the shiitake fungus to produce mushrooms, which is why a cold water soak combined with a cold overnight environment is a standard induction technique.

Reishi is the warm outlier. Field and Forest Products gives 70 to 80 degrees Fahrenheit as ideal fruiting for reishi, with 65 to 80 degrees Fahrenheit as an acceptable setup range. That warm requirement is why a mixed gourmet fruiting room at 65 degrees Fahrenheit will leave reishi blocks sitting unproductive while everything else is pinning normally.

If your pins initiated and then growth stopped suddenly after a room change, a cold snap, or a heat spike, the temperature is almost certainly to blame. Match the actual thermometer reading against the target for your specific species before assuming the block is done.

Is Your Mushroom Block Not Pinning? Colonization Must Be Complete

I keep a mental picture of a shiitake block a customer cut open at four weeks because the outside "looked done." Inside it was still white and soft, and within days it soured instead of pinning. The simplest rule in mushroom growing is also the one most frequently violated: do not open a block or cut a bag until colonization is fully complete. Cornell Small Farms states that fruiting initiation happens after the substrate is fully colonized, and the University of Florida Extension says oyster mushroom bags are ready to fruit after the whole bag has turned white, which takes about two to three weeks from inoculation.

For shiitake on sawdust, Cornell notes that blocks should sit six to seven weeks minimum, until most of the block has popcorned and browned on the outside. That brown rind is the visual cue that the mycelium has finished consolidating and has enough energy reserves to support fruiting. Cutting or fruiting a block that is still white and soft inside is likely to produce no pinset, or a weak scattered one, followed by contamination as the exposed uncolonized substrate gets taken over by competitors.

A good liquid culture from a clean, vigorous strain speeds up colonization considerably, and I can usually spot a weak start from the colonization pattern alone. Slow colonization that stalls and restarts is a sign of weak inoculum or contamination pressure, and the resulting block is a poor fruiting candidate even when it does eventually fill in. Starting with quality spawn is the prevention step here. You can browse Out-Grow's liquid culture collection to see what is currently available and in season.

Can You Mist Mushroom Pins Directly?

A grower once told me proudly that he misted his oyster pins every couple of hours to keep them happy, then wondered why every cap had turned a blotchy yellow-brown. He had done everything with good intentions and created the exact problem the misting was supposed to prevent. A lot of growing guides say to mist your pins whenever they look dry. The problem is that this practice invites one of the more frustrating problems in mushroom growing: bacterial blotch. Penn State Extension describes it specifically. Mushroom caps that remain wet for four to six hours or longer after watering are at high risk for Pseudomonas fluorescens infection, which produces pale yellow lesions that deepen to golden or chocolate brown. The lesions are only two to three millimeters deep but they stop growth and ruin the cosmetics of the flush entirely.

The risk is even higher for king oyster mushrooms at warm temperatures. Field and Forest Products advises against direct watering for king oyster and notes that bacterial contamination risks increase above 65 degrees Fahrenheit. The correct approach is to maintain humidity by misting the chamber walls and the air around the fruit bodies, not by spraying the pins directly. Fine mist from a distance is better than a direct spray, and the surface should be able to dry between events rather than staying continuously damp.

Do Mushroom Pins Need Light to Grow?

I once had a grower swear his lion's mane block was defective because it kept throwing distorted, stalled fruits, and the whole time the block was sitting in a closet he opened only to check on it. We moved it near a window on a timer and the next flush came in clean. Yes, for most gourmet and functional mushroom species, light matters. It does not function the way sunlight does for plants; mushrooms do not photosynthesize. But light serves as a developmental signal for proper morphology, and without it, many species produce distorted or elongated fruit bodies even when humidity, temperature, and fresh air exchange are all correct.

Cornell Small Farms says specialty mushrooms need light for proper morphology. The University of Florida Extension specifically says that fruiting of many oyster mushrooms requires light and lower temperature. Ohio State University Extension says shiitake fruiting requires light. Field and Forest Products gives 8 to 12 hours per day of indirect sun or supplemental light for lion's mane, and 10 or more hours per day for king oyster and reishi.

Regular shop lights, grow lights, or even the ambient light from a window on a light schedule work fine. The key is consistency and duration. Running fruiting in a closet with the door closed all day and only opening it occasionally is not enough light exposure for most of these species.

Mushroom Pin Not Growing: Symptom, Cause, and Fix

Most of the diagnosing I do starts with one question: what does it actually look like right now? The visual tells you more than any theory. The table below covers the most common presentations I get asked about and what each one usually means. Use it alongside the cause list above to narrow down what is happening in your grow before deciding on a fix.

How to Fix Stalled Mushroom Pins: Step-by-Step Diagnosis

The worst thing I watch panicked growers do is change five things at once, then have no idea which one helped or hurt. When pins stall, work through this sequence instead, one change at a time. Changing multiple variables at once makes it impossible to know what actually fixed the problem, and the wrong fix can make things worse.

Relevant Out-Grow Supplies

If you are troubleshooting pins in an all in one grow bag, the most common fix point is the fruiting environment after the bag is opened, not the bag itself. Our 5-pound all in one grow bag and 10-pound all in one grow bag both carry sterilized grain and substrate in a single sealed unit, which eliminates contamination risk at the inoculation stage. If your colonization was healthy and your pins are still stalling, the environment is the issue, not the bag.

Starting with clean, vigorous spawn is the prevention step that most home growers overlook. Slow or weak colonization produces blocks with lower energy reserves and poorer pinsets even when fruiting conditions are ideal. Our liquid culture collection covers the gourmet and functional species we carry and is the starting point for reliable colonization from the first inoculation. For general supplies including thermometers and hygrometers for monitoring fruiting conditions, see our mushroom supplies collection.