Mushroom Humidity and CO2 Targets by Species: A Reference Table
I keep a laminated card taped to the wall of my grow room, and this article is basically that card. For most gourmet and functional mushrooms, you fruit at roughly 85 to 95 percent relative humidity with carbon dioxide held below about 1,000 parts per million, ideally 500 to 800, and you let it climb far higher, 10,000 to 20,000 parts per million, during the colonization stage when the mycelium is running through the substrate. The exact humidity and carbon dioxide numbers a species wants shift with how thin-skinned and fast it fruits, and that is what the reference table below sorts out species group by species group.
Out-Grow Quick Answer
- Fruiting humidity for nearly every gourmet species sits between 85 and 95 percent relative humidity. Push oysters and lion's mane to the top of that band.
- Fruiting carbon dioxide belongs below 1,000 parts per million, with 500 to 800 as the target for the fussiest species. Long stems and small caps mean it is too high.
- During colonization the mycelium tolerates 10,000 to 20,000 parts per million carbon dioxide. That is normal, not a problem to fix.
- Oysters and lion's mane are the most carbon dioxide sensitive. Shiitake and reishi tolerate more, and reishi's form is deliberately shaped by carbon dioxide.
- Treat every number as a range and watch the mushrooms. Fruit body shape tells you more than any single sensor reading.
The rate at which stale, carbon-dioxide-heavy air inside a fruiting chamber gets replaced with fresh air. Too little traps carbon dioxide and gives you long stems and small caps. Too much dries out forming pins.
The gas mushrooms give off as they respire. It builds up in a closed chamber and acts like a shaping signal. High carbon dioxide drives stem elongation, low carbon dioxide with good fresh air lets caps spread normally.
The Mushroom Humidity and CO2 Reference Table by Species
The first time a customer sent me a photo of oysters with stems like drinking straws and caps the size of a fingernail, I did not need a sensor reading to diagnose it. That is a carbon dioxide problem, every time. He had the humidity dialed to 95 percent and thought more sealing meant more success, so the tub was airtight and the gas had nowhere to go. One row of holes and a twice-daily fanning later, his next flush came in with proper caps. The table below is the reference I wish he had taped up before he started.
| Species group | Fruiting temperature | Fruiting humidity | Fruiting carbon dioxide | Fresh air need |
|---|---|---|---|---|
| Oyster species | 50 to 75°F (10 to 24°C) | 85 to 95% | 500 to 800 ppm | High |
| Lion's mane | 64 to 75°F (18 to 24°C) | 90 to 95% | 500 to 800 ppm | Moderate to high |
| Shiitake (block) | 61 to 70°F (16 to 21°C) | 80 to 90% | Tolerant, up to about 1,000 ppm | Moderate |
| Reishi and Ganoderma | 70 to 81°F (21 to 27°C) | 90 to 95% | Shapes form, tolerant of higher | Low to moderate |
| White button (Agaricus) | 57 to 64°F (14 to 18°C) | 85 to 95% | Under 1,000 ppm (500 to 800) | Moderate |
Two things about reading that table. The temperature column is the fruiting range, not colonization, and colonization runs warmer for almost everything, around 75 to 80 degrees Fahrenheit (24 to 27 degrees Celsius). And the carbon dioxide column is fruiting only. Every one of these species will happily sit in 10,000 to 20,000 parts per million while it colonizes, and that is by design, which is the part beginners trip over most.
Mushroom Humidity Targets by Species: How High Is High Enough
A grower on the dry side of Colorado emailed me one January with cracked, leathery oyster caps and a chamber reading 60 percent. His house heat had pulled the room bone dry and the pins were dehydrating faster than they could grow. We got him a bigger water reservoir and a fan running on a shorter cycle, and the humidity came up into the high 80s. The next flush was soft and full. That is the whole game with humidity: keep the surface moist enough that pins never dry out, without drowning them.
Mushrooms are roughly 90 percent water, so the air around a forming pin has to stay close to saturated or the pin aborts. The practical band for almost every gourmet species is 85 to 95 percent relative humidity. The bar chart below shows where each group I get asked about most likes to sit.
Notice how tight the spread is. Every group lives inside a ten point window. That is why I tell beginners not to chase a species-perfect humidity number. If you hold 85 to 95 percent and read the fruit, you are covered for the whole table. The range meter shows where the trouble starts on each side.
The other end matters just as much. I nudge humidity down slightly as caps open and start dropping spores, because holding a chamber pinned at 95 percent with weak airflow is how you invite bacterial blotch, those slimy tan patches on the caps. The rings below are the two settings I actually run: high through pinning, eased back for the finish.
Mushroom CO2 Targets by Stage: Why the Number Flips
The single most common mix-up I untangle over email is a grower who read that mushrooms need 20,000 parts per million carbon dioxide, sealed the tub tight for fruiting, and got a jungle of leggy stems. That 20,000 figure is real, but it belongs to colonization, not fruiting. During colonization the mycelium is spreading through grain or substrate in a closed bag, and it does not care about high carbon dioxide. In fact the stable, low-airflow conditions keep contamination down, which is exactly why we leave it sealed. Then the number flips hard once you want mushrooms instead of mycelium.
Carbon dioxide works like a shaping signal. High levels tell the mushroom to reach up toward fresh air, so it puts everything into stem and almost nothing into cap. Drop the carbon dioxide with real airflow and the mushroom spreads a proper cap instead. The timeline below is the arc every indoor grow follows, from sealed colonization to open fruiting.
Mushroom Environmental Parameters Chart: Sorting Species by Fussiness
Back when I was cloning my way through the culture library, I learned fast that not every species punishes a bad carbon dioxide reading the same way. Set a pink oyster and a shiitake in the same slightly stale tent and the oyster comes out deformed while the shiitake shrugs it off. That difference is the most useful thing to internalize from the whole reference table, because it tells you which grows to babysit and which to leave alone.
Oysters, in every color I have grown, and lion's mane sit at the sensitive end. They want the highest humidity, the lowest carbon dioxide, and the strongest fresh air exchange in the table. Get lazy on airflow and lion's mane grows coral-like tendrils instead of a solid head, and oysters go leggy. The scorecard below is how I rank the sensitive group against the environment.
Shiitake and the reishi family sit at the tolerant end. Shiitake handles moderately higher carbon dioxide and slightly lower humidity, down toward 80 percent, without complaint. Reishi is the odd one out in the best way: carbon dioxide does not just get tolerated, it gets used. Growers who want the branching antler form of reishi deliberately keep carbon dioxide high, and growers who want the flat conk shape open things up. Same species, two looks, decided by one gas.
How to Dial In Humidity and CO2 Targets for Any Species
The best troubleshooting call I ever had lasted four minutes. A grower described short fat mushrooms drying at the edges, and I knew before he finished that his airflow was fine but his humidity had crashed. We did not touch a single setting except the water. That is the workflow in miniature: figure out the stage, set the starting band, then let the mushrooms correct you. Here is the sequence I walk every beginner through for hitting humidity and carbon dioxide targets regardless of species.
Common Bad Advice About Mushroom Humidity and CO2
Most of the bad numbers I see repeated come from copying a chart without understanding the stages behind it. One marketing article I ran across claimed a mushroom house should hold 25 percent carbon dioxide, which works out to 250,000 parts per million. That is not a growing condition, that is a level that would fail occupational safety limits, which sit near 5,000 parts per million for an eight hour day. The real spawn run figure is 10,000 to 20,000 parts per million, and fruiting belongs below 1,000. When a source contradicts itself and cites impossible numbers, throw out the whole source.
More humidity is always better, so keep the chamber at 100 percent.
At or near 100 percent with poor airflow you get bacterial blotch and slimy pins. Aim for 85 to 95 percent with visible surface moisture but no standing water.
More fresh air exchange is always better for every grow.
Too much airflow dries out forming pins and drives aborts. Fresh air has to be balanced against humidity, enough gas exchange to fruit without drying the surface.
All gourmet mushrooms want the same humidity and carbon dioxide.
Oysters and lion's mane are far more carbon dioxide sensitive than shiitake or reishi. Adjust the target by species group, not one setting for the whole shelf.
Out-Grow Supplies That Help You Hit These Targets
You do not need a rack of instruments to hold these ranges, but a few things make it easier, and I only point people at gear that actually earns its place. The honest truth is that the substrate and the container do more for your environment than any gadget.
- An all in one grow bag handles the colonization stage for you, staying sealed at high carbon dioxide until you are ready to open it and fruit. Good fit for beginners who do not want to manage a separate spawn run.
- Our mushroom grow bags with micron filter patches let carbon dioxide and moisture balance passively during colonization, which is most of the environmental control you need at that stage.
- Ready-to-use mushroom substrate hydrated to field capacity is the real humidity engine in a small tub, since a properly moist substrate keeps the surface from drying between mistings.
Related Out-Grow Guides
The Short Version to Tape to Your Wall
Fruit at 85 to 95 percent humidity with carbon dioxide under 1,000 parts per million, let it run high during colonization, and push oysters and lion's mane to the demanding end while shiitake and reishi forgive more. Every number in the reference table is a starting band, not a law. After more than 20 years at this, the thing I trust most is still the mushroom itself. Set the range, watch the caps, and correct one variable at a time. None of these ranges guarantee a yield, but they put the odds firmly in your favor.
About the author
Mike Wiberg is the founder of Out-Grow, a mushroom cultivation supply company he has run since 2009. He developed the original all in one mushroom grow bag in 2014, a format now used across the hobby, and over more than 20 years of hands-on mycology he has grown hundreds of species and researched substrate formulations from the lab bench. He maintains one of the largest commercial culture libraries online, including a working 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 has helped growers troubleshoot cultivation problems for nearly two decades, and he writes from the lab and the grow room, not from a keyword list.