Quick answer: Most cultivated mushrooms grow at high relative humidity, roughly 80–95 %, with the highest humidity during pinning and fruiting. Oyster mushrooms, for example, colonize at about 24–25 °C and 80–90 % RH and fruit at about 20 °C and around 90 % RH (University of Florida IFAS). Commercial rooms hold these levels with humidifiers, control CO₂ with fresh air, and use dehumidification to stop condensation and wet caps after watering and during cooling.
Quick answer
- Humidity target: high (about 80–95 % RH), set by species, strain and stage.
- Humidify to stop casing and caps drying; ventilate to keep CO₂ down; dehumidify to remove free water and stop condensation.
- Measure at the crop and at cold surfaces, not only at one wall sensor.
Humidity and temperature targets by stage
Targets vary by species, strain and grower procedure. The values below come from extension guidance and are a starting point for your own protocol.
| Crop and stage | Air temperature | Relative humidity | CO₂ and air |
|---|---|---|---|
| Oyster, spawn run (colonization) | 24–25 °C (75–77 °F) | 80–90 % | Dark; little fresh air needed |
| Oyster, fruiting | about 20 °C (68 °F) | about 90 % | Good air exchange to avoid CO₂ build-up; light |
| Button (Agaricus), spawn run | compost 24–27 °C (75–80 °F) | High | Closed room |
| Button, pinning | Air lowered about 1 °C (2 °F) per day after casing | High, enough that the casing does not dry | CO₂ lowered to 0.08 % (800 ppm) or less |
| Button, cropping | 14–17 °C (57–62 °F) | High enough to minimize drying of the casing | Fresh air to hold CO₂ |
Sources: University of Florida IFAS, Grow Your Own Oyster Mushrooms (SS662); Penn State Extension, Six Steps to Mushroom Farming.
Why humidity control matters for mushrooms
- Too dry: the casing layer and caps lose water, pins abort, caps crack and yield falls.
- Too wet: free water sits on caps and surfaces, which leads to blotch, brown spots and bacterial problems, and water drips from ceilings and cold pipes onto the crop.
Most problems come from swings, not the average. A room that runs at 90 % RH but drops to 75 % every time the fresh-air damper opens, or reaches 100 % with condensation every night, will crop worse than one held steady.
The three tools: humidify, ventilate, dehumidify
| Tool | What it does | When you need it |
|---|---|---|
| Humidifier | Adds water vapor | Fresh air or heating dries the room; casing or caps start to dry |
| Fresh-air ventilation | Replaces CO₂-rich air with outdoor air | Pinning and cropping, when CO₂ rises |
| Dehumidifier | Removes water vapor and free water | After watering, during cooling, in humid seasons, and to stop condensation |
They work together. Fresh air brings CO₂ down but also changes humidity: dry winter air lowers it, humid summer air raises it. The humidifier and dehumidifier then bring the room back to target without changing the ventilation the crop needs.
Humidification
Ultrasonic humidifiers produce a fine mist that evaporates quickly without wetting the caps, and their output steps easily with the controller. Size the humidifier for the driest condition: maximum fresh air at the lowest outdoor humidity. Use treated water (reverse osmosis or softened) to prevent mineral dust on the crop and scale in the unit. Our industrial ultrasonic humidifiers cover 3–102 kg/h.
Dehumidification
Use dehumidifiers to:
- remove water after watering, so caps dry within a few hours;
- stop condensation on ceilings, cold walls and pipes during cool-down and at night;
- hold humidity when outdoor air is warm and humid and ventilation would push the room above target.
Refrigerant dehumidifiers suit rooms above about 15 °C. Below that, or for cold storage of harvested mushrooms, a desiccant dehumidifier keeps working without coil icing. See our grow room dehumidifiers and desiccant dehumidifiers.
Stop condensation before it drips
Condensation happens on surfaces colder than the dew point of the room air, often before the room sensor alarms. At 16 °C and 90 % RH the dew point is about 14.4 °C, so any surface just 2 °C colder than the air will get wet.
| Room air | Dew point | Surfaces at risk |
|---|---|---|
| 20 °C, 90 % RH | about 18.3 °C | Outside walls, cold-water pipes, cooling coil housings |
| 16 °C, 90 % RH | about 14.4 °C | Ceilings and doors in cool weather |
| 14 °C, 95 % RH | about 13.2 °C | Almost any surface slightly cooler than the air |
Keep cold surfaces insulated, move air gently across ceilings, and run dehumidification during cool-down so the dew point falls with the temperature.
Measuring humidity in a mushroom room
- Place sensors at crop height on more than one shelf level, away from humidifier nozzles and cooling coils.
- Log temperature and humidity together, plus the temperature of the coldest surface.
- Calibrate or check sensors often; many sensors drift at humidity above 90 %.
Humidity for harvested mushrooms
Harvested mushrooms keep best close to 0 °C at about 95 % RH, which slows weight loss and browning (USDA Agriculture Handbook 66). Cold rooms at this condition need humidification for the dry cooling air, and good drainage for defrost water.
FAQ
Frequently asked questions
What humidity do mushrooms need?
Generally 80–95 % RH. Oyster mushrooms colonize at about 80–90 % and fruit at around 90 %. Button mushrooms need humidity high enough to keep the casing from drying.
Can mushroom humidity be too high?
Yes. Near 100 % RH with free water on caps, mushrooms develop spots and bacterial blotch. Aim for high humidity without surface water.
Do I need a humidifier or a dehumidifier for a mushroom room?
Usually both. Humidifiers cover dry periods and fresh-air intake; dehumidifiers remove water after watering and stop condensation during cooling and humid weather.
How do you lower humidity in a mushroom grow room without hurting the crop?
Use a dehumidifier sized for the room's moisture load, and increase air movement across surfaces gently. Avoid lowering humidity with large blasts of dry outdoor air, which can dry the casing and caps.
Sources
- University of Florida IFAS Extension, D.I.Y. FunGuide: Grow Your Own Oyster Mushrooms at Home (SS662).
- Penn State Extension, Six Steps to Mushroom Farming.
- USDA Agricultural Research Service, The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks (Agriculture Handbook 66).
