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Humidity is one of the three things that make or break a mushroom harvest, alongside fresh air and light. Too dry, and pins can stall or caps can crack. Too wet, and surfaces stay soggy. To manage humidity, you need to measure it, and that means a hygrometer.
The problem is that cheap hygrometers can be surprisingly inaccurate, and mushroom growing pushes them to the top of their range, where they struggle most. This guide explains the different types, what accuracy you can expect, where to put one, and how to check it at home with nothing more than table salt.
For the bigger picture on why humidity matters, see our guide to humidity, fresh air and light.
What a hygrometer measures
Most hygrometers show relative humidity (RH). That is the amount of water vapour in the air, as a percentage of the most the air could hold at that temperature. Because warm air can hold more water than cool air, RH changes with temperature even when the amount of water stays the same. That’s why most hygrometers also show temperature, and why the two readings belong together.
How much humidity mushrooms need
Targets vary by species and stage, and you should follow your spawn supplier’s figures first. Two examples from sources we’ve used in our growing guides show the range:
| Species and stage | Humidity target | Source |
|---|---|---|
| Golden enoki, while pins form | 98 to 100% | North Spore |
| Golden enoki, as mushrooms develop | 88 to 95% | North Spore |
| Wood ear (Auricularia cornea), fruiting | 75 to 85% | Walker et al. 2023 |
Many fruiting targets sit near the top of the scale. That matters, because it is exactly where many hygrometers are least reliable. You can read more in our guides to growing enoki and growing wood ear.
Types of hygrometer
A University of Kentucky Cooperative Extension publication, Measuring Relative Humidity in Agricultural Environments, compares the main options for farms and greenhouses. The same tools are used by mushroom growers.
| Type | How it works | Typical accuracy and cost (University of Kentucky, 2005) | Pros and cons |
|---|---|---|---|
| Digital hygrometer | An electronic sensor with a display | About ±5% RH, “the norm for low-cost devices”. About $25 to $40 | Cheap and easy to read. Accuracy can fade |
| Dial hygrometer | A mechanical dial | Similar accuracy to a basic digital unit. Up to $85 | No battery, but can’t record readings |
| Sling psychrometer | Two thermometers, one with a wet wick, spun through the air | Read from charts using the two temperatures. About $45 to $70 | No electronics to drift, but slow and manual |
| Data logger | An electronic sensor that records readings over time | Some rated at ±2% RH. About $129 to $249 | Shows what happens overnight and over days |
Prices date from 2005, so treat them as a rough guide to the relative cost rather than current prices.
Which type suits a home grower?
For most people, a digital hygrometer that also shows temperature is the practical choice. If you want to know what your fruiting chamber does while you’re asleep, a data logger is worth the extra cost. Whatever you buy, assume it needs checking.
Using a sling psychrometer
A psychrometer is worth knowing about because it has no electronic sensor to drift. The University of Kentucky describes how to use one:
- Wet the cotton wick on one thermometer bulb with clean water.
- Move air across both bulbs, either by twirling the psychrometer for about a minute or by holding it in the draught from a fan.
- Read the dry-bulb and wet-bulb temperatures.
- Use the chart or sliding scale supplied with the psychrometer to convert the two temperatures to relative humidity.
Keep the wick clean and wet before every reading. A psychrometer is awkward to use inside a small fruiting tent, but it makes a handy cross-check for an electronic hygrometer in a larger grow room.
Why readings drift
The University of Kentucky publication warns that new devices may be accurate to ±2 to 5% RH, but that dust, moisture, fog and gases can degrade the sensors. It adds that cheaper devices often lose accuracy within six months to a year if used carelessly. It also advises avoiding sustained humidity above 95% and nighttime fog where possible, because both seriously affect accuracy.
That is a challenge for mushroom growers, who often run chambers in the high 90s. A field test of electronic humidity sensors by K. C. Wong of the Meteorological Service of Canada shows what can happen. After long periods of saturation, the sensors were slow to recover once the air became drier, by around 40 to 90 minutes on average depending on the model. The author suggests this was likely because condensation had formed on the sensor and had to dry off first. In a misted fruiting chamber, a hygrometer that has been soaked may read “100%” for a while after the air has dried.
Where to put your hygrometer
The University of Kentucky gives some placement rules for farm and greenhouse use that translate well to a grow space:
- Keep it out of direct sunlight.
- Put it where air is moving past it, rather than in a dead corner.
- Give it time. Allow 15 to 30 minutes for the reading to settle. A reading taken after a minute or two is inaccurate.
- Protect it from water and contaminants. In a fruiting chamber, don’t put the sensor directly in the path of a mister or fogger.
Place the sensor at the height of your mushrooms, not at the top of the chamber. Our guide to building a simple fruiting chamber covers chamber design.
How to check your hygrometer with salt
You can check a hygrometer at home using a saturated salt solution. It works because a sealed space above a saturated solution of a given salt settles at a known humidity. The reference values come from a 1977 paper by Lewis Greenspan at the US National Bureau of Standards (now NIST), which is still widely cited.
| Salt | Relative humidity at 20°C (Greenspan 1977) |
|---|---|
| Magnesium chloride | About 33% |
| Sodium chloride (table salt) | About 75.5% |
| Potassium chloride | About 85% |
| Potassium sulphate | About 97.6% |
Sodium chloride is the easy one, because it’s in every kitchen. Greenspan’s data also show that its value barely changes with temperature, staying close to 75% between 10°C and 40°C.
Step-by-step: the salt test
- Make a salt slurry. In a small open container, such as a bottle cap or small dish, put a layer of salt and add just enough water to make a wet slurry with no standing water. A guide from the collections-care organisation Connecting to Collections Care suggests about an inch of salt, wetted until it’s fully saturated.
- Seal it in. Put the small container and the hygrometer inside a larger airtight container or a sealed food bag. Don’t let the hygrometer touch the salt.
- Keep the temperature steady. Leave it somewhere with a stable temperature, out of the sun.
- Wait. The University of Kentucky says to wait until it stabilises. Connecting to Collections Care suggests leaving data-logging sensors for at least 96 hours. For a quick home check, leave it for at least several hours or overnight, and longer if the reading is still moving.
- Read and record the difference. Compare the reading with 75%. The University of Kentucky gives an example: if the hygrometer reads 77% over a 75.5% salt standard, it reads 1.5 points high, so subtract 1.5 from future readings.
Making sense of the result
- The University of Kentucky says most hygrometers can’t be adjusted mechanically, so you apply the correction to the readings yourself. Some digital models do have a calibration setting.
- A single calibration point is most reliable near that value. The University of Kentucky suggests about 10 to 15% RH either side. Because mushroom chambers run so humid, a 75% check doesn’t prove the hygrometer is accurate at 95%.
- For a check closer to fruiting conditions, potassium sulphate is the better reference, at about 97.6% according to Greenspan. Connecting to Collections Care recommends testing with more than one salt where possible, because sensors can read differently at high and low humidity.
- Accept small errors. Connecting to Collections Care points out that a sensor rated at ±5% is working as designed if it reads 70 to 80% in a sodium chloride test.
Other ways to check
- Compare with a known good instrument. The University of Kentucky’s other method is to compare against a high-quality calibrated device in a steady environment.
- Compare several hygrometers. If you have three cheap units and two agree, the third is probably the odd one out.
- Recheck regularly. The University of Kentucky advises rechecking instruments after humid or dusty use and before the next growing period.
Don’t chase the number
A hygrometer is a guide, not a target to hit to the decimal point. Mushrooms will also tell you what they need. Dry, cracked caps and stalled pins suggest the air is too dry. Wet, slimy surfaces suggest it is too wet or too still. If your mushrooms aren’t fruiting at all, our fruiting troubleshooting guide works through the other usual suspects.
Sources and further reading
- Duncan, G., Gates, R. and Montross, M. (2005). Measuring Relative Humidity in Agricultural Environments (AEN-87). University of Kentucky Cooperative Extension Service
- Greenspan, L. (1977). Humidity fixed points of binary saturated aqueous solutions. Journal of Research of the National Bureau of Standards 81A(1): 89–96
- Connecting to Collections Care: Saturated salts experiment (checking humidity sensors)
- Wong, K. C. Humidity sensor field test. Meteorological Service of Canada, presented at WMO TECO-2012
- North Spore: Golden enoki grain spawn growing parameters
- Walker, A. et al. (2023). Optimal conditions for mycelial growth and nutritional values of the Auricularia cornea. Studies in Fungi 8: 19
Featured image: hygrometer by Sally E J Hunter via Flickr (CC BY 2.0).
