Mushroom Fruiting Chamber Plans

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How to construct a humidified fruiting environment on a small scale.

This method uses a cool-mist humidifier and a pet terrarium to conduct both the air exchange and the moisture levels required for good fruiting of mushrooms Below is a short list of pros and cons regarding various humidifying devices used by hobbyist cultivators.

Ultrasonic Humidifier

Ultrasonic Fogger


  • The flowing mist just looks awesome!
  • Plenty of gas exchange. (When they are turned on.)


  • The misty cloud on the terrarium floor just looks awesome!


  • Expensive
  • Ultrasonic transducer part is prone to failure
  • Creates a dense mist that quickly over-saturates the mycelium. This implies then using a periodic operation thereby not providing a constant gas exchange in the fruiting chamber.


  • Expensive
  • Ultrasonic transducer part is prone to failure.
  • Creates a dense mist that quickly over-saturates the mycelium. This implies then using a periodic operation.
  • Takes up a lot of space inside the fruiting chamber.
  • There is no gas exchange.

Wet Perlite Layer

Aquarium Bubbler


  • Cheapest of all solutions.
  • Does not over-saturate mycelium via mist.


  • Fairly inexpensive.
  • Provides constant gas exchange. (Although somewhat limited according to pumps diaphragm size.
  • No mist over-saturation of mycelium.
  • Easy to clean and maintain


  • No gas exchange.
  • Messy to work with.
  • Perlite needs to be frequently cleaned or replaced as it is a good breeding ground for microbes.


  • Noisy to operate.
  • Does not provide enough gas exchange to mimic the natural environment.

Non-Evaporative Cool-Mist Humidifier modified with a Garden Hose!


  • High rate of constant gas exchange simulating the natural habitat..
  • No mist or over-saturation of mycelium. (From a properly constructed hose attachment.)
  • Filtered air intake.


  • Some additional minor building steps required.
  • Some models of Cool-Mist Humidifiers do not have an automatic shut-off feature when the water reservoir goes dry. (Potential safety-hazard.)

So, where do you find a cool mist humidifier?

If you live in the United States, most K-mart stores carry a small model by Kaz (HealthMist). This is what I use and it is perfect for what we need. I find that it can drive two hoses without a problem. Regardless of the brand, it should not have a wick inside! It should not have the word "evaporative" anywhere on the box. The inside should have a spinning disk surrounded by a circular splash comb.

Note: This photograph is of an older unit. The new ones look similar except that the top is a gray color and the filter is moved to the back of the unit.

Here's how to set it up:

  1. Purchase a small cool mist humidifier, an 11/16th inch drill bit (5/8th inch will also work), duct tape, and the cheapest 5/8th inch garden hose you can find.
  1. Cover the original output slot on the humidifier with tape or permanently seal it with caulking.
  1. Cut off any attachments that may have come on the garden hose. Cut a bare minimum of 1.5 feet of hose (preferably longer). It must be long enough to connect the humidifier to your terrarium and also allow the mist to have a lot of contact with the inner hose surfaces.
  1. Drill a hole in the corner of the humidifiers upper surface. If you did not find the 11/16th inch drill bit and are using the 5/8th inch drill bit instead, you may need to wiggle the bit around while drilling to enlarge the final hole.
  1. Use your full body weight (it will be a tight fit) to try and squeeze a 1/4 to a 1/2 inch of hose into the hole you made.
  1. If you would like additional outputs (most people report two is all the Kaz unit can handle), then repeat the above two steps to insert another length of hose into another corner.
  1. With however many hoses you attach to the unit, take the ends and attach them to your terrarium. It is recommended that you try to run the hose in a wavy fashion to increase the mist hitting the inner surface of hose material. This will allow water droplets to collect inside the hose and flow back down the hose into the humidifier. Vertical loops are to be avoided as they can create water traps blocking air flow from traveling through the hose and into your terrarium. A coil-shaped length of hose gently rising and then running straight and into your terrarium is ideal (just think of recreational water park slides). This is very important for the first few feet of hose because if you do not shape your hose this way, water will drip into your terrarium causing major problems with contamination. If you want to be safe, you could also attach a small cup under the hose end that is entering the terrarium to catch any water drops.
  1. Fill the humidifier's water reservoir. I'd also recommend adding an 1/8th to a 1/4 cup of store bought 3% hydrogen peroxide to keep the water fresh.

If you need a 90 degree turn into the top of your terrarium, I'd recommend using a 1/2" PVC elbow (about twenty cents). You can push in the garden hose to one end and place the other end into the terrarium.

When I first read about using a cool mist humidifier, I read about using a 1" diameter PVC tubing. That doesn't work well. Such a large tube allows free mist to reach the other end, which means you are back to not having the humidifier running constantly to avoid saturating the mycelium. That is the same problem one has with the ultrasonic humidifiers. Finally, if you used more than two such tubes, you would be running out of water very fast. The garden hose limits the output sufficiently so that given one tank water with one hose output, the tank should last several days without need for a refill (even in a dry room). Also, the relatively thin diameter of the garden hose and the turns run the mist against the inner hose walls before it reaches the output. A cool mist set up simply works GREAT!

To emphasize the point, I'm currently running such a humidifier with one output on some elm oyster mushrooms. These mushrooms will not grow using Perlite or an aquarium bubbler since they require a lot of gas exchange. Yet using this set up, a single garden hose output provides plenty of gas exchange for two and a half square feet of cropping surface and keeps humidity levels above 90% RH even when operating in an air-conditioned room with air as dry as 25% RH!

I would recommend placing an air exit hole and hose / PVC pipe at the opposite end of the terrarium and close to the bottom if not in the bottom itself. This will allow for any built up condensation and CO2 gasses to vent to the outside of the fruiting chamber.

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