Constructing Your Terrarium/Fruiting Chamber

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Note: Do NOT waste your money on a digital hygrometer for your terrarium or mushroom growing house. They may be accurate in a home at normal ambient household humidity, but in a terrarium at near saturation humidity, they will not read properly. Get a quality analog hygrometer from a cigar shop and calibrate it by wrapping in a damp towel for an hour, and then adjust the screw on the back so the dial reads 99%. Repeat weekly.

How to Make a Terrarium for Growing Mushrooms

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This page explains the construction of a simple terrarium, also known as a fruiting chamber, for growing mushrooms. The function of a mushroom fruiting chamber, or terrarium as they're sometimes referred to, is to provide an environment with near saturation humidity, while allowing plenty of fresh air to enter and circulate. My terrarium design has become widely referred to as the shotgun terrarium due to the abundance of holes on all six sides. The ideal mushroom fruiting chamber will be constructed in such a way that stale CO2 laden air can exit, or be forced out, while a constant supply of fresh moving air is allowed to enter. While the terrarium needs to provide this supply of fresh air, it also needs to maintain a humidity level at or above 95% to promote mushroom formation and growth.

Over the years, I've seen many mushroom fruiting chamber designs come and go. Those that hold a high humidity often get saturated with stale air, and the mushrooms develop fat, tough stipes (stems) and tiny caps. Since with many species of mushrooms, it's the cap that we eat, the entry of fresh air is vitally important. Other terrarium designs use complicated systems of humidifiers and fans, and while these often provide the fresh air that's required, they sometimes fail to keep the humidity levels high enough, resulting in dry substrates and poor mushroom fruitbody formation. In addition, the use of mechanical or electrical devices introduces the possibility of failure, either by over saturation or drying out, due to equipment malfunction. That's why I advise to keep it simple. The terrarium I present here can be left unattended while you're at work all day, and you won't have to worry about losing your mushrooms.

BRF/PF Tek Part 4 - "Fruiting Mushrooms in a shotgun terrarium" -

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The terrarium system that I designed and demonstrate here is what I consider to be the best combination of easy construction, excellent function, and low maintenance. To watch a video of the terrarium under construction, see part 3 of the BRF/PF tek video. It's been dubbed the shotgun terrarium, and the name seems to have stuck. The name comes from the appearance of up to a few hundred holes that are drilled into the plastic storage bin.

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Shotgun Terrarium Theory of Operation

The theory of operation for the shotgun mushroom fruiting chamber is that natural air currents travel from areas of high pressure to areas of low pressure. Cool air has the molecules closer together than warm air, thus cool air is at a slightly higher pressure than warm air. When we put several inches of damp perlite in the bottom of our terrarium, we create an area with a slightly cooler temperature than the air above, which is exposed to lights that create heat, and our mushroom substrates, which are often at up to a few degrees warmer than the surrounding air due to thermogenesis.

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This temperature differential, however slight, results in enough of a pressure gradient that it causes air to flow up through the perlite, absorbing moisture as it travels, and into the relatively lower pressure air within the fruiting portion of our fruiting chamber.

This air then exits through the holes in the upper section of the terrarium, carrying the excessive CO2 produced by our mycelium out with it. With this design, no electrical or mechanical equipment is required. Regular misting helps to keep our brf cakes or other substrates moist, and also serves to replace the moisture that evaporates from the perlite.

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When using this system, drill holes as shown in the pictures and video clip on all six sides. I use a 1/4" (6mm) drill bit. Use of a larger drill bit is not recommended. It must be remembered that a 1/2" hole has four times the area of a 1/4" hole. (A 12 mm hole has four times the area of a 6 mm hole) If the holes are too large, your terrarium will not be able to maintain the high humidity necessary for growing mushrooms.

This system also requires an ambient humidity in your house of 30% or above, which incidentally will ensure your own comfort as well. In desert climates, and especially in cold climates during the winter months, indoor humidity is often very low. If you get shocked from touching another person, or a light switch, etc., after walking across the carpet in your home, it means your humidity is too low. The answer is to run a cool mist humidifier 24/7 in your home. Moist air holds heat much better than dry air, so you'll actually lower your utility bills by doing so. If you'll ensure that the humidity in your growing room is at least 30%, this design terrarium will easily maintain 95% humidity on the inside where your mushrooms need it.

Of course, it should go without saying that for this system to work properly, the terrarium must be elevated at least 1" above the table it's sitting on. Use blocks of wood, shot glasses, or whatever you have around the house to raise the terrarium off the table so air can circulate under it. It should also be noted that 90% of all airborne contaminants in a room are located near the floor, so make sure your terrarium has a home on a table or shelf.

Lighting Requirements of Mushrooms

Some mushrooms, such as the Agaricus species commonly found in grocery stores require no light at all. However, those commonly grown by hobbyists, such as Pleurotus ostreatus (Oyster Mushrooms), Lentinus enodes (Shiitake), Psilocybe cubensis, a hallucinogenic mushroom, and Hericium erinaceus (Lion's Mane) all require light to produce abundant, normal sized fruits. Experience has taught us that the light best suited for primordia formation and the development of fruitbodies is bright light with a color temperature of 5,000 Kelvin to 7,000 Kelvin. Fortunately, this type of light is easily obtainable at your local home improvement center in the form of fluorescent fixtures. For a small terrarium as described in this chapter, a single CFL (compact fluorescent) that screws into a standard light bulb socket will work very well. These can often be found in grocery and drug stores in every neighborhood. 15 watt CFLs will do the job well, but the package will probably have a large 60 stamped on it, indicating they produce light "equivalent" to a 60 watt incandescent light bulb. They're referring to lumens of output, not the frequency. Incandescent light bulbs are the worst possible choice for growing mushrooms, since they emit a 'red' light in the 3,000 Kelvin color temperature range.

The higher the color temperature, expressed in Kelvin, the closer to the 'blue' end of the spectrum the emitted light is. The lower the color temperature the 'redder' the light is. If you have a choice of fluorescent lamps, purchase those labeled 'daylight' since these have a somewhat higher color temperature than cool white. Daylight, sometimes called 'natural daylight' fluorescent tubes generally emit light in the 6,500 Kelvin range, while cool white fluorescent emits light at around 5,000 Kelvin.

If you have several terrariums stacked or otherwise near each other, you can use larger 2 to 4 tube fluorescent fixtures. These come in 48" and 96" lengths. Place the fluorescent lamps as close as you can get them to your terrariums without causing excessive heating. Species such as Shiitake and Oyster mushrooms prefer to fruit at temperatures in the upper 50's to mid 60's Fahrenheit (15C to 20C), while Psilocybe cubensis prefers to fruit at a temperature in the mid 70s to about 80 Fahrenheit (23C to 27C)
Most mushroom species don't mind a slightly warmer temperature during daytime than at night, so if your grow room is a bit colder than the temperature ranges given above, a little warming from your lights during the daytime won't hurt at all, provided you don't let the air in your terrarium get too dry. For cakes, try to keep the humidity above 95%.

Cased substrates are a bit more forgiving, but still try to keep your humidity above 90%. 12 hours on, 12 hours off has proved to be a great combination over a wide range of species. Of course, if you have a bright window near your terrarium, that will suffice, but direct sunlight for more than a few minutes per day should be avoided.

Disregard outdated advice in old books which is constantly repeated on the internet to colonize mushroom substrates in total darkness. Experience and rigorous peer reviewed studies have proved that exposure to low level ambient indoor lighting during spawn run and substrate colonizing will speed up the process, leading to full colonization up to a few days earlier than the same substrate would if colonized in darkness. In addition, mushroom mycelium develops a day/night circadian rhythm, so exposure to light from day of inoculation sets this process in motion, leading to earlier fruiting and harvest.

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