Shotgun fruiting chamber

Cost

$20–30

Build time

1 hour

Capacity

Several small cakes or jars

Skill level

beginner

A shotgun fruiting chamber — SGFC — is a clear tub perforated with dozens of small holes, with a layer of wet perlite in the bottom providing humidity.

It predates the monotub as the standard home setup and has largely been replaced by it. It remains genuinely useful for a specific case: fruiting several small containers separately rather than one bulk surface.

The principle

Perlite holds a large amount of water across an enormous surface area, evaporating steadily to keep humidity high without any water sitting where mushrooms are.

The holes — many, small, on all six faces — provide passive fresh air exchange. Warm humid air rises and leaves through the top holes, drawing fresh air in through the bottom ones. It is a continuous convective exchange rather than the batch exchange of fanning a monotub.

That continuous exchange is the SGFC’s real advantage: it is very hard to under-ventilate.

Build

Tub: 50–66 qt clear storage tub with lid.

Holes: ¼″ diameter, spaced 2 inches apart in a grid, on all four sides, the bottom, and the lid. That is 300–500 holes. It takes a while.

Use a sharp bit and drill slowly — brittle plastic cracks. Deburr afterwards.

Stand: the tub needs to sit on blocks or a rack so air reaches the bottom holes. Sitting flat on a table defeats them.

Perlite: 4–5 inches in the bottom. Rinse it first — perlite dust is a respiratory irritant and you do not want to breathe it. Wet it, then drain thoroughly. It should be damp with no standing water.

Put fruiting containers on a rack or foil squares above the perlite, not directly in it.

Running it

• Mist the walls when the sides stop showing condensation, usually 1–2 times daily.
• Re-wet the perlite every few days by pouring water through and draining.
• Fan it 2–3 times a day. Less critical than a monotub because of the passive exchange, but it helps.
• Indirect light, 12 hours.

SGFC vs monotub

	SGFC	Monotub
Air exchange	Continuous, passive	Batch, needs fanning
Humidity source	Perlite reservoir	Misting the walls
Best for	Several small containers	One bulk surface
Build effort	300+ holes	6 holes
Yield per footprint	Lower	Higher
Drying out	More prone	Less prone

Use a monotub for bulk substrate. More yield, far less drilling, better humidity retention.

Use an SGFC when you want to fruit several separate small containers, or when you are fruiting something that needs unusually high fresh air and you would rather not fan four times a day.

Common problems

Drying out. The most frequent SGFC complaint. All those holes exchange air well and lose moisture just as well. Re-wet the perlite more often and mist the walls more.

Not enough holes. Under about 200 the convection does not establish properly and it behaves like a badly-ventilated tub.

Sitting flat. Bottom holes blocked, no convection.

Perlite waterlogged. Standing water at the bottom goes stagnant and starts smelling. Damp, drained, not submerged.

Contamination in the perlite. It happens over time. Replace it between rounds rather than trying to sterilise it — it is cheap.

Worth knowing

The SGFC is a good demonstration of the central fruiting trade-off — humidity against air exchange — because it solves it structurally rather than by scheduling. Understanding why it works makes the monotub’s hole placement make more sense.