DIY fruiting chamber: shotgun + Martha tent builds

May 30, 2026 Grow

Most cultivation guides hand-wave the fruiting chamber and then act surprised when your first grow produces sad, stalled pins. The chamber is half the equation. Substrate determines whether your mushrooms grow; the chamber determines whether they finish well.

This post is two builds, both genuinely useful, with real materials and real timing.

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Which build for you?

	Shotgun chamber	Martha tent
Build cost	~$40 CAD	~$200 CAD
Capacity	1–4 grow bags or buckets (≤30 L of fruit block)	4–15 grow bags or buckets (≤200 L)
Footprint	60 × 40 × 40 cm — fits in a closet	60 × 60 × 150 cm — fits in a corner
Effort to operate	Manual mist + fan 3× daily	Mostly automated
Best for	First-grow oyster on straw, single lion’s mane block, casual hobbyist	2nd+ year hobbyist, multiple species at once, year-round production
Difficulty	Drill some holes	Source 6 parts and wire a humidistat

Most Canadian growers start with the shotgun, then upgrade to the Martha tent when they realize they want to run 3+ blocks simultaneously.

Build #1: The shotgun fruiting chamber (SGFC)

The cheapest reliable fruiting chamber in cultivation. Designed for passive fresh-air exchange via 200+ small holes, with a humidity reservoir of wet perlite at the bottom.

Shopping list (~$40 CAD)

Clear or translucent plastic tote, 60–110 L — Rubbermaid Roughneck (Canadian Tire, ~$20), IRIS USA, or Sterilite Show-Off. 80 L is the sweet spot — big enough for 2–4 grow bags, small enough to manage.
Perlite, 2 bags (8 L each, total ~16 L) — garden centre or Canadian Tire (~$15). Look for “horticultural perlite,” not pool filter perlite.
Spray bottle — dollar store, $2.
N95 mask — pharmacy, $2 per mask.

Tools

Drill with a sharp 1/4 inch (6 mm) bit
Tape measure or ruler
Permanent marker
Bucket and sink (for soaking perlite)

Build it (1 hour)

Mark the drill pattern. On each side, the lid, and the bottom of the tote, mark a grid of dots spaced 5 cm (2 inches) apart in both directions. Yes, the bottom too. The bottom holes let air enter from below as it warms inside the chamber.
Drill every hole. A sharp 1/4” bit cuts through Rubbermaid plastic cleanly — push gently. You’ll end up with 200–400 holes total depending on tote size. This sounds like a lot. It is. Don’t skip holes.
Soak and drain the perlite. Pour the perlite into a bucket, cover with water, let sit 30 minutes. Then drain in a colander or directly through the bag’s bottom (the perlite holds onto plenty of water). Final consistency: wet to the touch, not dripping.
Fill the bottom. Pour the drained perlite into the bottom of the tote, 5–8 cm (2–3 inches) deep. This is your humidity reservoir.
Add a barrier. Lay foil, plastic mesh, or a wire cooling rack on top of the perlite. Your fruit blocks sit on this, not directly on the wet perlite (waterlogged substrate breeds bacteria).

Operating the SGFC

Mist the inside walls of the tote 2–3 times a day with a spray bottle. Don’t spray the mushrooms directly — spray the chamber and let humidity equalize.
Fan the lid for 30 seconds, 2–3 times daily. This swaps fresh air through the holes plus the lid-fanning. Passive FAE is enough for oyster, king oyster, and chestnut at this scale.
Re-soak the perlite every 5–7 days when it dries out. Drain and return.
Temperature: 13–22 °C ambient (basement temps are perfect).
Light: indirect light from a window, or a 6500K LED bulb on a 12-hour timer.

Where it falls short

The SGFC’s passive FAE caps out around 4 ACH effective air exchange — fine for oyster (which tolerates ~800 ppm CO₂) but inadequate for lion’s mane (which needs <500 ppm). If you grow lion’s mane in an SGFC, expect antler-like branching growth instead of round fruiting bodies. Use our fruiting chamber calculator to confirm your chamber is sized right for your species and number of blocks.

Build #2: The Martha tent (active FAE + humidification)

The Martha tent solves both shortcomings of the SGFC: it scales to many blocks at once, and it has active humidification + active FAE so lion’s mane and shiitake work properly.

Shopping list (~$200 CAD)

Wire mini-greenhouse rack — 4 or 5 shelves, ~150 cm tall. Canadian Tire, Walmart Canada, or Home Depot have these for ~$50.
Clear plastic sheeting — 6 mil, enough to wrap the rack on all four sides. ~$10 at any hardware store.
Ultrasonic cool-mist humidifier with a humidistat or compatible external humidistat. A basic 4 L Honeywell or Levoit unit, ~$50.
4-inch inline duct fan with controller — 100–150 CFM. Available on Amazon.ca (~$50).
Flexible duct, 1.5–2 m — ~$15.
Power strip + 24-hour mechanical timer — ~$20.
Tape and zip ties — $5.

Build it (2 hours)

Assemble the wire rack per its instructions.
Wrap the rack in plastic sheeting on all four sides plus the top, leaving the bottom open. Use tape or zip ties along the seams. Don’t seal the bottom — you need a vent path.
Cut a 4-inch hole near the top of one side of the plastic, and another 4-inch hole near the bottom of the opposite side. The top hole becomes the inlet; the bottom becomes the exhaust.
Install the inline fan on the top hole. Connect the flexible duct to its exhaust and run the duct out of the room (window, attic, anywhere that’s not the room you live in). Or terminate the duct in a HEPA filter.
Place the humidifier on the bottom shelf, pointed up into the chamber. Run it on a humidistat set to 90% RH (or use a humidifier with built-in humidistat).
Wire the fan to a 24-hour timer: 15 minutes on every hour gives ~6 air changes per hour for a chamber that size. Lion’s mane wants closer to 8 ACH — run the fan 20 minutes on every hour.

Operating the Martha tent

Humidity: 85–95% RH, controlled by the humidistat. Refill the humidifier every 1–2 days.
FAE: Fan-on-a-timer handles it. Adjust if mushrooms grow irregularly:
- Antler branching on lion’s mane → more FAE
- Cap edges drying / papery → less FAE or more humidity
Temperature: Whatever your room ambient is, plus 1–2 °C from the fan motor heat. A basement at 16 °C ends up around 17–18 °C inside the tent.
Light: A 6500K LED bulb on a 12-hour timer, mounted to the top of the rack.

Capacity reality

A 4-shelf Martha tent at ~150 L total internal volume comfortably runs:

6–10 oyster grow bags
4–6 lion’s mane blocks
4–6 shiitake blocks (post-browning)
3–4 king oyster bags
Or a mix

Past about 12 blocks, the humidifier struggles to keep up and the fan size becomes the bottleneck.

Common shotgun chamber problems

Symptom	Cause	Fix
Pins emerge but never grow into mushrooms	Humidity too low	Mist more frequently; check that perlite isn’t dried out
Mushrooms grow long and stringy	CO₂ too high (FAE too low)	Fan the lid more often; crack the lid open for 30 sec at a time
Mould or wet rot on substrate	Too wet, not enough air exchange	Drain perlite slightly; fan more
Pins dry up before maturing	Humidity not high enough at the block surface	Spray the inside walls more, not the mushrooms directly
Mushrooms cap edges drying	Humidity too low OR FAE too high	More misting; less fanning

Common Martha tent problems

Symptom	Cause	Fix
Humidifier fills the chamber with mist clouds	Run-on time too long, or humidistat set too high	Lower setting to 88%; or use a shorter on-cycle
Lion’s mane still grows antlers	Fan-on time insufficient	Run fan 25 min on / 35 min off
Pools of water at the bottom of the tent	Humidity too high, no drainage	Cut a small drain hole in the bottom corner
Mushrooms grow toward the fan duct	Air current is too direct	Add a baffle plate, or rotate blocks

Spore safety reminder

Whichever chamber you build, wear an N95 mask when opening it — especially during harvest. Run your fan’s exhaust outside your living space if at all possible, or through a HEPA filter. Repeated unmasked exposure to mushroom spores can cause hypersensitivity pneumonitis (“mushroom worker’s lung”).

This is more relevant for the Martha tent than the shotgun chamber — SGFCs typically hold less total fruit block (less spore output), but they’re still spore-emitting environments.

Next steps

Once your chamber is built and you’ve run one cycle, the next things to dial in:

CO₂ measurement — a cheap CO₂ meter (~$60 on Amazon.ca) tells you exactly when to fan. Useful especially for lion’s mane.
Humidity-controlled humidifier — instead of a humidifier on a basic timer, get one with a built-in humidistat that holds 90% reliably.
Insulation — if your basement runs cooler than 13 °C in deep winter, line the back wall of the tent with foam board insulation.
Multiple chambers — for serious growers running different species at different stages, two small chambers beat one large one.

Related: Fruiting chamber calculator — size your build for your species and block count. How to grow oyster mushrooms in Canada, How to grow lion’s mane mushrooms in Canada, How to grow shiitake mushrooms in Canada — the cultivation guides these chambers serve.