Shotgun Fruiting Chamber: DIY Build and Martha Tent Setup

Indoor air usually sits somewhere between 30 and 60% relative humidity, and fruiting mushrooms want 85 to 95%. A fruiting chamber closes that gap while still letting in the fresh air gourmet species need. Below are the two builds we recommend on the MushRealm bench: a shotgun fruiting chamber you can make in an afternoon, and a martha tent for when one box isn't enough.

Do you need a fruiting chamber?

Not always. A single oyster grow bag with slits can fruit on a shelf with a loose clear plastic bag tented over it, misted twice a day. It works, but humidity swings and you will lose some pins.

• One bag: a humidity tent, or a shotgun chamber for steadier results.
• A few bags or blocks at once: a shotgun fruiting chamber.
• More than that, or a new grow every few weeks: a martha tent.

Materials list

How to build a shotgun fruiting chamber

1. Choose a clear box. Clear sides let light reach the blocks. Wash it with hot soapy water first.
2. Mark a grid. With a marker and ruler, put a dot every 5 cm (2 in) across all four sides, the bottom and the lid.
3. Drill the holes. Drill a 6 mm (1/4 in) hole on every mark, using a sharp bit at low speed with a scrap of wood behind the plastic so it doesn't crack. It comes to several hundred holes, so take breaks.
4. Clean up. Brush off the plastic burrs, wash the box again and wipe it with 70% isopropyl alcohol.
5. Prepare the perlite. Wear a dust mask, because dry perlite is dusty. Rinse it in a bucket or colander until the water runs clear, let it soak, then drain it until it stops dripping.
6. Fill the base. Spread 5 to 8 cm (2 to 3 in) of damp perlite across the bottom.
7. Raise it. Stand the box on four risers over a drip tray, so air can move underneath and excess water can drain out of the bottom holes.
8. Load it. Set your grow bags or blocks on top of the perlite, on a square of foil or a clean plastic lid so the substrate doesn't sit in the perlite. Leave space between them and the walls.
9. Place it. Pick a spot with indirect light at the right temperature for your species, away from radiators, air vents and direct sun.

The wet perlite releases moisture constantly, while the holes let fresh air move in and out passively. That balance is why the design works without a fan.

Running a shotgun chamber

• Mist the walls and perlite, then fan with the lid for 10 to 20 seconds, 2 to 4 times a day.
• Keep the hygrometer at block height and aim for 85 to 95%.
• If humidity drops, pour some water over the perlite and let it drain through.
• In a very dry room, tape over some of the lid holes. If stems start to stretch, take the tape off again.
• Rinse or replace the perlite between grows so it doesn't build up mould.

Martha tent setup for more bags

A martha tent is a small greenhouse frame of wire shelves, usually four, with a clear zip-up plastic cover. A common size is about 70 x 50 x 160 cm (27 x 19 x 63 in), which takes several grow bags per shelf.

1. Set it up in a room with a steady temperature, on a waterproof tray or mat. It will drip.
2. Add humidity. Stand an ultrasonic humidifier on the floor inside, or outside with its tube feeding in near the bottom. Fill it with distilled or filtered water; hard tap water leaves white mineral dust on everything.
3. Automate it. Plug the humidifier into a humidity controller with the probe at middle-shelf height. Set it to switch on at around 85% and off at around 92%.
4. Add fresh air. Cut a hole high on one side of the cover for an 80 to 120 mm fan blowing out, and a slit low on the opposite side as an intake. Run the fan on a timer, for example 1 to 2 minutes every 15 to 30 minutes, and run it more if stems stretch.
5. Add light. An LED panel or strip on a 12 hour timer. It can sit outside the clear cover.
6. Load the shelves. Space the bags so air can move between them, and keep them out of the direct fog stream. Mist settles, so the bottom shelves run wettest: put species that like it wettest, such as lion's mane, low down.

Wipe the shelves and the inside of the cover with alcohol between grows.

Humidity control tips for any chamber

• Calibrate your hygrometer. Mix a capful of table salt with a few drops of water into a damp paste, seal it in a zip-lock bag with the hygrometer for 8 to 12 hours, then read it. It should show 75%; note the difference and correct for it from then on.
• Measure in the right place: at block height, not next to the humidifier or right above the perlite.
• Read the surfaces. A fine sheen of droplets on the walls and blocks is right. Puddles or water on the caps mean too wet; dry, yellowing pins mean too dry.
• Mind the seasons. Heating dries indoor air, so expect to mist more from autumn onward.

For target numbers by species, see our fruiting conditions guide. For what happens once the bags go in, follow the oyster mushroom walkthrough, then the second flush guide. The rest of the kit you need is in our equipment checklist, and a grow kit bag fruits happily in either setup.

Quick checklist

• Clear 50 to 70 litre box with 6 mm (1/4 in) holes every 5 cm (2 in) on every side.
• 5 to 8 cm (2 to 3 in) of rinsed, drained perlite in the base.
• Risers and a drip tray underneath.
• 85 to 95% humidity on a calibrated hygrometer.
• Mist and fan 2 to 4 times a day.
• Martha tent: humidifier on a controller, a timed exhaust fan and an LED on a 12 hour timer.

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