What is fresh air exchange (FAE)?

FAE matters because falling CO2 is one of the signals that tells mycelium to fruit, and high CO2 deforms the mushrooms that do form. North Spore explains that as mycelium reaches the edge of its substrate, "it senses a lower CO2 concentration which signals it to produce mushrooms" (North Spore booklet). Cornell's farm guide says a closed fruiting room's CO2 gets too high for proper fruiting in about 10 minutes, and mushrooms then grow "long stems and little caps" (Cornell Small Farms and Fungi Ally).

Most sources want CO2 under about 1,000 ppm while mushrooms fruit, and lower for oysters. UC Cooperative Extension keeps oysters under 600 ppm with 4–6 air changes an hour while they crop, and gives shiitake 2–4 (UC Cooperative Extension). Cornell's farm guide exchanges all of a grow room's air at least every 10 minutes and sees deformed oysters above 800 ppm (Cornell Small Farms and Fungi Ally).

For a Martha tent, GroCycle, a company that sells growing kits and courses, starts the exhaust fan at 2 minutes every hour, or 15 minutes every 4 hours, and adjusts from there (GroCycle). A monotub or shotgun chamber breathes through holes in its walls instead of a fan. Our CO2 and fresh air guide gives starting setups by chamber and targets by species and stage.

Colonizing blocks don't need fresh air. UC lists no air exchanges during the oyster spawn run, with CO2 around 20,000 ppm, and says growth is stimulated up to 28,000 ppm (UC Cooperative Extension). North Spore likewise notes that the CO2 around colonizing mycelium is "very high" (North Spore booklet). Keep blocks closed through incubation, and bring in fresh air once the block is fully colonized and ready to pin.

• Drying the block out. Fresh air and humidity pull against each other. North Spore notes that after increasing fresh air exchange, "you need more humidity" (North Spore booklet). Recheck relative humidity after every change.
• Blaming CO2 for dim light. Cornell's farm guide says light that is too dim also gives long stems and little caps (same guide as above).
• Overdoing it. GroCycle says CO2 below 800 ppm gives mushrooms that are "too small and numerous" (GroCycle). Our CO2 guide covers where sources disagree.

Hypha organizes grows by room and zone, so you can see which blocks shared a tent when you changed its fresh air.