Fresh Air

Mushrooms breathe too

Your oyster kit looks like a quiet block, but the fungus inside is very much alive. Like you, a mushroom takes in oxygen and gives off a gas called carbon dioxide (CO2 for short). You do the exact same thing every time you breathe out.

Here is the problem. If a room has no way to swap old air for new, the CO2 the fungus breathes out just piles up around it. Mushrooms hate that. They need what growers call fresh air exchange, or FAE — old, stale air moving out and clean, oxygen-rich air moving in.

Why CO2 tricks the mushroom

A baby mushroom uses the amount of CO2 in the air as a clue. High CO2 is nature's signal for "you are still buried deep underground — keep reaching upward to find the surface."

So when CO2 builds up around your kit, the mushrooms get fooled. They grow long, thin, stringy stems and tiny caps, stretching up to escape air that never gets fresher. Oyster mushrooms are especially fussy about this. Growers aim to keep CO2 low, often below about 800 parts per million, so the fungus knows it has reached open air and can grow into the wide, healthy caps you want to see.

Fanning helps

The easiest fix is also the most fun: give your kit fresh air on purpose.

• Fan the kit. Waft a piece of card, a book, or your hand across the opening a few times, two or three times a day. This sweeps stale CO2 away and pulls fresh air in.

• Crack a window or leave the room door open so air can move gently through the space.

• Do not seal the kit up airtight. A closed bag or box traps CO2 fast.

Fanning does a second job too. It brushes off spent air without letting the surface dry out completely, which matters once we add humidity in the next lesson.

Two clues to watch

When your caps are wide and your stems are stocky, your fresh air is good. When stems grow long and skinny with pinhead caps, that is your fungus asking for more fresh air. Fan a little more often and watch it change.

Safety note: your kit grows a known-safe oyster mushroom. Never pick or eat wild mushrooms you find outdoors — some are poisonous, and only a trained expert can tell them apart. And always grow and cook your harvest with an adult.

Air is need number one. Next up: keeping your fungus perfectly damp.