Why Mushrooms Need Wet Air

You are about to build the weather

In this course you become an engineer. Your project is a fruiting chamber — a clear box that holds the perfect damp, breezy weather a mushroom needs to grow. Growers also call it a humidity tent.

But before you build a machine, a good engineer asks one question: why? Why do mushrooms need wet air at all? Let's find out, because everything you build later depends on this answer.

As always, wash your hands before you handle any growing gear, and keep a grown-up on your team.

Mushrooms are mostly water

Here is a fact that surprises most people: a fresh mushroom is roughly 90% water. Squeeze one and you can almost feel it. A mushroom is less like a plant and more like a soft, water-filled sponge that pushes up fast.

Because a mushroom is so full of water, it dries out easily. When the air around it is dry, water escapes from the mushroom into the air. This escaping is called evaporation — liquid water turning into invisible vapour and drifting away.

If too much water evaporates, the baby mushrooms crack, crust over, and stop growing. Dry air is a mushroom's biggest enemy indoors.

What "humidity" really means

Humidity is the amount of water vapour floating in the air. We measure it as relative humidity, written as a percentage. It tells you how full the air is with moisture compared to the most it could hold at that temperature.

• 40% humidity means the air is fairly dry, like a heated room in winter.

• 90% humidity means the air is almost as damp as it can get, like a foggy morning.

When the air is already close to full of moisture, water can barely evaporate out of the mushroom. So the mushroom stays plump and keeps growing. That is exactly the trick your chamber will pull off.

The magic number: 80 to 90 percent

Oyster mushrooms fruit best when the relative humidity sits around 80 to 90 percent. If it drops much below that, the tiny mushrooms — called pins — often fail to form or dry up before they grow.

Your whole engineering job in this course is to build a small space that holds 80-90% humidity steadily, day after day. You will do it with three simple things: a plastic box, wet perlite (a puffy volcanic rock), and a measuring tool called a hygrometer.

A quick reminder for every mushroom lesson: we only grow known-safe kit mushrooms, we never pick or eat wild mushrooms, and we never eat any mushroom until a grown-up has cooked it.

Next, we gather the materials for your build.