Does your grow kit smell sour, of alcohol or of forest? How to read the smell

At the nursery we smell everything. Jars of grain spawn, bags of colonised substrate, plugs on their way into cold storage. Not out of habit, but because the nose is faster than the eye: a block going wrong can be smelled days before it can be seen. You can use that skill at home too. Here is what the smells of a grow kit mean, from good news to reason to act.

Fresh, like forest floor or button mushrooms: healthy

This is how a block should smell. A healthy oyster mushroom block smells like an autumn wood after rain: earthy, fresh, faintly mushroomy. Some species have their own accent. A lion's mane block smells sweetish, almost of flour, and maitake has something spicy about it. As long as it stays fresh, nothing is wrong. Action needed: none.

Sweet and of alcohol or yeast: too little oxygen

A smell reminiscent of beer, yeast or disinfectant comes from the mycelium itself. Starved of oxygen, in a tied-up bag or a sealed container, it switches to a kind of emergency metabolism and produces alcohol in the process. The block is not broken, it is suffocating a little. Action: more air. Open the bag further, leave the container ajar, take the block out of the cupboard. The smell fades within a few days. If it lingers, the block is probably also too warm, because heat speeds this process up.

Sour, like vinegar or old milk: bacteria

A sharp sour smell points to bacterial growth in the substrate, usually in wet, smothered spots. Often you will also see wet, dark or slimy patches. This is the smell that does require action. Action: find the wet patch. On the outside and small: cut it away, move the block somewhere airier and cooler, mist less. Throughout the block: lost, and then we want a photo with your order number via the contact page. A sour block has almost always been too warm and too wet, and sometimes that started in transit; in that case we sort it out.

Musty, like a cellar or damp clothes: too little air and too wet

Musty air is the stage before sour. The block has been standing too long in still, humid air, often in an enclosed space. Action: the same measures as for the alcohol smell, plus a day without misting. Usually it is fresh again within a few days.

Of ammonia: spent or overripe

A sharp ammonia smell comes from protein breaking down. Two causes. The first: mushrooms left on the block too long that are starting to spoil; harvest them and that is that. The second: a block at the end of its life whose substrate is being broken down by bacteria. That block has given what it had. Action: harvest whatever is still standing, then crumble the block into the garden. How other signs tell you a block is spent is in our piece on a block doing nothing.

Of rot: past the line

Genuine rotting smell, the kind you know from forgotten vegetables, means the substrate is breaking down in a way that has nothing to do with mushrooms anymore. Usually together with black, wet patches and a block that feels limp. This block goes in the bin, in a sealed bag. Wash your hands and send us the photo. Fortunately it is rare.

A smell you cannot place?

Describe it in an email, as oddly as you like. "Smells like wet dog" or "like the cheese counter" often tells us more than a photo. We smell substrate all day and recognise most descriptions immediately. And the rule of thumb to remember: forest is good, yeast wants air, sour wants action, rot is over.