Home mushroom growing kits

A mushroom growing kit is a block of substrate already permeated by fungal mycelium. Understanding why it is sold in that state — rather than as spores and a bag of straw — explains most of the biology involved.

Two phases, and only one of them is risky

Cultivation has a colonisation phase, in which mycelium grows through the substrate consuming it, and a fruiting phase, in which it forms mushrooms. These have opposite requirements. Colonisation wants darkness, high CO₂, no air movement and stable warmth; fruiting wants light, low CO₂, fresh air and high humidity.

Colonisation is also where cultivation fails. A freshly inoculated substrate is a rich, moist, uncontested food source, and the desired fungus is in a race against moulds and bacteria that grow faster than it does. Once the mycelium has fully permeated the block, it has consumed the available nutrients, acidified its environment and physically occupied the space — the block is defended, and contamination becomes far less likely.

Selling a pre-colonised block therefore transfers the risky phase to a facility with sterile technique, and hands the buyer the phase that mostly requires patience and humidity.

Pasteurised, not sterilised — and why

Straw and similar bulk substrates are usually pasteurised rather than sterilised: held at around 65–80 °C, or treated with lime to raise pH, rather than autoclaved.

The reasoning is competitive rather than hygienic. Full sterilisation creates a biological vacuum, and if any contaminant lands afterwards it has no competition at all. Pasteurisation instead removes the fast-growing competitors while leaving a background of harmless thermophilic organisms that occupy the substrate and resist invaders — selective, not absolute, and better matched to a process that cannot be kept sterile. Supplemented substrates such as grain or sawdust with bran are nutritious enough that they must be fully sterilised, because no partial treatment would hold.

The characteristic contaminant is Trichoderma, a green mould that grows faster than most cultivated fungi and can parasitise them. Green patches on a block are the usual sign that the race was lost.

Fruiting is triggered, not scheduled

Mushrooms form when the fungus receives signals indicating it has reached a surface. Three matter.

Carbon dioxide is the primary one. Mycelium respiring inside a substrate raises local CO₂ substantially; a drop indicates open air. High CO₂ during fruiting produces the classic malformation — long thin stems and small caps — because the fungus is still being told it is buried and keeps extending.

Light is a signal rather than an energy source; fungi do not photosynthesise. Blue light in particular triggers primordium formation and orients the fruiting body, which is why kit instructions specify indirect light and why mushrooms grow toward it.

Humidity must be very high, typically above 85 percent, because the developing fruiting body is mostly water and has no cuticle to prevent evaporation. Drying at this stage aborts the pins outright.

Some species additionally need a cold shock to initiate fruiting, which in nature corresponds to autumn.

After a flush, the block rests and can fruit again from remaining reserves, with each successive flush smaller as the substrate is exhausted.