Colonisation

The mycelium runs through the bulk substrate, digesting it and binding it into a solid block. This is the longest passive stage in the process and the one where the correct action is almost always nothing.

Conditions

Temperature: 70–78°F. The single variable worth controlling.

Warmer is not faster in any useful sense. Above 80°F, contaminants — especially Trichoderma and Rhizopus — gain more than your mycelium does. Below 65°F everything slows.

Measure at the substrate, not on the wall. A tub on a cold floor sits several degrees below room temperature.

Light: none needed. It does no harm, but there is no reason for it.

Humidity: irrelevant at this stage. The tub is sealed and the substrate carries its own water. Fruiting humidity is a later problem.

Fresh air: minimal, but not zero. Mycelium respires and needs some oxygen. A completely airtight container eventually stalls. A monotub’s filter holes or a bag’s filter patch are sufficient — do not tape them over.

CO₂: high is fine, and helpful. Elevated CO₂ during colonisation suppresses premature pinning and encourages the mycelium to finish the substrate first. This is why monotubs are run sealed until colonisation completes.

What’s happening

Hyphae extend from every grain kernel outward, secreting enzymes that break down the substrate externally and absorbing what dissolves. Where hyphal fronts from different kernels meet, they fuse, and the whole thing becomes a single connected network.

By the end, the substrate is bound tightly enough to lift out of the tub as one block. That binding is the real signal that colonisation is complete — visible whiteness comes earlier.

What to check

Look through the side of the tub. That is it.

Day 3–5: white beginning to spread from grain kernels. Patchy is normal.

Day 7–10: fronts merging, substrate visibly whitening across most of the tub.

Day 12–18: fully white including the base, substrate consolidated.

Do not open the tub. Every opening is an introduction of room air onto a partially-defended surface. Curiosity at this stage is the most common self-inflicted contamination route there is.

If you cannot see the bottom of your tub, that is a reason to use a clear tub next time, not a reason to open this one.

Timing by species and rate

Situation	Typical
Oyster, 1:2 spawn rate	7–10 days
Oyster, 1:4	12–16 days
Lion’s mane, 1:4	14–21 days
Shiitake sawdust block	30–60 days, plus browning
King oyster, 1:3	14–18 days

Shiitake is genuinely different — the block must also go through a browning phase before it will fruit, and that adds weeks.

When it goes wrong

Nothing happening after a week. Usually temperature. See stalled mycelium.

Colour appearing. Any colour that is not white needs identifying immediately, and the tub needs isolating from your other grows while you do it.

Pins forming before full colonisation. Side-pinning happens when the tub is getting light and fresh air prematurely, often through a poorly sealed lid. The pins will be small and the flush disappointing. Let it finish rather than switching to fruiting conditions early.

Sour smell. Bacterial. The tub is likely finished.

Knowing it’s done

Three signs together:

1. Fully white, including the bottom and the corners
2. Consolidated — the block holds together as one mass
3. Slight surface texture change — the flat white starts to look fuzzier as the mycelium moves into a fruiting-ready state

Then wait 3–5 more days. A slightly over-colonised block fruits better than a slightly under-colonised one, and many disappointing first flushes are simply tubs put into fruiting conditions too early.

Next

Fully colonised and consolidated, the tub is ready for fruiting conditions.