Fruiting conditions

Introducing fruiting conditions is a deliberate act. A fully colonised block will sit indefinitely without fruiting if nothing changes, which is exactly why commercial growers can hold blocks until they want them.

What triggers fruiting is not a magic number. It is change — a shift in conditions that signals to the mycelium that it has run out of substrate and should reproduce.

The four variables

Fresh air exchange

The most important and the most misunderstood. Dropping CO₂ is the primary pinning trigger for most species.

During colonisation CO₂ sits at 5,000–20,000 ppm. Fruiting wants it under about 1,000. That drop is the signal.

Insufficient FAE afterwards produces long stems and tiny caps — the most common fruiting complaint there is.

Monotub: open the holes, remove any tape, fan the lid 2–3 times daily
Martha tent: inline fan, 5 minutes per hour as a starting point
Bag: cut it open

Humidity

90–95% during pinning, easing to 85–90% once fruits are established.

Pins are extremely vulnerable to drying — they have almost no protective surface. Developed fruits are less so, and holding 95% around mature mushrooms invites bacterial blotch.

Mist the walls of the chamber, not the substrate and not the mushrooms. This is the single most useful habit in fruiting. Water on caps causes blotch; water on the substrate surface causes cobweb; humidity in the air causes neither.

Temperature

Species-dependent, and generally 5–10°F below colonisation temperature. That drop is itself part of the trigger for many species.

Species	Fruiting
Blue oyster	55–65°F
Pink oyster	70–85°F
Lion’s mane	60–70°F
Shiitake	55–70°F
King oyster	55–65°F

Stability matters more than the exact figure. A tub swinging ±10°F daily will abort pins regardless of its average.

Light

Weaker than the internet suggests, and it does not control stem length.

What it does control: where pins form, and cap pigmentation. Mushrooms grown in darkness are pale and pin unpredictably.

12 hours a day of indirect light is plenty. A north-facing window, or any ordinary room light. Direct sun overheats and dries. There is no reason to buy a grow light for mushrooms.

The sequence

Confirm full colonisation, plus 3–5 days of consolidation.
Case, if you’re casing. Half an inch of moist coir over the surface, then wait for the mycelium to run through it — usually 5–7 days.
Introduce conditions. Open the holes, drop the temperature, start misting the walls, give it light.
Wait. Primordia appear in 3–10 days depending on species.
Do not adjust anything for the first three days. The most common error at this stage is changing settings before the previous change has had time to produce an effect.

Species that need something extra

Shiitake requires the block to brown fully — a hardened, tan outer layer — and then a cold water shock: submerging in cold water for 12–24 hours. Without it, most strains will not fruit.

King oyster wants a genuine cold shock, around 50°F for 48 hours.

Reishi is CO₂-controlled by design: high CO₂ gives the antler form, low CO₂ gives flat conks. Neither is wrong, and growers choose deliberately.

Reading the tub

Days 1–3: surface may look slightly different — a fuzzier, whiter aerial growth. Normal.

Days 3–7: tiny bumps, then recognisable pinheads. This is primordia formation.

Days 5–14: pins develop into fruits. Growth accelerates dramatically toward the end — most species double in size in the final 24–48 hours.

When nothing happens

If two weeks pass with no pins on a fully colonised block:

Not enough of a change. The single most common cause. Fan more, drop the temperature further, actually open the holes.
Too dry. Surface crusted over. Mist the walls more, consider casing.
Not actually finished colonising. Check the base and corners.
Species needs a shock. Shiitake and king oyster, above.
Too warm. Many species will not initiate above their range even though the mycelium looks perfectly healthy.

Once fruits are developing, timing the harvest becomes the thing that matters.