What is pinning?

Pinning sets up the flush: where pins form and how many decides where the mushrooms grow and how crowded they are. Not every pin becomes a mushroom. GroCycle says "the fungus selects the healthiest, most viable pins" (GroCycle), and a review of button mushroom research says far more primordia form than mushrooms (Molecules, via PMC). Our primordia page covers what pins are.

A change in conditions triggers pinning in a fully colonized block. North Spore names fresh air, humidity, light and temperature as the changes that start fruiting (North Spore booklet). For oysters, UC Cooperative Extension's pinning conditions are:

Factor	Oyster pinning target
Humidity	95% RH
Air temperature	55–60°F (13–16°C)
CO2	Under 600 ppm
Fresh air exchange	4 air changes an hour
Light	Diffuse natural light is enough

The table is from UC's guide (UC Cooperative Extension). UF/IFAS says many oysters need "a lower temperature of 20°C (68°F) and light" to start fruiting (UF/IFAS). A cold drop isn't a universal trigger; our temperature guide covers where it helps. Shiitake is started differently: UC submerges its logs and blocks in cold water.

Pins usually show within one to two weeks of fruiting conditions. GroCycle gives 3 to 14 days from the move into fruiting conditions (GroCycle). UC gives oysters 7–14 days to pin, and shiitake 7–14 days after its soak (UC Cooperative Extension). Our timeline guide lists more species.

Keep humidity at its highest until pins have formed. North Spore lowers it by 5–10% only after that (North Spore booklet).

A block that won't pin is usually too dry, short of fresh air, at the wrong temperature or not fully colonized. Our pinning troubleshooting guide works through each cause. Three related problems have their own names:

• Aborts: pins that stop growing. North Spore says pins usually abort "when there isn't enough humidity around the block" (North Spore help center).
• Side pinning: pins forming along the sides of a bag or tub instead of the open surface.
• Overlay: a thick mat of mycelium that keeps growing instead of pinning.

Hypha gives each bulk block a reminder to move into fruiting, and its photo journal keeps the block's photos together, so you can see the day pins first showed.