Casing layer — why it dramatically improves yields and how to apply it

What casing does: it creates a moisture reservoir above the substrate, which triggers pinning by creating a humidity gradient. It also hides the substrate surface from direct light, which matters for pin formation. And it provides a neutral-pH environment for pin initiation (untreated substrate surface can become acidic, inhibiting pinning). The improvement in pin density is dramatic — expect 2-4x more pins in a well-cased tub.

Simplest recipe: CVG — coconut coir, vermiculite, gypsum. Hydrate a coco coir brick with boiling water (pasteurizes it), mix in half as much coarse vermiculite by volume, add a tablespoon of gypsum. Let cool to room temperature. Field capacity (no water drips when squeezed hard). Apply 1/2 inch layer over fully colonized substrate. That's it. This has been the standard for 15 years because it works.

Key tip I wish I'd known: apply the casing when the mycelium is fully colonized but BEFORE you initiate fruiting conditions. If you apply it after you've already started fruiting conditions, the substrate may pin through the casing unevenly. The casing should be the last step before you start fanning and misting.