Fully colonized rye grain spawn jar showing dense white mushroom mycelium growth What grain spawn actually is, why it outperforms spore inoculation for Psilocybe cubensis cultivation, and how to prepare it correctly — including the sterilization and hydration steps where most growers go wrong. This article is part of the Spores Lab Substrate Preparation pillar — a complete guide to choosing, preparing, and optimising your growing medium for Psilocybe cubensis and gourmet mu