Millet, hyacinth bean, and sorghum outperformed standard lab media (like PDA) in supporting fast, dense mycelial growth of oyster mushrooms (Pleurotus ostreatus). At higher concentrations (50–70 g/L), these locally available grains helped mushrooms fully colonize in just 9 days versus 15 days on PDA. This low-cost, high-efficiency method could empower small farmers and incubators to scale mushroom cultivation in resource-limited settings.