A plastic film covers the compost in a mushroom bed during the spawn run step of the mushroom growing procedure. It is during this step that the spawn begins to colonize the compost with mycelium. The plastic cover is vented by having a pattern of symmetrically arranged holes throughout its area so as to prevent or minimize moisture loss and prevent airborne pathogens from landing on the compost. The vented cover also reduces the CO.sub.2 level while increasing the oxygen rate.