Casing material for growing mushrooms comprising a biomass of plant origin treated to remove cell content, e.g., by extraction in an aqueous slurry. The biomass may comprise non-woody lignocellulosic biomass, such as mown field vegetation, such as mown grass from lawns, verges or pastures, and/or grass-type vegetation. The treated biomass is mixed with marl and/or lime and/or peat.