What is a casing layer?

A casing layer matters because, for button mushrooms, it is the only way to make the mycelium fruit. Penn State Extension says the casing triggers the switch from vegetative to fruiting growth, though no one knows exactly how. It also holds water and supports the mushrooms. Fine mycelium fuses into thick strands called rhizomorphs as it grows through the casing, and pins form on them: "Without rhizomorphs, there will be no mushrooms" (Penn State Extension).

Some gourmet species gain from casing too. Cornell's Small Farms Program says king oyster sometimes needs scratching or casing to start fruiting (Cornell). In a Penn State study, a casing layer raised king oyster yield by 141% over uncased substrate (Penn State).

Most farms make casing from sphagnum peat moss buffered with limestone. Peat has a pH of 3.5 to 4.5, and limestone brings it up to 7.5. Pasteurized clay loam soil, weathered spent compost and coir fiber are used too. Soil, spent compost and coir must be pasteurized first; peat doesn't need it, since it carries no mushroom disease spores or pests (Penn State Extension).

Material changes yields. In a 2020 button mushroom trial, commercial casing soil gave 27.03 kg per square meter and coco peat 4.43 kg (Sarhad Journal of Agriculture).

Apply casing once the substrate is fully colonized, at an even depth and moisture, so mycelium reaches the surface everywhere at once and mushrooms form together (Penn State Extension).

• Depth. The same 2020 trial tested 2, 4 and 6 cm, and 4 cm (about 1.6 in) gave the heaviest individual mushrooms and the widest caps (Sarhad Journal of Agriculture).
• Water. Penn State waters a little at a time to bring the casing up to field capacity before pins form, and calls watering the most delicate job in mushroom growing.
• Timing. Button mushroom harvest begins 15–21 days after casing. Mixing colonized substrate into the casing cuts that by 5–7 days (Penn State Extension).

• Uneven depth or moisture, which makes mushrooms come in at different times. Penn State notes that wetter casing is harder to spread evenly.
• Unpasteurized soil, compost or coir, which can bring in insects and disease (Penn State Extension).
• Letting it dry out. Mushrooms take up water rapidly from the casing as they grow from pins to maturity, Penn State says.
• Mistaking overlay for growth. A thick mat of mycelium on the casing that won't pin is overlay; our pinning guide covers it.

Hypha's photo journal keeps each block's photos with notes alongside and offers side-by-side growth comparisons, so you can watch the casing for rhizomorphs and pins.