Red Sorghum

If you’ve ever seen pictures of a button-mushroom farm, you might be asking yourself … why does it look like all the mushrooms are growing out of the soil? Aren’t mushrooms supposed to grow from a substrate, different from the way that plants grow?

Many moons ago, button mushroom growers discovered that adding a layer of organic material on top of the compost could have a huge impact on yield. This layer of “soil” on top of the mushroom beds is referred to as a “casing layer” and has a number of important benefits.

In fact, some mushrooms won’t fruit at all without the application of a casing.

What is a casing layer?

In basic terms, a casing layer is a layer of moist material – either organic, inorganic, or a combination of the two – placed on top of a colonized substrate prior to fruiting. The main function of the casing layer is to aid with moisture retention in the substrate. Some casings also contain biological components, microorganisms that are necessary for fruitbody formation in some species.

Colonized substrates exposed to the environment have a tendency to dry out rapidly if the humidity falls below a certain threshold. Even small fluctuations in humidity can prevent a fruiting block from achieving a full and even pinset.

A casing layer is able to provide a consistently humid environment at the interface of the substrate and the environment (where pins form), which can often make up for less than ideal environmental conditions. Once the mushrooms pin completely- and start to develop into more defined fruitbodies that protrude through the casing- they are less susceptible to fluctuations in humidity.

A a good casing is one that is excellent at holding moisture and slow to contaminate.

Casing layers do involve an extra step in the growing process, which means more time and resources to get the job done. Since not all mushrooms really benefit from a casing layer, using one can sometimes be counterproductive.

They can also be susceptible to contamination, especially in fruiting environments with limited amounts of fresh air. Typically, casing layers used for moisture retention should be either pasteurized or treated with hydrated lime in order to ward off contamination.

For certain species, such as Wine Caps (Stropharia rugosoannulata) the casing layer should only be lightly pasteurized- in order to retain the beneficial bacteria that is necessary to initiate fruiting.

Eventually, any casing layer will contaminate if it remains moist for long enough – but the grower should be able to get a number of flushes before this happens in most situations.

Types of Casing Layers for Home Growers

Many different materials can be used for a casing layer- including garden soil, sphagnum moss, coco coir- but the most commonly used is material for casing is peat moss. Peat moss is cheap and widely available, and is excellent at retaining moisture.

Another excellent casing material is vermiculite. Vermiculite is an inorganic material typically added to soil to help potted plants retain moisture. It’s also very resistant to contamination, meaning it can be added directly to the top of the substrate without any consideration for sterilization or pasteurization.

Our personal favorite casing layer  is a 50-50 mixture of peat moss and vermiculite, moistened to “field capacity” and pasteurized for 30-40 minutes.

Peat moss is naturally acidic, so growers often add calcium carbonate to the mixture in order to buffer the pH. This helps to mitigate the chances of contaminates flourishing in the moist casing layer. A typical ratio of calcium carbonate would be 2-3% calcium carbonate by wet weight of the casing layer.