If you’ve spent any time researching how to grow mushrooms, then you’ve likely come across the term “casing layer”. But what is it, why do some growers use a casing layer, where others do not, and what are the benefits? This article describes the benefits and drawbacks of using a casing layer, casing layer recipes, and the process to make a casing layer. 

What Does a Casing Layer Do?

A casing layer helps to provide an ideal micro-climate for primordium (pins) to form. Additionally, the casing layer helps the colonized substrate retain its moisture as well as prevent contamination from competing organisms. 

Benefits and function of a casing layer.

According to their bestselling book, The Mushroom Cultivator. Stamets and Chilton describe the four essential functions (benefits) of the casing layer as:

  • Protecting the colonized substrate from drying out.
  • Providing a humid micro-climate where mushroom primordia can form and develop.
  • Creating a water reservoir for the mushrooms as they grow and mature.
  • Supporting the growth of beneficial microorganisms. (Chilton and Stamets)

The Micro-climate:

At first glance, the casing layer appears to lay flat over the colonized substrate. However, at a micro-level, the casing layer actually provides small pockets of air between the colonized substrate and the casing materials. These “caverns” maintain high humidity levels and allow for slight fresh air exchange, which creates ideal conditions for pins to form. 

Can you Get Larger Flushes When Using Casing Layers? 

Yes! A casing layer can definitely help. 

By optimizing your growing conditions, which includes using a casing layer, yields can be significantly increased. In fact, according to Stamets, the yield potential for P. Cubensis is approximately 2-4 pounds of fresh mushrooms per square foot, per 5-week growing period (Chilton and Stamets).

Drawbacks of a Casing Layer

Time – The primary drawback of a casing layer is the additional step in the cultivation process and the subsequent time for the mycelium to adapt and react to the new conditions. While this may seem to be a minor inconvenience, many home growers opt to skip this step entirely. 

– Another drawback is the potential for complete overrun of the casing layer by the underlying mycelium. This condition, referred to as “overlay”, can result in a dense layer of mycelium growing on top of the casing layer which hinders the formation of pins and can halt growth altogether. Therefore it is important to remain diligent in maintaining ideal fruiting conditions.

Over-Colonization/Overlay may be caused by the following: 

  • Carbon dioxide levels that are too high (not enough air exchange)
  • The casing surface is too dry
  • The relative humidity of the air is too low
  • The temperature in the fruiting chamber is too high

If your substrate is already overlayed, you may scratch the surface of the mycelium using a fork or other object to promote pinning. 

over colonized tub
Example of over-colonized substrate

Quick Note: Pay Attention to pH Levels

Before making a casing layer it’s important to understand that your casing layer should have a higher pH level than the substrate used for colonization. A higher pH will help to inhibit molds from growing on the casing layer, as well as deter the mycelium from over-colonizing the casing layer. The ideal pH should be between 7-8.5 (Agriculture and Natural Resources 234).

pH levels in casing layer
Check the PH of your casing layer

Substrate Casing Layer Recipes

Below are three of the best casing layer recipes we’ve come across. Keep in mind that the ingredients you choose may have varying pH levels. Always test your casing layer using pH strips prior to use.

Stamets Casing Mix


50/50 Casing Tek


60/40 Casing Tek


While these are just a few examples, you may modify or create your own recipe depending on what you have on hand. Some growers use a 100% Coco Coir Casing Layer, or a pure Vermiculite Casing Layer with generally good results. Just remember to hydrate your ingredients to the proper capacity and always check the pH!

How to make a Monotub Casing Layer

Your casing mix can be used in a variety of different growing chambers and the process will be the same no matter the recipe you use. Keep in mind that the casing layer should not be “soupy”, but rather the drier the better. The water content of the end product should be roughly 60%-70%. See this post to learn how to find your substrates’ water content amount.


  1. Select a casing layer recipe and weigh out all of the dry ingredients.
  2. Add all of your dry ingredients into a 5-gallon bucket or another mixing container.
  3. Slowly add distilled water to the mixture, stopping to mix the ingredients together using a large wooden spoon, a clean hand, or another utensil. Important – wear gloves if mixing by hand.
  4. Continue this process until you are only able to squeeze out a few drops when squeezing the casing. This will be in the general range of ideal moisture content of 75%. However, for precise moisture content measurement, you can use a hot oven or microwave method to determine the saturation level. Learn how to measure moisture content here. Add vermiculite or additional peat moss if the mixture is too moist.
  5. Pull out a handful of casing mix and squeeze it over a pH strip. The ideal pH level is 7-8.5. Add pH adjusting ingredients as necessary. For reference: Water has a pH of 7, peat has a low pH and should be used to reduce the pH, and calcium carbonate and hydrated lime are used to increase the pH.  
  6. Pack your casing mixture into an oven-safe bag (large Ziploc bags seem to work) and place in the oven or water bath at a temperature of 140F to 170F for 1-2 hours.
  7. Store in a dry place until ready for use. 

When should you use the casing layer?

The casing layer should be used just before or just after the mycelium has fully colonized the bulk substrate. The casing layer colonization step takes place between the substrate colonization and fruiting steps. 

How to apply a casing layer

  1. To use the casing you will first need to remove the lid from your monotub or growing chamber. 
  2. Using latex or surgical gloves, grab a handful of the casing and begin to spread a thin layer of casing over the top of the colonized substrate. Try to keep the casing layer at an even and uniform depth no greater than a half-inch deep.
  3. Replace the lid of the growing chamber.
How to use a casing layer

Next Step: Many growers have their own preferences on how long they will wait to place into fruiting after applying the casing layer. This is considered the post-casing/pre-pinning stage and can take anywhere from a few days to a week. During this time it is best to keep the grow chamber in a dark place, maintaining a relative humidity of 95% and an incubation temperature (75 – 80°F) (Shroomery)

After 1-3 days you should notice aggressive mycelium stretching upwards from the valleys in the casing layer. This is an indication that the mycelium is ready to be placed in pinning/fruiting conditions.

How to promote mushroom pinning:

  • Reduce CO2 buildup by introducing fresh air (fan or open vents)
  • Reduce the temperature to 70°-75°F
  • Introduce ambient light. No direct sunlight
colonized casing layer ready for fruiting conditions

And there you have it! Everything you never knew you wanted to know about mushroom casing layers. 

Do you use a casing layer, and if so, what do you use?

I’d love to hear from you. Leave a comment below. 

Works Cited

Agriculture and Natural Resources. “Casing layer and effect of primordia induction in the production of Agaricus subrufescens mushroom.” Agriculture and Natural Resources, vol. 51, no. 4, 2017, pp. 231-234. Science Direct, https://www.sciencedirect.com/science/article/pii/S2452316X17305082. Accessed 27 3 2021.

Chilton, Jeff S., and Paul Stamets. The Mushroom Cultivator: A Practical Guide to Growing Mushrooms at Home. Agarikon Press, 1983. The Mushroom Cultivator on Amazon, https://amzn.to/3NjALWe. Accessed 27 March 2021.