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Mushroom caps splitting and cracking issue

Mushroom caps and stems cracked
Mushroom caps and stems split in mushroom growing

Split or cracked mushroom caps: main reasons and how to fix them

Split or cracked mushroom caps (stems as well) can occur in any active or edible mushroom species. Let's explore the main causes and how to address this issue.

Cracked shrooms caps
Cracked shrooms caps

Main causes of cracked or split mushroom caps

As you probably know, mushrooms consist of about 90% water and require high relative humidity (RH) of around 90% for pinhead initiation and fruit body growth.

The mushroom cake/block may be moist enough, but the air outside the cake/block is too dry. Usually, room RH varies from 30% to 70%. This means that for mushrooms, you need to maintain higher humidity than in your room. That's why we use fruiting chambers as closed environments with specific climate conditions for mushrooms (such as monotubs, shoeboxes, growboxes, dub tubs, plastic bottles, grow tents, and grow rooms).

🔴 Fluctuations in relative humidity (RH). The sharp difference between the required humidity (90%) and the low humidity for mushrooms (lower than 85%) causes a fast evaporation rate, moisture loss, and mushroom dehydration.

Mushroom cap crack
Mushroom cap split
Caps cracks
Caps cracks due to dehydration - low RH and dry mushroom cake


🔴 Moisture loss also occurs due to active fresh air exchange (FAE) and frequent aggressive fanning. As a result, we experience rapid moisture loss, leading to cracks on caps and stems as well.

Mushroom stem cracked
Mushroom stem split

🔴 Another reason for split caps is a high temperature in the fruiting chamber. In this case, yellowish pigmentations and secondary metabolites usually appear on mushrooms and mycelium.

Imbalanced or incorrect fruiting parameters can impact the development of mushroom fruit body

🔴 Lastly, cracked mushroom caps and abnormal growth can be a result of genetic factors, mutations.

APE mushroom caps are cracked
APE shroom caps are cracked. Photo: Mycelia P

Some mutations are triggered by sharp changes or improper fruiting conditions. Certain genetic mutations are already present in cells (for instance, mushroom spores with genetic mutations) from the very beginning of mycelium development. Sometimes mutations look funny.

Mushroom cap cracked due to genetic mutation
Mushroom cap cracked due to genetic mutation

❗️ Pay attention! If cracked caps and/or split stems have color changes (yellow, brown, gray, black), have spotting or necrotic symptoms, abnormal deformations, then it can be a sign of contamination!

How to fix mushroom cracking

You cannot fix it if it has already happened; however, it is possible to prevent it in future flushes.

Maintain a balance between fruiting parameters

Track your fruiting parameters using a simple device (available on Amazon): relative humidity, temperature, and CO2 level (optional).

Maintain relative humidity

To maintain high RH, use ultra-fine water mist. The Flairosol spray bottle (available on Amazon) is a cheap and effective solution for manual misting. Mist the walls of your tub! Don't spray on mycelium and mushrooms directly!

A fogger/humidifier (available on Amazon) can also be used, although it is more suitable for large tubs or grow tents. Avoid water pools on mushroom cake.

Provide gentle fresh air exchange to reduce CO2 level

Passive natural FAE through air vents in your tub or gaps between the tub and lid is sufficient. Avoid vigorous fanning as it can dry out your cake and mushrooms.

Active FAE involves using a fan or waving a lid, while passive FAE relies on allowing air to naturally flow and diffuse

An aquarium air pump (Amazon) or any small PC fan (Amazon) are good solutions for additional FAE in a growbox.

If you use a fan, turn it on a few times a day for 1-2 minutes. Mist immediately after fanning.

Proper temperature for the mushroom fruiting period

Each mushroom species requires a specific temperature range for normal fruiting, and the temperature difference can vary significantly between species. For edible mushrooms, the temperature ranges from 50°F to 82°F (10°C to 27°C), while for active mushrooms, it can range from 50°F up to 100°F (10°C to 40°C).

Fresh air exchange, relative humidity, and temperature should be balanced during the fruiting period

Genetic isolation

Starting your growing journey with a spore syringe is always like a lottery. You never know what you'll end up with. Many mushroom colonies coexist, resulting in a wide genetic variety within the same mushroom cake/block.

You cannot fix genetic mutations during the cultivation process. However, you can work on genetic isolation (strain isolation, selection, cloning) to achieve perfect mushroom fruits in the future.

To sum it up

Mushroom splitting and cracking are neither contamination nor a disease. Such mushrooms are safe to consume.

However, you should check your fruiting parameters: relative humidity (should be 85-95%), temperature (check this parameter for each species), and fresh air exchange (natural FAE or active FAE with a timer and schedule).

All these factors should coincide and be balanced.

If you find this guide helpful support my work with subscription or one-time donation on ☕️ buymeacoffee

Join Shroomok Community on Discord or Reddit for questions and sharing your experience.

Have a happy growing and healthy shrooms!

Peace, Shroomok ❤️

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