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Category: Contams & Troubleshooting

Psilocybe growing problems - overlay, abortions, etc

Overlay and Stroma in Mushroom Cultivation

The process of cultivation psilocybin mushrooms is not only joy and pleasure. There are also difficulties and failures. Sooner or later every mushroom cultivator comes across them. Here we are going to show you one of the most common problem in the growing process — overlay, stroma and related issues.

What is overlay and stroma

Here we have 2 news.

The good one — overlay and stroma are not contaminants. And there are chances to fix the problem.

The bad one — growing conditions for pinhead initiation have not been met. So, you might not get harvest at all!

Overlay and stroma look alike. Cultivators don't distinguish the difference between these two definitions. And it makes sense.

❗️Paul Stamets shares two definitions. He admitted overlay caused by wrong fruiting conditions and stroma caused by genetic changes

Overlay is a dense mycelial growth that covers the casing surface and shows little or no inclination to form pinheads. Mainly it caused by prolonged mycelial growth into the casing layer. Substrate and casing layer are 100% colonized are at such a risk. However, there are chances to fix the problem

Stroma is a dense, mat-like growth of mycelia or cushion-like aggregation of mycelium forming on the surface of composts or casings and indicative of somatic (vegetative), not generative growth.

🧬 Such mycelium can absorb nutrients, but can't produce mushroom fruit bodies. At the fruiting stage the cultivator won't be able to influence such genetic changes in any way. Although inappropriate fruiting conditions can be a trigger for genetic changes.

How to identify overlay

🔴 Visually it is a dense snow-white layer of mycelium resembling fluffy cottony clouds.

1 - start of stroma overlay. 2 - mushroom cake covered by stroma overlay
Step 1 - Start of stroma overlay. Step 2 - mushroom cake covered by stroma overlay
Overlay on psilocybin mushroom cake
Overlay. Psilocybin mushroom cake after rehydration
Overlay and mycelium products
Overlay and mycelium products
Mycelium overlay. Growing psilocybe cubensis
Mycelium overlay. Growing psilocybe cubensis

The difference between normal and overlayed mushroom cakes.

Difference between cakes with fluffy/down-like, Tomentose Mycelium and fruiting cakes with a wiry-type structure, Rhizomorph Mycelium
Difference between cakes with fluffy/down-like, Tomentose Mycelium and fruiting cakes with a wiry-type structure, Rhizomorph Mycelium
Overlay on the cake with blue psilocybin spots. Problems in growing psilocybe Cubensis
Overlay on the cake with blue-gray psilocybin spots. Problems in growing psilocybe cubensis

🔴 To the touch neglected form of overlay like cotton pads or polyurethane foam. It can turn into a dense dry crust that block mushroom cake and doesn't allow air and moisture to pass through. Overlay often accompanied by Mycelium secondary metabolites or mushroom pee (phase 2 on the photo).

1 - start of overlay. 2 - mycelium turned into crust of overlay
1 - start of overlay. 2 - mycelium turned into crust of overlay
Strange cottony mycelium or mycelium crust
Strange cottony mycelium or mycelium crust
Stroma overlay
Mycelium crust of overlay stroma.

Main causes of overlay

⛔️ Prolonged colonization period for grain spawn in incubator and overlay can form in a spawn jar. However, it's not a problem. Remove mycelium overgrowth and use grain spawn for the next step (Spawn to Bulk).

Overlay in spawn jar
Overlay in spawn jar

⛔️ Prolonged time for top layer colonization after Spawn to Bulk. Provide fruiting conditions when 70-80% of the top layer is covered with mycelium.

It's too late start of fruiting conditions for such mushroom cakes.

Fluffy cakes. Overlay. Psilocybe Cubensis F+
Fluffy overlayed cakes. Psilocybe Cubensis F+

⛔️ Too dry or too wet top layer or bulk substrate. Control field capacity to have perfect moisture balance of the substrate.

⛔️ Imbalanced fruiting conditions: CO2, temperature, relative humidity

Use air monitor to track basic fruiting parameters during mushroom fruiting period : temperature, relative humidity, CO2 level

All-in-One: Digital СO2 detector
Air Quality monitor: Digital СO2 detector + Thermometer + Hygrometer

Shroomok's Choice on Amazon:

Mini Air Analyzer 3-in-1: CO2, Temperature, Humidity $45.99

8-in-1: CO2, Air Pollution, Temperature, Humidity $59.98

⛔️ High carbon dioxide (CO2) concentration (above 800-1000 ppm). Recommended CO2 concentration for Psilocybe Cubensis is 600-800 ppm.

⛔️ Low relative humidity in the fruiting chamber (less than 85%) and high CO2 concentration.

⛔️ High temperature, 77F or 25°C and above for Psilocybe Cubensis.

⛔️ High evaporation rate, high RH and high CO2. The humidity level in the fruiting chamber is high, but it is provided by high evaporation from the top layer, thus mushroom cake dries out quickly.

⛔️ High substrate acidity. To prevent substrate acidification add buffering agents like hydrated lime (Amazon) to the casing layer and bulk substrate. Details are in Spawn to Bulk and Casing

⛔️ Strain degeneration, old genetic. Some genetic may be capable only for vegetative growth. Mycelium can absorb nutrients, but is unable for pinning and fruit bodies development.

⛔️ Aged mycelium. If the grain spawn was stored for too long (over 3 month) this can cause genetic changes and stroma growing.

⛔️ Overlay and stroma is often a complex of several factors:

  • dry mycelium due to high evaporation rate + high CO2 = overlay
  • high CO2 + high RH and water drops on mycelium = overlay
  • high t° + low RH + high CO2 = overlay
  • low RH + high CO2 = overlay
  • dry top layer + high CO2 = overlay
  • dry top layer + low RH + high CO2 = overlay
  • any of parameters + genetic issues = stroma

If you don't decrease the temperature in the growbox/monotub, don't balance relative humidity (RH) and fresh air exchange, mycelium continue vegetative (mycelial) growth and won't form pinheads

🔴 Recommended: Fruiting period. Pinhead initiation and mushrooms growth parameters for Psilocybe Cubensis

How to fix overlay and initiate pin setting

It is impossible for mushroom pins to break through the thick layer of mycelium and start fruiting. Because the top layer of mycelium is literally dead.

Stage 1 on the photos below are mushroom cakes at the start of overlay formation. Such a problem can be handled by increasing fresh air exchange every 4-6 hours (but not aggressive fanning) or by connecting an aquarium air pump for gentle fresh air exchange 24/7.

Stage 2 - is already overlayed cake

1 - start of stroma overlay. 2 - mushroom cake covered by stroma overlay
Step 1 - Start of stroma overlay. Step 2 - mushroom cake covered by stroma overlay
1 - start of overlay. 2 - mycelium turned into crust of overlay
1 - start of overlay. 2 - mycelium turned into crust of overlay
1. Fix fruiting conditions

First stage (phase 1 on the photos above) of overlay is easy to fix by balanced fruiting conditions.

Increase the FAE to reduce CO2 level. Gently introduce fresh air every 4-6 hours manually. Just open the lid of the tub for about 30 seconds.

Maintain relative humidity around 90%. The walls of fruiting chamber should be covered with small water drops.

Air temperature in the fruiting chamber should be around 70-74°F or +21°..+23°C.

In 5-10 days first pins should appear

Great thing to use is aquarium air pump for constant FAE in the growbox. For sufficient FAE use air pump for 60 Gallons per hour and higher. This would be enough for the fruiting chamber of 30-80 Qt

Aquarium air pump for introducing FAE (180 L/hour)
Aquarium air pump for introducing FAE (180 L/hour)

The aquarium air pump should be switched on all the time 24/7 during this hard period. This way fresh air move in the growbox all the time and don't worry about high CO2 level. There is a rock at the end of the hose, it should be dropped into a jar of water. This will filter the incoming air against contams.

Aquarium air pump for FAE
Growbox with aquarium air pump for FAE
Monotub with air pump for FAE
Fruiting chamber with air pump in the corner

Shroomok's Choice on Amazon:

Quiet Oxygen Pump for 20-100 Gallon - 2 Outlets - $15.99

Powerful Air Pump - for 20-200 Gallon - 2 Outlets - $22.89

2. Fork tek and Casing

You may think that this method is too cruel, but it works great for significant overlay, stage 2.

Step 1. Scratch the mycelium with sterile fork aka Fork Tek.

The fork should be sterile! Wipe it with iso alcohol. Otherwise, you increase the risk of contamination

Overlay and fork-tek
Overlayed mushroom cake and fork-tek

In this way we remove the crust and give an air access for mushroom cake. But don't scratch it off completely!

How to fix overlay? Fork scratch
How to deal with overlay? Mushroom cake after fork scratching

Step 2. Make a casing or pseudo-casing layer

Casing layer recipe and advantages

Grain for spawn is a highly nutrient substrate for producing mycelium, till too much nutrients mycelium continues colonisation phase and don't want to produce mushroom fruits.

Bulk substrate (coco coir mix) is a source of water and less nutrient.

True casing layer is a non-nutrient substrate that promotes pinning. The mycelium thinks it’s getting choked off and fights to fruit so it can deliver its spores and live on. Casing layer is a sign for mycelium "No more nutrients! Hurry up! Go go go, save your genes!" And boom! Fruits are coming for sporulating!

The recipe for casing layer:

  • sphagnum aka peat moss - 2-3 cups
  • vermiculite - 1 cup
  • hydrated lime aka calcium hydroxide - 0,5-1 tbsp
  • water - 1-1,5 cup

Sterilization in a pressure cooker or instant pot at 10-15 psi for 1 hour.

Let the jar cool down to room temperature.

Pseudo-casing layer recipe

Less effective method, because coco coir is relatively nutrient. However if you don't have a peat moss it works as well.

The recipe for casing layer:

  • coco coir - 130g or 1/5 coco brick
  • vermiculite - 1.6 cups
  • hydrated lime aka calcium hydroxide - 1 tbsp
  • water - 3.6 cups or 0.9 qts

Check field capacity (balanced moisture level). A good way to test it is to grab a handful of the substrate mixture and squeeze it in your hand.

⛔️ If you are unable to squeeze out any water, it is too dry. Add more chilled boiled water.

⛔️ If a light squeeze causes a stream of water to come out, it’s too wet. Squeeze the substrate before use.

✅ You want to be able to squeeze the mixture as hard as you can, and only have a few steady drops come out to have a field capacity moisture level. This is perfect!

Make sterilization or pausterization as for bulk substrate in bucket or pot. Or you can fill the jar with ready substrate and put it in pressure cooker at 10 psi or instant pot for 1 hour or pausterize in a microwave. Let the jar to cool down to room temperature.

Step 3. Cover the cake with casing layer

Mushroom cake right after the fork tek.

Mushroom cake right after fork tek
Mushroom cake right after fork tek

Add 1/4-1/2 inch or 0,5-1,5 cm of casing layer on top of the cake. Casing doesn't require colonization conditions, provide fruiting conditions right after casing

Mushroom cake covered with pseudo casing layer
Mushroom cake covered with pseudo casing layer. Photo: Prof. Frags

Usually in 3-7 days first pins appear

Mushroom fruiting after fork tek and casing
Mushroom fruiting, 2-3 days after after fork tek and casing. Photo: Prof. Frags

Mushroom blooming!

Active fruiting after overlay
Active fruiting after overlay. Photo: Prof. Frags
3. Fork tek and rehydration with cold shock

This method will be useful for dry cake with overlay.

Scratch the overlay

Overlay and fork-tek
Overlay and fork-tek

If overlay wasn't removed, then rehydration is absolutely useless. Mycelium will work like a film and won't absorb water. The demonstration in the photo below

Magic mushroom cake with overlay in the water
Magic fungi cake with overlay in the water. Mycelium is watertight

Soak mushroom cake/block in cold water with added ice for 2-4 hours. Cold shock works as a trigger for pinning, can inhibit the aggressive mycelium growth and stimulate pin setting.

Casing layer after soaking and cold shocking is optional, but highly recommended!

Recommended: All you need to know about cold shock for magic mushrooms

Recommended: How to rehydrate mushroom cake

Useful notes for overlay treatment and prevention

🔴 Usually overlay indicates faults in the growing process. Check your growing conditions first. Early overlay is possible to fix by changing some fruiting parameters.

🔴 For fork tek scratching use sterile fork (treat it with alcohol). Don't touch casing layer with your hands. This can be a vector of contamination. Use sterile gloves (treated with alcohol) when work with mushroom cake.

🔴 After fork tek you may notice that mushroom cake is completely dry (more than 3 weeks after providing fruiting conditions). In this case rehydration is recommended.

🔴 Casing layer is optional, but highly recommended. Use casing layer 1/4-1/2 inch or 0,5-1,5 cm after bulk colonization to prevent overlay and stimulate pinning. Casing also works as a treatment for overlay and pinhead initiation.

🔴 Fork-tek is stressful for mycelium. It takes about 1-2 weeks for recovering and start of pinning. Be patient.

🔴 Overlay is a frequent companion of Tomentose mycelium (fluffy, cottony, cloudy). Rhizomorphic mycelium is less prone to overlay.

🔴 If you notice aggressive cottony mycelium growth in spawn jar/bag then mix spawn to bulk in 1:3 or 1:4 ratio. Less nutrients is better.

🔴 Even if you haven't made mistakes during the colonization and fruiting phase the cause of stroma overlay may be due to strain genetic. In this case the mycelium is only capable for vegetative growth. It will actively consume nutritions, but will not be able to form fruiting bodies. Alas, you won't get mushrooms from such cake even in ideal fruiting conditions.

About stroma and overlay in Shroomok growing experience

A prime example of overlay in my growing experience was Psilocybe Cubensis F+. It was aggressive colonization of grain substrate. The first signs of mycelium in the jars were already 4 days after inoculation. When casing was 60% colonized with mycelium the fruiting conditions were provided.

Overlay happened at the fruiting stage in the growbox at +21°...23°C, humidity 95% and FAE 4-6 times a day - fruiting conditions that were perfect for other strains.

Eventually, I haven't got mushrooms from these cakes at all.

I played with different options to promote pinning. Each mushroom cake with stroma overlay was subjected to a number of experiments: fork-tek, fork-tek with rehydration, cold-shocking. Casing layer promote some hyphal knots that stalled later.

None of the cakes were successful. They covered with a new white fluffy mycelium. No pins. No shrooms. Probably it was an example of genetic stroma.

After 75 days after inoculation and all experiments I said goodbye to three Psilocybe Cubensis F+ cakes.

Psilocybe Cubensis cake 1, F+ strain. 75 days after inoculation
Psilocybe Cubensis cake 1, F+ strain. 75 days after inoculation
75-th day after inoculation. Cake before disposing
Psilocybe Cubensis cake 2, F+ strain. 75-th day after inoculation. Cake with overlay before disposing
Psilocybe Cubensis cake 1, F+ strain. 75-th day after inoculation. Cake before disposing.
Psilocybe Cubensis cake 3, F+ strain. 75-th day after inoculation. Cake before disposing.

Accompanying problems with overlay

🔴 Check out: Mushroom aborts

🔴 Check out: Mycelium metabolites or mushroom piss


Check out the section Contams & Troubleshooting to explore cultivation issues.

More photo examples in 📸 Growing Problems photo gallery

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

If you find this guide helpful, please support me with a cup of coffee on ☕️ buymeacoffee

You can also leave your comments on this page below ⬇️

Have a happy growing and healthy shrooms!

Peace, Shroomok ❤️


I read your whole article. Haven't found much micellium yet. It's dry in So Cal, Los Angeles, San Fernando Valley. So I mist them and alcohol them. Try not to get to clumpy or soggy. Hard to find a happy medium. But I found some old grow dung dirt with old spores and hooked them up outside and the freshies will grow in my towel closet in the bathroom. Then I took some pieces of P.E. shrooms and,1/2 top and 1/2 stem and crumbs from bottom of bag, and it has micellium growing. That's in my bathtub. If I have success,I will have 9 kids of shrooms. I'll be praying hard. ❤ I'm a 67 year old lady baby boomer. I love shrooms for every reason....💋🤙🏼💎💃🏼🎬

Donald Gordon

I think you need to be more clear and concise about it's very confusing you have a bunch of extra information in there that you don't need to have and so therefore the conclusion is we we still have a bunch of unanswered questions

Shroomok avatar Shroomok

Your claim definitely deserves Oscar for concise. 230 symbols of complete uncertainity.
Please, go ahead Mr. Gordon, ask your questions here or on Discord / Reddit

big henry

I've followed the tips in this article. Unfortunately I lost 2 cakes to aggressive overlay, 1 cake to contamination, and it looks like I'm going lose 2 more :( Live and learn. The last set of 2 has just come back with a vengeance. Oh well. Maybe I should stick to spores instead of LC.

Shroomok avatar Shroomok

@big_henry please share photos of your problem on Discord or Reddit? I'll check them out and hope we can save some of your cakes; and figure out what's causing the problem to avoid mistakes in the future

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