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Side and bottom pins

Bottom pins and side pinning
Bottom pins and side pinning

Mushroom growth only on the sides and bottom of the cake

Question: Are my side pins caused by light?

What are these guys doing here? Side pins
What are these guys doing here? Side pins

For a long time, mushroom cultivators believed that side pinning occurred by light. Light influence on the pinning process of Basidiomycetes Fungi, but it is a secondary trigger, that mostly influence on active phase of fruit body development. Actually side pins are mostly from microclimate, not light. Basically when your substrate colonizes and pulling away as it dehydrates, the tiny gap between the cake and tub creates the perfect microclimate for side pins and even for bottom pins, where enough moisture.

Question: Why my mushrooms are only growing around the edges of the cake or underneath and not growing evenly across the entire cake?

Why are my mushrooms fruiting only on the sides and underneath and not growing on the top?
Why are my mushrooms fruiting only on the sides and underneath and not growing on the top?
Bottom pins fruiting
Bottom pins fruiting

It is not a problem when mushroom cake has rich fruiting 360 degrees or just a single side pin. The real issue, when mushrooms don't fruit on the top at all, but on the sides or at the bottom only. Zero pins on the top layer and side pins can be an indication that your surface level field conditions aren’t right!

Each environment is unique. Season of the year, climate zone, thus temperature and humidity fluctuations are different. Hence each mushroom cultivator should adjust growing conditions according to shrooms needs. Although there are some basic mistakes to avoid.

Common newbie mistakes causing side pinning and zero pins on top layer

Improper field capacity of bulk substrate
Only side growing shrooms
Only side growing shrooms

Mistake from the start. Perhaps you used too dry bulk substrate on spawn to bulk step. This moisture was enough for bulk colonization, but not enough for further mushroom fruiting.

It's a huge mistake and you should learn this basic lesson to avoid it for the future!

What to do?

  1. Check the field capacity of bulk substrate BEFORE using it. Field capacity means a perfect substrate moisture conditions. A good way to test for this 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. If a light squeeze causes a stream of water to come out, it’s too wet. 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.
  2. Harvest the side growing mushrooms and rehydrate (soak) mushroom cake in water for a few hours. Rehydration helps to saturate the mushroom cake with missed water.

Recommended guide: Rehydration aka soaking mushroom cake

Low relative humidity in fruiting chamber

When relative humidity (RH) in the fruiting chamber lower than 90% substrate looses moisture faster. Moisture evaporation from the top layer is higher than from the sides. Mushroom cake shrinks, hence the gap between the walls of the tub and mushroom cake appears.

If you use liner (plastic bag inside) mushrooms appear near the walls, where cake more moisturised. Cracked caps are also the sign of low humidity environment.

Dry environment - cracked caps and mushroom fruiting near the walls of the tub
Dry environment - cracked caps and mushroom fruiting near the walls of the tub

This is a common problem for grow tent setup (especially for the first run) or while you testing new tek, new equipment.

Side pins fruiting near the walls of fruiting chamber and zero mushrooms on the top surface
Side pins fruiting near the walls of fruiting chamber and zero mushrooms on the top surface

But it is more common case for the impatient growers, who open the fruiting chamber every hour to check the progress and they literally drying out their cake.

What to do?

  1. Monitor RH level with any simple hygrometer. Place the sensor inside the tub and the screen outside the tub.
  2. As in natural environment humidity fluctuations is totally normal and even beneficial for promoting pinning. You don't need to maintain stable humidity all the time. However if you see on hygrometer 85-89% it means you need additional misting (mist the walls of your tub) or use humidifier/fogger, especially after each opening the tub.
Excessive evaporation rate, air flow and fanning

Very often newbie growers show me a common case, when relative humidity in tub is 90-99%, there are condensation water drops, but top layer of the cake looks dry. Mostly this humidity level maintained by extreme evaporation from the substrate. The more moisture evaporates in the air the less moisture inside the substrate. It's conrintuitive sometimes.

Side pinning and side growing mushrooms
Side pinning and side growing mushrooms

Using fan, ventilation, air conditioning in your room where you have a fruiting chamber can create additional air currents that drying out the substrate through air vents in tub.

Excessive water loss, thus extreme shrinkage, creates the gap between cake and tub. Hence, there are perfect microclimate for fruiting in this gap, along the edges of the tub, while the top layer becomes a desert for mycelium

As mushrooms are 90% water, the better places for growing are more hydrated areas where water loss are less. Mycelium think: "We need more moisture to form fruit bodies. Too dry here. Let's search for a better place. The sides and at the bottom are pretty good conditions. Here are more chances for development fruit body and producing mature spores for our next generation. Let's do it!"

That's why we have pins and then shrooms on the sides, sometimes at the bottom, where climate conditions are more favorable.

What to do?

  1. Adjust FAE with fan. If you use fan for Fresh Air Exchange (FAE) avoid extreme air flow and avoid fanning for too long. In this way you remove moisture and dry the cake. Adjust your FAE if you noticed that top layer drying out. Change the time and frequency, i.g. 2-3 times a day for 20-30 seconds. Or don't use fan at all.
  2. Misting or Humidifier. Don't forget to mist the walls of your tub or use fogger after each opening/fanning to maintain proper humidity level without extreme evaporation from the substrate. Avoid misting/spraying water on mycelium directly.
  3. Adjust ventilation air holes in tub. If you have air holes in your fruiting chamber you can adjust the natural airflow, that remove moisture out of your tub. Use tape to adjust for more or less FAE as needed. Sometimes closing air holes by half saves situation. Sometimes closing air holes completely and flipping the lid of the tub is a great FAE adjustment as well. Sometimes you need to close the air vents completely and get your hands out of your tub. Don't open it every hour to check how's it going. Often LITFA tek creates perfect microclimate balance!

LITFA - Leave It The F*ck Alone - common acronym among mushroom cultivators for impatient newbie growers

Vermiculite top layer

Only vermiculite as a top layer for mushroom cakes often works bad. Vermiculite retain water worse, tends to dry out faster than true casing layer or thin coco coir mix layer. It is a common problem for growkits.

Vermiculite top layer for growkit
A few side growing mushrooms and zero shrooms on the top layer of growkit

What to do?

  1. Use coco coir mix or peat moss (sphagnum + lime) as a main ingredients for top layer (1/3 inch or up to 1cm). They keep moisture much better. You can use it for your growkit as well.

How to prevent side pinning

Inside liner for preventing side pinning

For most of growers inside liner is just a personal preference to use it or not.

It can be any plastic trash bag. The only downside of it that you can't see signs of contamination if the color of this bag is black (or any other color).

Much better option for inside liner is transparent painters plastic. It sticks to the substrate very well, it's super cheap and easy to work with it. You can still see if you get side and bottom pins or signs of contamination.

As long as those inside liner sticks to the cake really well it will prevent from side and bottom pinning.

When you line the tub inside, the lining shrinks with the mushroom cake and shrooms won't have such conditions between tub and cake. As far as side pinning is because the microclimate between the cake and bin is ideal for fruiting conditions. The purpose of the liner is to adhere to the cake and move with the shrinking cake thus preventing or reducing the microclimate between the two surfaces/sides.

Trim the liner after Spawn to Bulk to match the substrate level. If the liner over the substrate it creates excess of moisture on mycelium and issues with Fresh Air Exchange (FAE).Liner is a pretty simple way to prevent side pins. Not really any cons to using them either.

Tip: Inside liner is also helpful for harvesting process and further soaking mushroom cake in water (mostly for big cakes). It helps against cracking the cake

Anyway, if something wrong with your environment mushrooms will grow close to the wall of your tub even if you use inside liner

Side pins and side growing mushrooms in tub
Side growing shrooms
Press down the bulk substrate on around the outside perimeter

It doesn't matter if you use or don't use an inside liner, pack the substrate down during Spawn to Bulk a little tighter, especially near the tub walls. It will prevent some of the side pinning on the initial flush. This tamping down the substrate enough keeps the cake against the wall longer.

I never line my shoebox tubs, because side pins are really easy to harvest in them. Just make sure you pack the substrate down pretty tight to avoid massive side pins for the first 1-2 flushes

Here is an example when bulk substrate wasn't compacted along the tub walls. Mushrooms grow on the top and from the sides.

Side growing mushrooms in monotub
Side growing mushrooms in monotub

Promote pinning on the top of the cake with Bubble Wrap

You can lay bubble wrap or food film (aka bubble wrap tek or blanket-tek) to promote pinning on the top of your mushroom cake. This method works if the top layer still have some moisture, not dried.

In this way, we save moisture inside the cake and replicate a climate conditions between the mushroom cake and the film, close to the conditions between the cake and the tub, that favorable for side pinning.

Spray some water on the piece of bubble wrap or food film and put it on top of the cake. Remove the film once a day to introduce fresh air and check the progress.

Bubble wrap tek or Blanket tek with food film
Bubble wrap tek or Blanket tek with food film

Typically in 3-5 days you can see first pins on the top. From this moment remove the film completely and provide normal fruiting conditions for pins transformation into mature mushrooms.

Are side pins bad

Question: Are side pins bad? Can you eat this? Does it have any potency?

Are side pins bad? Can you eat this? Does it have any potency?
Are side pins bad? Can you eat this? Does it have any potency?

Side and bottom growing pins that are able to develop into mature mushrooms are totally healthy shrooms. They may look ugly, curved, may be too wet. But they are totally normal for harvesting, potent and save to consume.

Side and bottom growing mushrooms
Side and bottom growing mushrooms

For newbie growers side pins and side growing mushrooms can be the only mushrooms to be harvested. Better than nothing! Isn't it?

Most of my mushroom fruits are sidepins
Most of my mushroom fruits are sidepins

Even if you are experienced grower you can get side pins only, when you are trying to adjust climate conditions, while playing with new tek or new equipment for your growing setup.

Still dealing with climate conditions in the tent. Side growing mushrooms
Still dealing with climate conditions in the tent. Side growing mushrooms

Cultivators that have mushrooms on top also can have side and/or bottom growing mushrooms. They are your bonus fruits!

It's totally normal to have side pins. Let them be!

Let me share a few examples and comments from experienced growers.

1. You would be surprised at the size a side pin can get if left alone. And canopy’s are not always going to happen.

Giant side pins growing in a bag
The side pins in grow bag grow like tar
Huge side growing shroom
Huge side growing shroom

2. I’ve said it maybe hundreds of times now… Some of the largest fruits I’ve ever measured have been side/bottom pins. Here is a side pin that is currently in my tent…

Huge side pin
Huge side pin

Honestly, some of the largest fruits I’ve seen from better growers than I am have been sidepins too! They aren’t a bad thing, and it’s a losing/winning battle.

Sidepins Happen! 125 g
Sidepins Happen! 125 g

3. This picture doesn’t do it justice the side pinning is crazy on this one

Testing a bottle-tek and have a crazy side pinning
Testing a bottle-tek and have a crazy side pinning

A few days later. Look at this side pin city!

Side pinning bottle-tek
Side pinning bottle-tek

Are bottom pins bad

Question: How bad can side pinning be? Well, I’d say bottom pinning. I look under my tub and see a canopy of bigger pins under tub. The cake is literally being lifted off of the bottom from all the mushrooms.

Many bottom pins
Many bottom pins

If I see that I’m getting a lot of side pins (mainly after 1st-2nd flush), I’ll put the cake on the racks or on the other side. If you have massive bottom pinning you can flip the cake upside down, thus shrooms will have more space for growth, will be more attractive and it will be easier to harvest.

Done! Flipped the cake upside down to let bottom pins grow!

Flipped the cake with bottom pins
Flipped the cake with bottom pins

I usually pick bottom pins before anything fun happens with the cap, because they get too wet or they abort and the caps just stop. But the side pins make it eventually, so I let them keep at it. This cake is much happier upside down xD

This mushroom cake is much happier upside down
This mushroom cake is much happier upside down

Side pins harvesting tips and issues

For initiating new flush of fruiting by soaking cake in water you should pick ALL shrooms on the top, all sides and bottom. Why?

If you leave pre-mature and mature mushrooms from the sides and at the bottom they might start to decay and rot in the nearest days after soaking. Your cake will be contaminated and wasted.

You can leave tiny pins on the cake. Some of them continue growing, some of them may turn into aborts.

On small tubs, shoeboxes or small mushroom cakes side pins don’t really matter since they’re easier to harvest. Just remove the cake from the box and harvest all shrooms.

Side growing mushroooms
After removing from plastic box you can see top and side growing mushrooms on small mushroom cake with pseudo casing layer. Preparation for soaking in water


For bigger mushroom cake it can be annoying to harvest side and bottom pins.

Side growing mushrooms in monotub ready for harvesting
Side growing mushrooms in monotub ready for harvesting

To make harvesting easy and to avoid crumbling/cracking cake during harvesting side and bottom pins follow the next steps:

  1. Pick all shrooms on the top. Twist mushrooms by rocking slightly to the sides and remove them gently. So that you won't damage mushroom cake.
  2. Put a thick surface on the top of the cake. Use the lid from the tub of smaller size, or ceramic plate, or baking tray that +/- fits your cake size.
  3. Put your hand on this lid/tray/plate.
  4. Flip the tub with the other hand and remove cake from the tub. So that you have a cake on the tray upside down.
  5. Voila! Now you can harvest all mushrooms grown on the sides and at the bottom without cracking the cake.
  6. After harvesting put the cake back in the tub in the same way. It is ready for rehydration step.

Question: I noticed I had a lot of side pins. So decided to move my cake to a bigger tub with a rack. But when I moved the cake, it broke in half, not fully to become two pieces, it’s still one piece with a big crack. Question is… have I just destroyed my grow?

It should be fine. If your cake cracked into 2-3-4 parts go ahead! Shrooms will grow even if the entire cake turns into many slices. Put them in a tub, provide proper fruiting conditions and wait. You'll get rich fruiting in your 'big crack' area.

Sum it up

When the cake pulls away from the wall (its normal natural process) it creates a tiny microclimate that is usually ideal surface conditions for pinning.

Mild cake shrinking is totally normal process due to evaporation and absorbing moisture by mycelium and mushrooms while growing. However it doesn't mean that any minimal gap between cake and tub is a problem to react.

It's amazing when your mushroom cake fruit everywhere! However fruiting from the sides or bottom and zero mushrooms on the top says you about mistakes in your climate conditions.

So the microclimate in the sides of your bins is perfect, right? So ask yourself, what do you need to do to mimic THAT in the whole tub?

First of all, avoid basic mistakes, such as improper substrate field capacity, excessive fanning, excessive evaporation, low relative humidity of the environment. Create favorable microclimate for your mushroom cake.

You can counter side pinning by packing your substrate down tight and using a liner inside the tub.

Side pins and bottom pins is not a problem at all. They are your bonus mushrooms!

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

Have a happy growing!

Peace, Shroomok

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