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What is the best ratio of spawn to bulk for mushroom growing

Let's discuss how spawn to bulk ratio influences the speed of mycelium growth and overall results!

🔴 User 1: I have a question, when it's time to mix with the bulk substrate, is there a disadvantage/advantage to use a sub 1:1 spawn:bulk ratio? Or 1:2?

Recommended ratio of grain spawn to bulk substrate for mushroom growing
What is the best ratio of grain spawn to bulk substrate - Shroomok Growers Community on Discord

🔵 User 2: Not necessarily bad but typically 1:2 - 1:3 is ideal.

🔴 User 1: First of all, thanks for answering, I have gathered info on multiple forums and they all recommend anywhere between 1:1 and 1:4, but the definition of ideal is a bit blurry for me. Ideal in the sense that we can 'dilute' the spawn into larger volumes of bulk without affecting too much the yield of a single tub, effectively meaning I would 'waste' spawn if my spawn ratio is higher? Or ideal in the sense, that it's the ratio that maximizes the yield in any given tub?

I have also read that much higher bulk ratios increase the risk of contamination by increasing the time of colonization post spawn-to-bulk transfer, so that's a no go since my environment isn't as sterile as I would like it to be...

🔵 User 2: Well your theory on that is correct, the longer it takes for your myc to colonize the higher chances something else can grow first. If you are using coir it's borderline impossible to contam the sub by itself, now if your spawn isn't clean, well that's a diff story. What im saying is that typically contam is introduced via spawn since coir has little to no nutrients for contam to thrive on. Now in regards to your question regarding the ratio of spawn look at it this way....the whole point of using a 1:3 for example over a 1:1 is that your cakes/spawn provides enough nutrients for your tub. The increased amount of sub gives your mycellium more surface area to colonize. More mycellium means you can bear more fruits. On the low end (1:1) you get less mycellium area but faster colonization as compared to 1:3, etc. The opposite is true for higher ratios. You get more surface area to colonize but it colonizes slower. I typically do 1:2 or 1:3 as I find it is a good mean choice between colonization speeds and surface area. Hope that answered all your questions.

Also I would like to mention that colonization speed is primarily determined by genetics, but when I refer to colonization speed in this sense it is simply due to it having to colonize more area to achieve full colonization, however the rate of colonization is the same.

Ex. ) 2 cars traveling at the same speed but different distances will reach their destination at different times

🟠 User 3: Agree with the 1:3

🔵 User 2: I use 1:5 sometimes. But mind you higher ratios, you HAVE to have good technique, as higher ratios allows much more leniency for contamination to take over before full colonization. Regarding all your discussion of additives, you could always try compost experiments are fun, but coco coir has already tried and true been used and it is for a reason! It is inherently not able to be contaminated (until nutrients and water are added). But basically just remember your substrate is there to provide a scaffold or surface area for the mycelium network to expand and to provide and hold water. Typically with ratios your nutrients are already in excess.

🔴 User 1: Thank you, I get your point. So as long as I can cover the whole surface area with the right depth requirement for fruiting to occur, I should be good no matter the ratio, as long as it stays in the accepted ranges. The question was asked because I have 'too much' spawn compared to what is typically used in a 66 qt mono tub. What I have seen is people using 3 lbs of spawn + 3-6 lbs of bulk for this size of tub, since I have 4.5 lbs of spawn, I was just wondering if using 3 lbs of bulk (on the low end) was problematic, from what I understand, it shouldn't be.

🔵 User 2: I dont think it would necessarily be problematic, more so a waste of spawn at that point.

🔴 User 1: Exactly what I thought, might just use some to inoculate my next spawn bags and use one of my tubs with the extra spawn to see if there's a difference

To clarify, I have three bags destined to three tubs, one would be the experiment

🔵 User 2: Send it

🔴 User 1: Sure will, if everything won't crash in my face until the end, I will share my results with you guys! Oh that gives me an idea, maybe we can start a spreadsheet with everyone's experiments and draw our own analysis for optimal growth! Data is my specialty

🔵 User 2: That's been done already not here but there is a journal like that

🔴 User 1: Alright! Will look it up, thanks!


Hi, I am just switching from cakes to bags. I had quite success with cutting the cakes (2-3 cm slices) and placing them on coco bulk and cover the slices also with coco (40 x 60 x 7 cm). Looks great and smells great so far, 0 contamination etc.

So my bags are ready in a couple of weeks and I was asking myself if I should "mix" the bags 1:2, 1:3 with substrate or if I should try to cut the bag-blocks similar to the way I proceed with the cakes.

Does anyone have experience or recommendations with "substrate-layer" instead of "substrate-mix" ?

Shroomok avatar Shroomok

Hi @Joe! Could you please clarify? Do you mean you cut BRF (brown rice flour) cakes for slices and then cover with bulk substrate (coco coir mix). If so, yes it works great and much better for fruiting than PF-tek cakes.

After full colonization wait a few days more to make sure grain totally covered with mycelium from all the sides.
1:2 or 1:3 ratio is up to you.

You can use "substrate-layering", no problem. I prefer to break up grains to separate sticked grains and MIX with bulk substrate (coco coir mix). In this way we create more points of growth. Therefore, the speed of bulk substrate colonization is faster. Also I highly recommend to add 1/2-1/3 inch (~1-1.5cm) of bulk substrate (coco coir mix) on top of your cake after mixing.

Let me know if you have any additional questions.

P.S. Hope I've got you right :)


Hi Shroomok!

yeah you got me right It's like you suggested: I have 440ml (sorry metric here) cakes based on rye berries. At the beginning of my growing approaches I directly growed from the cakes, currently I switched to slicing the cakes and putting them into coco coir mix - my rye berry based bags (~2 KG) unicorns look great and should be ready soon. Bags seem to colonize very well and I am about to quit the cake business :)

So: I will try a 1:2.5 and 1:3 ratio and MIX the stuff. The top coco layer sounds great.

A little negative side effect I currently experience from using the "sliced-cake" method is that the shroom clusters are pretty tough sticking together and I "accidentally" supercarefull-harvest "small bystanders". I was a little bit more flexible and could be more selective when harvesting from the cakes directly.

I have one additional question:

Whats the ideal filling height of the 1:2, 1:3 mix + the coco topping ?

I wanted to try, please correct me if dead wrong:

4 cm MIX + 1cm coco topping
5 cm MIX + 1cm coco topping
6 cm MIX + 1cm coco topping

have a nice day!

Shroomok avatar Shroomok

Hey @Joe!
Glad to hear we are on the same page :)

1) Yes, you're right! Moving from pf-tek cakes to Spawn2Bulk + top layer (casing or pseudo casing) is a game changer in increasing yield.

2) When picking clusters, small shrooms/pins come together with the whole cluster, it's fine.

3) Regarding the height of your mushroom cake (grain spawn + bulk substrate + top layer) it should be within 5-10 cm (2-4 inches). +/- 1 cm is not critical. You don't need a tower, at the same time cake should be thick enough.

There is also one more recommendation. Your cake should take up no more than 1/3 of the volume of the fruiting chamber (shoebox, bottle, monotub). For instance, if your plastic box = 24 cm in height, then your mushroom cake should be up to 8 cm (24/3=8).

So, all the options you've listed above are right.

Don't afraid to make different cakes (different S2B ratio, different height etc.) and compare results.

Good luck! Waiting for good news from you 🤞


Shroomok more than good news. "Just" the cutting cakes to slices and putting them into coco with coco top layer increased the yield incredibly. I would love to tell you more about my "Spawn2Bulk + top layer" experiences but the current "cakes to slices" just had a huge 2nd flush and I think they ll still grow the next 7-10 days :) Huge healthy shrooms :)

Thanks for the recommendation about the fruiting chamber volume. I took a quite different very nerdy tech approach. So here are my growing details. I am currently collecting more data and I am trying to get more sensors but I would love to publish my results together with all the data. Maybe it would be possible to publish it here in a small post or article. I am not a fan of photos or social media but we ll find a way :)

Liquid Culture: malt extract 4% (a bit peptone ex soja)
Grain Spawn: organic rye berries, 10% vermiculit, 2% gypsum
Bulk substrate: 65-75% coco coir, 35-45% vermiculit, 5% gypsum

Fruiting tent(!):
Tent: HOMEbox 60x60x160cm
Humidity: The House of Hydro - 3 Disc mist maker (op), about 18L water tank (aquarium heater optional)
Ventilation: PrimaClima, PK 125-2, Carbonactive activated carbon filter (the model that can handle humidity of 80%+)
Fruiting-Boxes inside tent: 60x40x12cm - 4 to 3 boxes
Sensors (high quality): CO2, Temp, Humidity
(note: the sensors a nice to have and I want to add more just for "scientific reasons" ;) - but in general I switched from a complex "PID Controller" setup to a simple time based activation of humidity and ventilation)

Climate and Stuff: So currently I found a nice setting. I activate the ventilation once an hour for about 5min, (wait 5 min) and then activate the mist maker for 4min. I have a constant humidity at about 82% (but no water at the bottom of the tent) at a room temp of 22-24C. CO2 is around 1300 ppm max and below 1000 ppm average. If the flush is starting the CO2 goes up like crazy without ventilation of the box (or with a closed window inside the growing room) a saw 4000-5000 ppm :)

ups didn't want to write that mutch... hmm who else to tell ^^

Shroomok avatar Shroomok

Hey Joe!
What a great news!
I’m glad you have a successful mushroom fruiting.

Did you rehydrate (soak) your mushroom cakes in water after the 1st flush?
Or maybe hard misting on the top layer after harvesting to initiate the next flush?
BTW, what strain(s) are you growing?

Grow tent is a great tech approach and new level in mushroom growing.
Thank you for sharing your journey with so many useful details. This is a complete report that others may find helpful.

And ofc thanks for sharing your fruiting conditions data, love it!
(Yes, CO2 changes are crazy at active stage of fruiting! But CO2 produced by oyster mushrooms will make you even more shocked)

P.S. If you’d like to share an “article by Joe” with more details, photos you can send it to me on e-mail shroomok [at] and I’ll create a separate write up ;)


Hi, I am finally running on 1:3 mix on four 60x40x12cm boxes inside the described tent. All my sensors are currently out of order. So in general I only use a "WLAN based 4 x power socket" running on "Tasmota Home automation" software (Fan-out CO2, and Fogger to get back humidity - hourly). I will send you a nice article/details/fotos soon, steam bath 3000 :)

q: Did you rehydrate (soak) your mushroom cakes in water after the 1st flush?
So my last "caked based" box - I even forgot to soak them before putting them in coco-mix. To be honest I did not experience any bad results from that. I did not manually mist the boxes at all - except about about 100ml per box when I "waited if there would happen anything more after the first flush" (I had no experience about when or how this (2nd + flushes) should happen and if it would be worth to wait instead of using new box).

q: what strain(s) are you growing?
GT, B+, I just can't get the APE running


current observations:
- After 3 weeks of spawning my 1:3 mix in the boxes were ready to go (maybe they even would take 1 week longer) and they are currently growing like hell (I am so interested in the 2nd flush results also). I even needed to get a 2nd big dryer hehe...
- I had one bad thing happen :( 3 out of remaining 4 spawn bag (2 ape) got the contam :( So yeah I will try to make a 1:4 or 1:4.5 mix out of necessity...

quick question about the cultivation boxes:
I noticed that about after 2 Weeks alot of mycel was ready on the "outer sections near the box edges" and even 1,3 small aborts could be seen - very few mycel in the center of the box. Is this normal ? I am using about 3 "air holes" per side of the box (holes smaller than in traditional monotube) and I closed them with micropore tape (1 layer). Are the aborts normal or a sign of "to mutch fresh air", maybe even only 2 holes per side or more layers of micropore!

regards Joe


sorry man. I missed your last post. Again awesome quality and detail. please forget about my last question you already answered it in detail with this post (

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