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Mushroom grow kit
Letterbox Monster
An all-in-one kit that posts flat through your letterbox, so it never lands with a neighbour. You inject your own gourmet culture, then colonise and fruit in the same bag. No grain stage, no mixing.
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For cultivating legal gourmet mushroom species only. A gourmet liquid culture or spore syringe is not included.
fits a standard UK letterbox, no missed parcels
arrives sterile or we replace it
any gourmet species, your choice
filled, sterilised and dispatched fresh to order
The short version
A single-stage, all-in-one gourmet mushroom kit that ships folded flat to fit a UK letterbox. You peel back the fold to expose a fresh sterile surface, inject up to 12ml of your own gourmet culture, seal the hole with a sticker, then stand the bag up to colonise and fruit in place. It carries 1000ml of our fast Substrate A, the same amount of substrate as the XL MycoBox, with no grain stage and no mixing.
How it works
How a Letterbox Monster grow works
This is the simplest kit we make. There is no grain stage and no mixing. You inject your own gourmet culture straight into the sterile substrate, seal the hole, then colonise and fruit in the very same bag.
Peel and inject
The bag arrives folded flat with the substrate spread out inside. Peel back the fold to open the bag fully and you expose a fresh, sterile surface to inject into. Have your syringe ready first, because the moment you peel it back is the cleanest it will ever be. Inject anywhere in that surface.
Seal the hole
Cover the tiny needle hole with the included sticker. There is no rubber port on this kit, the sticker is what keeps the bag sealed and clean. An alcohol wipe is included if you want to wipe the surface first, though it is barely needed if you peeled and injected straight away.
Stand it up and colonise
Let the substrate drop into the gusseted bottom so the bag stands vertically, then leave it somewhere steadily warm. Aim for around 24 degrees, though anywhere from 18 to 24 is fine as long as it holds steady. Skip the heat mat and watch for condensation. The HEPA filters handle the air exchange, so leave them alone.
Fruit in the same bag
Once the substrate is solid white all the way through, drop the temperature slightly to around 20 to 22 degrees. The mushrooms fruit out of the bag while it stands upright, no transfer needed. The moisture comes from the substrate, so you do not mist it. Harvest each mushroom as the veil under the cap begins to tear.
Do not cut the bag open before it is fully colonised. An exposed surface before the mycelium has claimed it is an open door for contamination.
Why this one
What makes the Letterbox Monster different
Two things set it apart: it posts flat through a letterbox, and it runs on Substrate A. Here is what each one actually buys you.
The substrate
About Substrate A
Substrate A is one half of our Substrate B, the grain-free half. We originally added grain into Substrate B because we did not think customers would accept a grain-free substrate as any good. The truth is that Substrate A is what does the heavy lifting, and it is why these bag kits run as fast as they do. The Letterbox Monster carries 1000ml of it, the same amount of substrate as our XL MycoBox, all folded flat to post.
Fast
Substrate A colonises quickly, even quicker than our MycMonster, which is itself no slouch.
Generous
1000ml of substrate gives plenty of mushrooms for the kit's flat size, on a par with our XL MycoBox.
Sterile to your door
Sealed and sterilised flat, ready for you to inoculate. The sterile state is exactly the point of the kit.
In the box
What's included
One complete all-in-one kit, sterile and sealed. The only thing it is missing is the living culture, which you add.
Letterbox Monster
Inject up to 12ml of culture
- HEPA-filtered tall, narrow grow bag (folds flat to post)
- 1000ml of sterilised Substrate A, pre-spread inside
- Sealing sticker for the injection hole
- Hygiene kit: IPA wipe, hand wipe
- One 18G sterile needle
- Online grow instructions
Fruits in the same bag
No port, no transfer, no second vessel
- Inject through the freshly peeled surface
- Seal with the sticker (there is no rubber port)
- Colonise standing upright in the bag
- Fruit straight out of the same bag
The one thing you add
What you'll need
The Letterbox Monster arrives sterile and ready to inject, so you supply the living part: a gourmet liquid culture or spore syringe, up to 12ml. We make gourmet liquid cultures you can inoculate it with.
Liquid culture has known, predictable genetics and tends to colonise faster and more reliably. Spore syringes are cheaper and offer more variety, but are never fully sterile, so incubate them slightly cooler to let the mycelium outrun any stragglers. Either way, the cleaner your injection, the better your odds. Most failed grows trace back to a sloppy inoculation rather than a bad kit, which is why we recommend peeling the fold back and injecting in one clean movement.
We stock the sterile gear that lowers the risk further, including a still-air box, wipes and gloves, in sterile and lab equipment.
Risk, reversed
The Sterile Guarantee
Every Letterbox Monster is made fresh and arrives sterile and sealed. If one shows contamination on arrival, before you have injected it, send us photos and we will replace it. What happens after you open it comes down to your technique and conditions, which no reputable seller can promise for you. So we stand fully behind the kit reaching you clean, and the troubleshooting below helps with the rest.
At a glance
The spec sheet
- Kit type
- All-in-one, single stage, inject and fruit in the same bag
- Vessel
- HEPA-filtered tall, narrow grow bag that ships folded flat
- Substrate
- 1000ml of sterilised Substrate A, pre-spread inside the bag
- Inoculation
- Peel back the fold, inject the fresh surface, seal with the sticker (no port)
- Culture needed
- Up to 12ml, not included
- Colonisation temp
- Around 24 degrees, 18 to 24 is fine if kept steady
- Fruiting temp
- A little cooler, around 20 to 22 degrees
- Colonisation time
- Often one to two weeks, depending on what you inject and the temperature
- Yield
- Plenty for its size, on a par with the XL MycoBox, over multiple flushes
- Delivery
- Posts flat through a standard UK letterbox
- Made
- United Kingdom, fresh to order
- Suited to
- Gourmet species: oyster, Lion's Mane, Shiitake and similar
When things look off
Common issues
Because you do the growing, the odd thing can go sideways. Here are the common ones and what they usually mean.
Nothing happening
Usually patience or temperature. Mark the mycelium edge and check again in a few days. Cold slows it right down.
Yellow liquid
Normal. That is metabolites, the mycelium's own waste, not contamination.
Green, grey or sour
That is contamination. Do not open the bag, bag it up to bin it, and keep it away from your other grows.
Thin stems, no pins
Not enough fresh air. The HEPA filters handle exchange, so check the bag is open at the top and add a little airflow in the room.
The full step-by-step lives in the Letterbox Monster grow guide, and the grow-kit troubleshooting guide goes deeper on contamination and temperature.
Other kits
Not sure this is the one?
Common questions
Frequently asked
Aim for about 24 degrees while it colonises; anywhere from 18 to 24 is fine as long as it is steady, and keep an eye out for condensation. Drop it a couple of degrees, to around 20 to 22, for fruiting. Avoid heat mats, they cause uneven heat and condensation.
On its own, yes, it ships folded flat and is designed for a standard UK letterbox. If you order several kits together, or bulky extras, the parcel may be too thick to post, so order it alone or with just a culture syringe or two if the letterbox fit matters to you.
No. You peel back the folded bag to reveal a fresh sterile surface, inject straight into it, then seal the needle hole with the included sticker. The sticker is what keeps it clean, so do not skip it.
Any legal gourmet species you have a culture for. It suits oyster mushrooms, Lion's Mane, Shiitake and similar. We sell gourmet liquid cultures you can inoculate it with.
No. You add your own gourmet liquid culture or spore syringe, up to 12ml. We stock gourmet liquid cultures separately.
The substrate often colonises in one to two weeks, depending on what you injected and the temperature, then pins follow within days of it going fully white. So you are typically looking at a few weeks from injection to your first flush.
Send us photos before opening it and we will replace it, that is our Sterile Guarantee. Contamination that appears after you inject comes down to technique and conditions, so check the troubleshooting first.
Plenty for its flat size, on a par with our XL MycoBox, over multiple flushes. The first flush is always the biggest.
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Questions and answers
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For cultivating legal gourmet mushroom species only.Sold as a sterile substrate kit for home cultivation. We trust you to be responsible.