Letterbox Monster Grow Kit

Difficulty: Beginner, one injection

Time to harvest: Often a few weeks, colonises in one to two

Yield: Plenty for its size, on a par with the XL MycoBox

Grows: Gourmet species, your choice

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.

1

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.

Up to 12ml of gourmet culture.

2

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.

3

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.

Often white in one to two weeks.

4

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.

First pins within days of full colonisation.

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.

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.

Letterbox Monster A standard box kit Delivery Folded flat, fits a standard UK letterbox A parcel, needs someone in or a neighbour Stages One, inject the substrate directly Often two, grain then bulk Substrate 1000ml of Substrate A, built for speed Usually plain coir Species Any gourmet culture you inject Often fixed if pre-colonised Best for A fast, no-fuss grow when you are rarely in Growers happy to wait in for a parcel

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.

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

What you'll need

The culture is not included.

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.

Liquid culture King Oyster Inoculate with this Liquid culture Lion's Mane Inoculate with this Liquid culture Pink Oyster Inoculate with this Liquid culture Shiitake Inoculate with this

The Sterile Guarantee

We guarantee the kit, not the grow.

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.

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

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.

Not sure this is the one?

Same method MycMonster Bag kit, Substrate B In a box MycoBox Inject the box, one stage Learn the method MonoBox Grain into bulk, fruit in a tent