XL Letterbox Monster Mushroom Grow Kit

Who's This Kit For

Won't be at home? Letterbox Monster fits through any standard UK letterbox*.

First time growing mushrooms? This all-in-one makes mushroom growing as easy as an Ikea cactus!

Need enough mushrooms to share? Letterbox Monster produces plenty of mushrooms for its size (as much as our XL Cylocybe Special Mushroom Grow Kit!).

Want mushrooms fast? This is even faster than our MycMonster Grow Kit (and that's fast).

*We call this the Letterbox Monster, not the Letterbox Miracle - if you're ordering multiple of these and/or other stuff too, then it might not actually fit through a letterbox. But if you are ordering just this and a gourmet liquid culture syringe or two, then it will fit absolutely fine (as long as it's a standard UK letterbox).

How Does A Grow Kit Fit Through A Letterbox?

Letterbox Monster is an all-in-one mushroom grow kit which we sterilise flat, with its substrate spread out evenly within a grow bag. This grow kit is suprisingly big, especially once the substate is all broken up and loose. From the post person's perspective (or your flatmate), it just looks like a thick envelope. Our latest innovation with this kit is the bag, it's actually double bagged and sterilised - because each bag is rated for high air exchange that's too much during colonisation stage, double bagging both dramatically reduces the airflow during this initial colonisation stage (exactly what you want), then at fruiting stage you will cut the top of both bags to release the outer bag which will then be flipped over the top of the inner bag which contains the colonised substrate - meaning high fresh air exchange will start happening - exactly what you want. This nifty idea came to Cylocybe.com in August 2026 and will hopefully impress you heck out of you fun guys - it even has as much substrate as our XL Cylocybe Special Mushroom Grow Kit. It's not just stealthy, it's big and it fruits like a mofo.

You will also find inside the hygiene kit and your gourmet culture syringe (if you order one of those that is!).

Of course, once you've opened it all out and injected it, its gusseted 'bottom' allows it to stand vertically on it's own. Again, the way it's fruited too, in the bag and sitting vertically (once it's finished colonising with mycelium, that is).

Our Advanced Sterile Substrate A

Substrate A is one half of substrate B (which customers have been telling us is brilliant - it's not just what we're saying). The main reason we even made substrate B, was because we didn't think customers were ready to accept a non-grain-based substrate as being any 'good', so we snuck grain in there too so it would be adopted easier. Truth is, our MycMonster wouldn't be the monster it is if it weren't for Substrate A.

Inoculation

You will see the grow kit bag is flat as a pancake, you will be wiping the area you intend to inject with the included wipes - then with the included polypropylene sticker you'll cover the hole. This sticker is water proof of course, and it's super sticky... After a day or two, trying to remove it would probably tear the grow bag (so don't). Inject 6ml-12ml of a gourmet culture syringe.

After you've injected the bag, you'll cover the tiny needle hole with the included sticker. Then you just leave it all to colonise somewhere warm and in a week or two depending on mother nature and what you injected, the substrate could be completely white with mycelium!

XL Letterbox Monster Grow Kit Contents

• Grow Container: HEPA-Filtered Tall / Narrow Mushroom Bag
• Substrate: 1000mL of Sterilised Substrate A
• Fruiting: In Same Bag
• Sanitation: IPA Wipe, Hand Wipe, 18G Sterile Needle

Not Included

• Gourmet liquid culture syringe - 6-12ml max

Helpful Advice

• Keep incubation temperatures between 23C (a happy medium)
• Incubation temperatures of 26C provides fastest colonisation results, but if you go over by even a degree your chances of contams really go up!
• Never place this kit on direct heat, like a heatmat - you will bake the water out of it and ruin it.
• We recommend inoculating your XL Letterbox Monster within 1-2 weeks of receiving it for the best results.
• The specialised HEPA filters on the XL Letterbox Monster manage clean air exchange - please don't remove them
• NEVER cut open the Letterbox Monster before it's fully colonised by mycelium, or contamination may ruin the kit

Shipping & Legal

We ship from the UK to almost anywhere in the world. Each kit is made fresh to order (Shipping & Dispatch page).

This product is for cultivating legal mushroom species only.