Letterbox Monster Grow Kit
New as of Sunday 30th November. Promo price during launch day only.
Letterbox Monster is a beginner-friendly, all-in-one, super simple Grow Kit built for blazing fast growth, big harvests and getting through UK letterboxes so your grow kit doesn’t get delivered to a neighbour when you’re not in!
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 MycoBox 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 folded-over grow bag. This grow kit is suprisingly big, especially once fully unfolded / extended. From the post person’s perspective (or your flatmate), it just looks like a very large, thick envelope, but once opened the kit will extend to nearly twice the length of the envelope and contain as much substrate inside it as our XL MycoBox Grow Kit. 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, you can let the substrate drop to its gusseted ‘bottom’ and stand it vertically as normal. That will also be 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 folded over, as soon as you peel this back to extend the bag fully you will have a large sterile area to inject your 6ml-12ml syringe anywhere you want. But, we recommend only peeling this back the moment you are ready with your syringe as you will never get a better, cleaner moment than this to inject (because it’s totally sterile).
We will still include an alcohol wipe for you to clean an area of the bag prior to injecting it, but it’s not really needed if you are peeling the bag back and immediately injecting it.
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!
So to summarise: Unfold / peel back the folded bag to reveal a fresh (sterile) surface, inject up to 12ml of solution from your gourmet spore or culture syringe and cover the hole your needle made with the included sticker. Leave somewhere warm to colonise.
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
- Online Instructions (coming within 24 hours)
Not Included
- Gourmet spore or liquid culture syringe – 12ml max
Helpful Advice
- Keep incubation temperatures between 20°C and 26°C
- Incubation temperatures of 26°C provides fastest colonisation results, but do not exceed this!
- Never place this kit on direct heat, like a heatmat
- We recommend inoculating your Letterbox Monster within 4 weeks of receiving it for the best results
- The specialised HEPA filters on the 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 and typically dispatched within our 3 working day aim (see our Shipping & Dispatch page for more info).
This product is for cultivating legal mushroom species only.
Grow Guide for Letterbox Monster Mushroom Grow Kit
Letterbox Monster Grow Kit Contents
- 1000ml Sterile Substrate A/strong>
- Hygiene Kit with 1x chlorhexidine wipes, 1x hand wipes, 1x sterile needles
- Post-Inoculation Sticker>
Required: 6 or 12ml gourmet spore or LC syringe
1. Inoculation of Letterbox Monster
Reducing the risk of contamination is crucial in successful mushroom cultivation, and you can achieve this by working in a small, clean, and draft-free space, ideally using a Still-Air Box (SAB) and taking thoughtful steps to reduce the chance of cross contamination.
Before starting, cleanliness is key. Shower, clean clothes. Your hygiene kit contains a face mask and wipes for cleaning your hands and forearms also.
Next, prepare your syringe by attaching a sterile needle and have your inoculation sticker to hand. Your Letterbox Monster will be folded in half, so peel back the plastic to reveal a nice clean and crease-free surface inject right into the substrate. Inject anywhere from 6ml - 12ml of solution and quickly apply the sticker that was included over the hole the needle made. Push the sticker down firmly.
Now, you can break up the substrate through the bag so that it alls to the bottom of the bag. It's now just waiting for it to colonise with white mycelium.
Inoculation Tips:
- Keep Letterbox Monster in its packaging until you're ready to use it to minimise contamination.
- After inoculation, ensure the sticker is firmly adhered to prevent contaminants from entering.
2. Colonisation of Letterbox Monsterâ„¢
The above picture shows a MycMonster kit, which uses the exact same bag as the LetterBox Monster. The only difference being that we put less substrate inside so that it fits through a letterbox and we send it flat - after inoculating the substrate, you'll break it up and sit it vertically like in the picture! Next up is about colonisation.
Colonisation Temperatures
Temperature is vital for healthy and fast mycelium growth. Use this table to find the ideal temperature for your inoculation method.
| Method | Ideal Temp | Expected Time |
|---|---|---|
| Spore Syringe | 24-26°C (75-79°F) | 10-14 Days (Sometimes takes longer) |
| AC / LC / G2G | 24-26°C (75-79°F) | 7-10 Days |
Universal Rules:
- Consistency is key. Avoid large temperature swings.
- Use an incubator or a warm cupboard. Don't ever use a heatmat with a mushroom grow kit!
- Patience is your best tool. These timelines are estimates and can vary.
For faster mycelium growth with Substrate A: when colonising at 26°C, on day 7 after inoculation thoroughly mix the substrate in the bag. Use both hands to rub the bag, breaking the contents and mycelium into a fine, loose crumble. The finer and more evenly mixed the crumble (achieved by repeatedly flipping the bag), the more uniform the mycelial distribution and the faster the colonisation. Do not repeat the step more times than this — repeating it can stall growth. Leave the bag to continue colonising afterwards.
Important: Using a heat mat is like trying to hatch an egg on a frying pan. It creates an environment that’s too hot and uneven, drying out the base of the grow kit while sending moisture upwards. This leads to soggy conditions and bacterial growth. For optimal results, create a stable, warm environment instead. Build or buy an incubator or put a radiator inside a small cupboard or just incubate at room temperature (slower colonisation is better than growing a bag of microbes).
3. Dunking for Humidity and Hydration
When the substrate inside the Letterbox Monster bag has fully colonised (completely covered in white mycelium), you should use a method known in the hobby as 'dunking' to get as much water into the mycelium cake as possible.
There are so many different ways to acheive this hydration, but the easiest is to simply add water to the Letterbox Monster bag. We'd suggest a litre of spring water (but tap can work too). Pour this over the myc cake, weigh it down so it stays submerged (as it will try to float!).
Fold over the top of the bag to keep everything as clean as possible. Leave your cake submerged like this for anything between 12-24 hours, depending on how much patience you have!
Dunking Troubleshooting Tips:
- Ensure the cake is 100% colonised before dunking.
- Make sure the cake remains fully submerged for the entire dunking period.
- Fold the top of the bag while dunking.
- Longer dunking periods (closer to 24 hours) generally produce better results.
After this dunking period, drain the water from the Letterbox Monster bag and give it all a good rinse with clean tap water (spring is fine too, but it's not necessary) and then drain thoroughly as we don't want stagnant water pooling.
4. The Best Bit... Fruiting Your Mushrooms!
Maintaining a temperature of around 21°C (give or take a couple of degrees) during the fruiting stage is ideal for best results with many popular species of mushrooms. Essential levels of humidity will naturally build up inside the grow tent or bag from the water you allowed to soak into the substrate during dunking.
Unlike some other grow kits, you don't need to mist your Letterbox Monster (unless it looks dry). Misting can help promote faster pinning, but once you see them - feel free to leave it all alone from that point. The only time you ever really need to open your grow bag after the onset of pins is when you're ready to harvest your fully grown mushrooms. Even peeking for a few seconds could be enough to lose some humidity if you aren't careful, which can sometimes cause growth defects.
Humidity and Air Exchange
Humidity and FAE (fresh air exchange) are extremely important for pinning and healthy mushroom development, as is temperature and light. They all need to work in harmony.
We suggest cutting a 1cm slit through both sides of the tent near the top and placing an electric fan somewhere in the room on a low setting (but don't go out buying one if you don't already have one). Even opening a window from time to time can be great for creating some air currents which will help increase the FAE inside the grow tent.
Light and Environmental Conditions
Your mushrooms will need light to grow. We recommend indirect sunlight or a 5000-6500K LED or fluorescent light a few feet away - if the light is heating the substrate, move it further away.
If you notice the inside of your bag looking dry, it may be due to the fan being too close, the holes you cut being too big, or grow lights (if using them) being too close and hot. Observe what's happening and adjust your setup as required. Fantastic results depend on it.
Pinning and Growth
After around 3-7 days, you'll hopefully begin to see lots of tiny white dots. These are hyphal knots that will develop into primordia, aka mushroom pins. It will take 4-5 days for them to grow into fully mature mushrooms ready for harvest.
During this time, they'll let you know if something isn't quite right. Fuzzy mycelium at the base of the mushrooms and/or thin stems are an indication that more FAE is needed. Increasing the fan speed or length of the cuts on the tent or bag might be something to try.
Don't worry about any mushrooms growing around the sides and bottom of the cake. You'll still be able to harvest them. If you want to give your mushrooms more room to grow next time, consider using a fruiting chamber instead of the bag.
Fruiting Troubleshooting Tips:
- Water pooling: If you see pools of water touching the substrate, dab them with a paper towel. Standing water can become contaminated quickly.
- Temperature issues: Maintain stable temperature between 18-24°C. Unstable temps can lead to excessive condensation.
- Fuzzy stems: This indicates insufficient fresh air exchange (FAE). Try increasing airflow slightly.
- Dry substrate: Only mist if the substrate looks dry; otherwise, humidity should come from the substrate itself.
- Aborted pins: Often caused by environmental changes. Maintain consistent conditions.
Harvesting and Subsequent Flushes
When the veil beneath one of your mushroom caps is about to tear off, this is when you should harvest your first flush. For best results, you should repeat the dunking process after each harvest. If you've taken good care of your grow kit, you should achieve multiple flushes of beautiful mushrooms before the nutrients are all used up.
Expected Yields
With proper care and optimal conditions, the Letterbox Monsterâ„¢ can produce impressive harvests across multiple flushes. Remember that each flush typically produces less than the previous one, with the first flush usually being the most abundant.
Happy shroomin' folks!
Last updated on 15 December 2025