Ditch your heat mats. Do this instead.
Heat is the variable most people get wrong, and it's the one they think about least. They sort their sterile technique, nail the inoculation, then sit the jar on a windowsill in a cold flat and wonder why colonisation crawls. Or worse, they slap it on a heat mat and cook the bottom of the grain dry.
The setup that fixed all of this for me costs about £55 and runs on pennies a day. A tube heater sat at the bottom of a cupboard, plugged into an Inkbird thermostat. It holds a whole batch at a steady temperature, evenly, without drying anything out. Here's why it beats a heat mat, how to put one together, and what it costs.
Why steady warmth matters
Mycelium colonises fastest in a fairly narrow band. The range you're after is 18 to 24°C, depending on the species you're growing. Get it right and the mycelium powers through the grain before any contamination gets a foothold. That race is the whole game.
Too cold and colonisation slows to a crawl, which hands mould and bacteria the time they need to move in first. Too hot and you stress the mycelium, and past a point you start killing it. The thing that matters as much as the number is keeping it steady and even. A jar that swings warm in the day and cold at night, or one that's hot on the bottom and cool on top, colonises badly. Stable and uniform beats hot.
The problem with heat mats
A heat mat warms by direct contact. Whatever sits on it gets heat conducted straight into its base, and nothing else does. That causes two problems.
First, it dries your grain. Concentrated heat on the bottom of a jar or bag drives the moisture out of the grain touching it. Mycelium can't colonise dry grain, so it stalls at that layer, and you get patchy growth or a hard dry crust at the base that never colonises at all. You spent ages getting the hydration of that grain right, and the mat quietly undoes it.
Second, it heats unevenly. The bottom layer cooks while the top of the jar stays cool, so you never get one consistent temperature through the container. And mats are hard to regulate. The surface of a mat often sits well above the air temperature around it, so even if a thermostat reads 22°C, the grain pressed against the mat can be a fair bit hotter, hot enough to stress the mycelium right where it's trying to take hold.
On top of all that, a mat warms one container. If you've got a batch going, you'd need a mat each.
The setup I use: a cupboard, a bar heater and an Inkbird
The fix is to stop heating the container and start heating the air around it. Warm the space, let the jars sit in it.
A tube heater (the slim bar heaters sold for greenhouses and airing cupboards) sits at the bottom of a cupboard. Heat rises, so warmth from the bottom convects up and fills the whole cupboard evenly. No hot spots, no contact drying. The heater plugs into an Inkbird thermostat, which switches it on and off to hold whatever temperature you set. You put the Inkbird's probe in the cupboard at jar height, set your target, and it handles the rest. One cupboard warms a whole shelf of jars or tubs at the same steady temperature.
That's the difference in a line: a heat mat heats one object from below and dries it, a cupboard setup heats the air around everything and holds it level.
How to set it up
- Get a tube heater and an Inkbird. A small tubular heater (40 to 80W is plenty for a cupboard) and an Inkbird ITC-308. The Inkbird has a heating socket and a cooling socket.
- Plug the heater into the heating socket. Ignore the cooling one. The Inkbird now controls the heater.
- Put the probe in the cupboard, roughly at the height your jars will sit, not touching the heater or a jar. That's the temperature it regulates to.
- Set your target and a small differential. Pick a target inside your 18 to 24°C window for your species, and a differential of around 0.5 to 1°C so it isn't clicking on and off every minute.
- Heater at the bottom, jars on a shelf above. Keep the jars off the heater itself and let the warm air rise past them.
- Close the door and let it settle for an hour, then check it with a separate thermometer to confirm the Inkbird's reading matches reality.
Safety, because it's a heater running unattended: use a tube heater rated for continuous use (they're built for exactly this), keep it clear of anything that can catch, never drape or cover it, and run it off an RCD plug. Tube heaters are low-wattage and run cool by design, which is why they suit a cupboard, but treat any heater with respect.
What it costs
Here's the breakdown against doing a heat mat properly. "Properly" matters, because a heat mat with no thermostat is the cheap route that causes half the problems above.
| Setup | Kit | One-off cost | Warms |
|---|---|---|---|
| Cupboard + bar heater + Inkbird (what I use) | Inkbird ITC-308 ~£32 Tube heater (40–80W) ~£25 Cupboard £0 (use one you've got) |
~£57 | A whole batch |
| Heat mat, done right | Heat mat ~£15 Separate thermostat ~£25 |
~£40 | One container |
| Heat mat, the cheap way (no thermostat) | Heat mat ~£15 | ~£15 | One container, dries and overheats it |
The cupboard setup costs about £17 more than a properly thermostatted heat mat. For that you get even heat, no drying, and a whole batch warmed instead of a single jar. Once you're running more than a jar or two, it's cheaper per jar, and it pays for itself by not losing you grows. Running cost is a few pence a day, since the heater only fires to top the temperature up.
When a heat mat is fine
I'm not going to pretend a mat never works. One jar, a folded towel between it and the mat to buffer the heat, and a thermostat set low, and it'll get you there. It's quick and cheap for a single grow. But the moment you care about consistency, or you're warming more than one container, the cupboard wins on every count that matters, and it's barely more money.
FAQ
What temperature should I aim for?
Somewhere in 18 to 24°C, depending on your species. Steady and even matters as much as the exact number.
Where does the Inkbird probe go?
In the cupboard air at jar height, not touching the heater or a jar. It regulates to whatever the probe reads.
What wattage tube heater?
40 to 80W is plenty for a cupboard. A bigger one just cycles off sooner, it won't push the temperature past your setpoint.
Is it safe to leave on?
Yes, if you use a heater rated for continuous use, keep it clear of anything flammable, and run it on an RCD plug. Tube heaters are designed for unattended use in greenhouses and airing cupboards.
Why not just use a heat mat?
Conductive heat dries the grain in contact and warms unevenly. Fine in a pinch for one buffered, thermostatted jar. Not for reliability, and not for batches.
My cupboard gets too warm even at the lowest setting?
The Inkbird won't heat past its setpoint, so an overshoot usually means the probe is in the wrong spot or too close to the heater. Move the probe up to jar height, and crack the door if it's still holding too high.
