Heat mat vs heated propagator: which is better?

You know your chilli and tomato seeds need warmth to germinate in late winter, and you've narrowed it down to two options. The heat mat vs heated propagator question comes up because both warm compost from below in much the same way, yet they look nothing alike and suit different set-ups.

The real difference isn't the heat. It's everything around it: whether the kit holds humidity, whether it controls its own temperature, and whether it fits the trays, pots and space you already have. This guide compares the two on those points, with real running costs, so you can buy the one that matches how you actually sow.

What's the difference between a heat mat and a heated propagator?

A heat mat is a flat, waterproof heating pad that you stand seed trays, modules or pots on. It warms the compost from below and does nothing else. There's no lid and no enclosure, so what happens to humidity and air temperature depends on what you put on top. Our propagation heat mats include thermostat-controlled aluminium mats designed to take compost up to a maximum of 35°C.

A heated propagator is a complete unit: a heated base, a tray and a clear, vented lid. The base does the same job as a mat, while the lid traps moisture and some of the warm air, so the compost surface stays damp and the seeds sit in a stable microclimate. Some run at a fixed output; others have a built-in thermostat.

In other words, a heated propagator is a heat mat with the humidity problem already solved. The two converge at the top end, too: the largest heated propagator in our range is effectively a 50 × 120cm heating mat inside a tall, zip-up cover with a digital thermostat.

Heat mats and heated propagators compared

These are the differences that should decide the purchase. Running costs use the October–December 2026 price cap average of 26.32p per kWh and assume the heat is on for all 24 hours, so treat them as a ceiling.

Two things stand out. First, a mat on its own solves only half the problem, because seeds on a warm open base dry out faster at the surface. Second, temperature control matters more than the type of kit: a mat or propagator with a thermostat is far less likely to cook seedlings under a lid on a sunny March afternoon.

Which should you choose?

Choose a heated propagator if…

• You're new to raising seed under heat and want warmth and humidity sorted in one box.
• You sow a few varieties at a time on a windowsill, where a compact unit fits neatly.
• You want everything to look tidy indoors, with no loose trays and covers.
• You'd like room to overwinter young plants, in which case a tall unit with headroom beats a bare mat.

Choose a heat mat if…

• You already own seed trays, modules and clear lids, and just need to add heat.
• You sow in varied containers — deep root trainers, 9cm pots, cell trays — that won't all fit one propagator base.
• You want to heat a section of greenhouse staging or a bench rather than a single tray.
• You take a lot of cuttings and want to move pots on and off the heat freely.

When you need neither

If you mostly sow hardy crops — broad beans, sweet peas, brassicas, lettuce, most hardy annuals — or don't start until April, you can skip bottom heat altogether. Those seeds germinate at cool temperatures, and lettuce can germinate poorly when kept too warm. An unheated propagator on a bright windowsill will do the job. And if your only growing spot is dim, spend on better light before heat: warm compost in low light produces fast, weak, leggy seedlings whichever option you pick.

Sizing and running costs

To work out the maximum running cost of any mat, propagator or cable, multiply the wattage by the hours it's on, divide by 1,000 to get kWh, and multiply by your unit rate. At 26.32p per kWh, that's about 6.3p a day for every 10W running non-stop.

A thermostat cuts those figures, because it switches the heat off once the compost reaches the set temperature. An enclosed propagator tends to reach its target sooner than an open mat, because the cover stops warm air escaping, so a like-for-like propagator often runs for fewer hours.

Getting the size right

Measure the trays and pots you'll use at your busiest point, usually March, and choose a mat or base that covers them fully. Seed trays that overhang the edge of a mat end up with cold corners and patchy germination. For a propagator, check the internal height as well as the footprint: shallow lids are fine for germination but crowd seedlings within a couple of weeks.

How to set up either option

1. Pick a level, bright spot. A windowsill, staging or a sturdy shelf near the light. Heat mats should lie completely flat with nothing folded or creased.
2. Use a fine seed compost. Fill trays with a free-draining seed compost, firm lightly and water with room-temperature water before sowing.
3. Cover mats for humidity. On a mat, fit a clear lid or stand an unheated propagator on top. A heated propagator already has one.
4. Set the temperature. Around 20–25°C suits chillies, sweet peppers and aubergines; tomatoes are happy at the lower end. Place a thermostat probe in the compost, not on the mat surface, so it reads what the seeds feel.
5. Ventilate as seedlings appear. Open vents or lift lids a little each day once shoots emerge, and wipe away heavy condensation.
6. Move seedlings on. Take trays off the heat, or lower the setting, as soon as most seeds are up, then harden off tender crops before planting out after the last frost, typically mid-May in most of the UK.

Frequently asked questions

Can I put a heat mat inside an unheated propagator?

Yes, and it is one of the most flexible set-ups there is. Stand the unheated propagator directly on the mat so its base sits flat, and keep the lid on for humidity. Use a mat with a thermostat, or add a plug-in one, so the compost does not overheat on bright days.

Do heat mats need a thermostat?

Not always, but it is strongly recommended. A mat without one runs at a fixed output, so compost temperature rises and falls with the room and can overheat under a lid in sunshine. A plug-in thermostat with a probe in the compost holds a set temperature and switches the mat off when it is not needed.

Which is cheaper to run, a heat mat or a heated propagator?

Running cost depends on wattage and how long the heat is on, not on the type. Every 10W of rated power costs up to about 6.3p a day at 26.32p per kWh. An enclosed propagator holds heat better than an open mat, so its thermostat usually switches off sooner.

Can I use a heat mat for cuttings?

Yes. Gentle bottom heat helps many cuttings root faster, and a mat lets you warm pots and trays of any size side by side. Cover the cuttings with a clear lid or propagator top to hold humidity, keep them out of direct sun, and check the compost does not dry out on the warm base.

Should seedlings stay on a heat mat after germinating?

Usually not for long. Once most seeds have emerged, lower the temperature or move the tray off the mat into the brightest spot you have. Warm compost combined with low winter light produces weak, stretched seedlings. Chillies and aubergines may appreciate a little gentle warmth for a few more weeks in a cold room.

Ready to start sowing? Whether you want a single all-in-one propagator or a flexible heat mat to slide under the trays you already own, Vitako stocks both, along with thermostats and seed compost, delivered across the UK. Browse our heat mats for seeds and cuttings and get your chillies and tomatoes off to an even start this winter.