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Heat mat for tomato and pepper seeds

Updated: October 2026

You are sowing tomatoes, sweet peppers, chillies or aubergines on a British windowsill, and the house falls below the packet overnight. A heat mat for tomato and pepper seeds is worthwhile for that fortnight of germination, and it is the wrong tool the morning the seedlings are up. The Royal Horticultural Society figures below are the ones this page follows. The packet still wins if it disagrees. Our [seedling heat mats](/hub/plant-accessories/seedling-heat-mat) comparison lists the pads. This page is the crop, from RHS guides and those listings, with no germination trial. A purchase through our amazon.co.uk links can earn a commission at no extra cost to you.

You are sowing tomatoes, sweet peppers, chillies or aubergines on a British windowsill, and the house falls below the packet overnight.

Our top picks

HBN HBN 17.5 W fixed heat mat

Best overall

HBN HBN 17.5 W fixed heat mat
7.8/10about £14.99
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WADEO WADEO 50 W probe heat mat

Rank 2

WADEO WADEO 50 W probe heat mat
8.0/10about £24.22
View offer
HBN HBN 17.5 W thermostat combo

Alternative

HBN HBN 17.5 W thermostat combo
6.4/10about £27.99
View offer
On this page
  1. 1.The temperatures on the RHS pages
  2. 2.When to sow in a British house
  3. 3.What a mat can and cannot hold
  4. 4.Off the mat once they are up

The temperatures on the RHS pages

Most seed, the RHS says, wants about 18°C to germinate. The packet overrides that general figure. Sweet peppers are a little warmer, at 18–21°C, and once the seedlings appear they want about 16–18°C. Chillies want about 21°C to germinate. As soon as they are up, the RHS says take them out of the propagator and keep them at 16–18°C in bright light. Aubergines need at least 21°C, sown from February, and then the same move to 16–18°C in good light. Tomatoes are sown warm, uncovered when the seedlings appear, and then kept at at least 16°C in as much light as you can give them. Emergence is often within a fortnight. That fortnight is the heat-mat window.

When to sow in a British house

From mid-February, the RHS February jobs page lists tomato, chilli and aubergine seed for greenhouse growing. Sweet peppers for a glasshouse are often mid-February to early March, and outdoor peppers late March or early April. Outdoor tomatoes, peppers and aubergines on the RHS grafting timing sit around late March. BBC Gardeners’ World has suggested January in a heated propagator. That January line is earlier than the RHS February start, and this page does not merge them. Sowing earlier can waste electricity if the plants cannot go out. Jenny Bowden’s RHS piece says wait until the end of March if an early windowsill sowing will only stretch. A mat does not replace light.
HBN HBN 17.5 W fixed heat mat

Testsieger

HBN HBN 17.5 W fixed heat mat
7.8/10about £14.99
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What a mat can and cannot hold

A fixed mat cannot be told to hold 21°C. The HBN 17.5 W and the Senua 20 W add a lift and stay on until you unplug them. The English Garden’s figure of about 8°C is not a measurement of this HBN. In a porch at 12°C that lift will not make chilli compost 21°C. Ranges here that include 21°C are the AOOWU claim of 20–45°C, the WADEO claim of 5–45°C, and the HBN combo at 5–42°C. You still check the compost. The Gsycle’s claimed band of about 40–45°C is the wrong band for these seeds. A lift versus a setpoint is on the seedling heat mat with thermostat page.

Off the mat once they are up

The day seedlings appear, the temperatures fall and the light has to rise. Chillies and sweet peppers move to about 16–18°C in bright light. Aubergines make the same move. Tomatoes come off the cover at emergence and want at least 16°C. A mat left on under a dome grows long stems and small leaves. Lid and probe setup is on how to use a seedling heat mat. If the only place after that is a cold dark sill, sow later. Keep lettuce and peas off the pad.

Common questions

What compost temperature do chillies, peppers, aubergines and tomatoes want?
Chillies want about 21°C to germinate, then 16–18°C in bright light as soon as they appear. Sweet peppers want 18–21°C, then the same 16–18°C. Aubergines want at least 21°C, then 16–18°C. Tomatoes want a warm sow, the cover off at emergence, and then at least 16°C in strong light. Those are RHS figures. If your packet prints a different number, follow the packet.
When does the RHS suggest sowing them in Britain?
From mid-February for tomato, chilli and aubergine if they are heading for a greenhouse. Sweet peppers for glass are often mid-February to early March, and outdoor peppers late March or early April. Outdoor tomatoes, peppers and aubergines sit around late March in the RHS grafting timing. A January start appears in other guidance. It is not the RHS February line this page uses.
Why might the end of March be safer than mid-February?
On a dull windowsill, an early sowing sits in a pot for weeks before it can be planted, and it stretches. The RHS warning on tomato sowing is that early heat is wasted electricity if the plants cannot go out, and Jenny Bowden’s RHS note says the end of March is safer when February trays only become leggy. A heat mat does not add the missing light.
Can a fixed mat be set to 21°C?
No dial on the fixed HBN or the Senua takes a setpoint. They add a lift above the room for as long as they are plugged in. You can aim a WADEO, an AOOWU or the HBN combo at 21°C only where the printed range includes 21°C, and you still measure the compost. The HYDGOOHO does not print that dial range. The Gsycle’s claimed 40–45°C band is too hot for these seeds.
When do you take tomato and pepper seedlings off the mat?
When they appear. Uncover tomatoes at emergence and grow them on at at least 16°C in bright light. Take chilli and sweet pepper seedlings out of the propagator straight away and keep them at about 16–18°C in bright light. Aubergines follow the same cooler, brighter step after germination. Leaving them on a hot mat under a closed lid is the leggy outcome these guides are trying to avoid.

Related pages

Seedling heat mat buying guide

Seedling heat mat

Best seedling heat mat: jobs in this UK listing set

How to use a seedling heat mat without cooking seed

Seedling heat mat with a thermostat or a fixed pad

Sources

Public UK sources checked for this comparison, plus the amazon.co.uk listing facts in the display file on 16 October 2026. Shelf prices and star lines are those UK figures. The Ofgem unit rate is the Direct Debit average for England, Scotland and Wales, not a Northern Ireland tariff and not a measured bill.

  1. RHS: how to sow seeds indoors (retrieved 16 October 2026)
  2. RHS: grow your own chilli pepper (retrieved 16 October 2026)
  3. Ofgem: energy price cap, 1 October to 31 December 2026 (retrieved 16 October 2026)
  4. Electrical Equipment (Safety) Regulations 2016, regulation 35 (retrieved 16 October 2026)
  5. The English Garden: heated propagators (retrieved 16 October 2026)
  6. BBC Gardeners' World: heated propagators (retrieved 16 October 2026)
  7. Thompson & Morgan: full-size seed tray (retrieved 16 October 2026)
  8. BSI: BS EN 60529, degrees of protection (IP code) (retrieved 16 October 2026)