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Seedling heat mat or greenhouse heater

Updated: October 2026

You are about to buy bottom heat for a tray of tomatoes when the greenhouse itself must stay above freezing, or the other way round. A seedling heat mat or greenhouse heater is not one product with two names. Heat the compost if you are germinating. Heat the air if the house must stay above freezing. A mat does not do the second job. The Products Compared editorial team set the listings in our [plant heater comparison](/hub/plant-accessories/plant-heater) against Royal Horticultural Society notes on sowing and on heating a greenhouse, on 16 October 2026. We did not measure a glasshouse overnight. A qualifying purchase can earn us a commission at no extra cost to you.

You are about to buy bottom heat for a tray of tomatoes when the greenhouse itself must stay above freezing, or the other way round. A seedling heat mat or greenhouse heater is not one product with two names.

Our top picks

HBN HBN 17.5 W seedling mat

Best overall

HBN HBN 17.5 W seedling mat
9.2/10about £14.99
View offer
EarlyGrow EarlyGrow 3-bay heated propagator

Rank 2

EarlyGrow EarlyGrow 3-bay heated propagator
9.0/10about £42.95
View offer
Airmaster Airmaster 520 W frost convector

Rank 3

Airmaster Airmaster 520 W frost convector
8.6/10about £25.94
View offer
On this page
  1. 1.What a mat heats
  2. 2.What an air heater does
  3. 3.Mat or lidded propagator
  4. 4.Why a tube is not a bigger mat

What a mat heats

A seedling heat mat warms compost in a tray. The RHS describes a heated propagator or heat mat as the efficient way to warm trays, and many seeds germinate around 18°C if the packet agrees. That is a few degrees in a small volume of compost, not the air in a 6 × 8 ft greenhouse. Plain mats here are about 17.5–20 W. The EarlyGrow propagator is 12 W. HBN mat reviews talk about trays and windowsills, not a glasshouse held through a frost.

What an air heater does

An air heater is the other side. The LightHouse tube is about 45–50 W. The Morris tube is 120 W. The Airmaster convector is 520 W with a thermostat. The RHS heating note treats an electric heater with a thermostat as the controllable option, and it points frost-free guidance at about 4°C. About 4°C in the air and about 18°C in the compost are different targets. A 20 W mat on a greenhouse floor does not keep pelargoniums. The THERMOFORGE T7 is not the greenhouse side of this choice: it claims about 1,000 W and prints 120 V, the wrong supply for a UK socket.
HBN HBN 17.5 W seedling mat

Testsieger

HBN HBN 17.5 W seedling mat
9.2/10about £14.99
View offer

Mat or lidded propagator

Gardeners’ World splits the propagation side into a lid, which holds humidity, and a mat, which is only the heat. The plain HBN mat is £14.99, 17.5 W, 230–240 V, and about 10 by 20 inches, with no dial. You supply the cover and the off switch. The Levson is £14.36, 20 W, 230 V, and IP65, also with no setpoint. IP65 does not make it an air heater. The EarlyGrow is the lidded example: £42.95, 12 W, a stated UK plug, 220–240 V, a vented lid, and a fixed band of about 16–19°C. You cannot turn it down, headroom is tight, and it is not a greenhouse heater. Choose the lid for one windowsill unit. Choose a bare mat when you will switch the heat yourself. A thermostat on a small mat is still tray heat.

Why a tube is not a bigger mat

A tube warms air, with the limit of its wattage. The LightHouse is about 45 W in the title and 50 W in another field, at 230 V, and reviews say there is no thermostat. Several say 45 W does not lift a greenhouse. Sliding that tube under a seed tray is not bottom heat. The Morris 2 ft tube is 120 W at 240 V and IP55, with a cut-out rather than a dial. The Airmaster is 520 W with a printed thermostat of 5–30°C. The spec says indoor and IP20, while the blurb names greenhouses. IP20 is not a wet-floor rating. About 1 kW for a small 6 × 8 ft greenhouse after insulation is a writer’s rule of thumb, not an RHS table. Every UK-voltage heater in this set sits under that figure. Degree targets belong on the greenhouse frost heater page. Filters are on the plant heater buying guide.

Common questions

Will a heat mat keep a greenhouse above freezing?
No. Mats here are about 12–20 W under a tray. The RHS treats that as germination heat, often around 18°C in the compost. Frost protection is an air temperature. A smaller mat will not hold it.
When is a lidded propagator enough, and when do you want a mat?
A lidded propagator is enough when you want one windowsill unit and a fixed band. The EarlyGrow is 12 W at about 16–19°C, with a stated UK plug. A bare mat suits a tray you already own if you accept no dial, as on the HBN 17.5 W mat. Either way you are heating compost, not greenhouse air.
What wattage here is bottom heat, and what wattage is air heat?
Bottom heat is about 12–20 W: the EarlyGrow, the HBN mat, and the Levson. Air heat starts with the LightHouse at about 45–50 W, then the Morris at 120 W, then the Airmaster at 520 W. A 120 W tube is not a large seed mat, and a 17.5 W mat is not a small greenhouse heater.
Can you put a greenhouse tube under a seed tray?
You should not treat a tube as bottom heat. It is built to warm air, often with no dial, so the compost temperature is an accident. Use a mat or a lidded propagator for seed. Keep the tube for the cupboard or shed it can take the chill off, and keep water off it.
Where does a 120 V tent heater sit in this choice?
In neither UK column. The THERMOFORGE T7 claims about 1,000 W and prints 120 V AC. UK mains are 230 V, and this file does not show a BS 1363 plug. It is a tent heater for a 120 V supply, not a UK greenhouse heater and not a seedling mat.
If you only have a windowsill, which side do you buy?
Buy the propagation side. A sill tray needs compost heat near the packet temperature, which the RHS puts around 18°C for many seeds. The EarlyGrow band is about 16–19°C. A greenhouse tube or a 520 W convector is the wrong shape for that sill.

Related pages

Best plant heater

Plant heaters

Plant heater buying guide: job, voltage, plug and IP

Greenhouse frost heater for pelargoniums and fuchsias

How to use a plant heat mat without cooking seed

Sources

Public UK sources for greenhouse heat, sowing temperature and electrical safety, plus the amazon.co.uk listing facts in the display file. Shelf prices and star lines are those captured figures, not a live re-check.

  1. RHS: heating greenhouses efficiently (retrieved 16 October 2026)
  2. RHS: electricity in the garden (retrieved 16 October 2026)
  3. RHS: sow seeds indoors (retrieved 16 October 2026)
  4. RHS: April greenhouse jobs (retrieved 16 October 2026)
  5. Electrical Equipment (Safety) Regulations 2016, regulation 35 (retrieved 16 October 2026)
  6. NICEIC: gardens and electrics (retrieved 16 October 2026)
  7. Gardeners' World: heated propagators (retrieved 16 October 2026)
  8. Thompson & Morgan: heating a greenhouse (retrieved 16 October 2026)