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Greenhouse frost heater

Updated: October 2026

A frost forecast and a greenhouse of pelargoniums, fuchsias, or cacti is an air temperature you can check, not a seed tray with a bigger plug. A greenhouse frost heater has to hold that air, with a thermostat, in a house you have already insulated. No heater here is shown to keep every greenhouse frost-free. A seedling heat mat is the wrong tool. You still need a min/max thermometer. The listings are in our [plant heater comparison](/hub/plant-accessories/plant-heater). The Products Compared editorial team set RHS and Thompson & Morgan notes against them on 16 October 2026. We did not leave a thermometer in a greenhouse overnight. A qualifying purchase can earn us a commission at no extra cost to you.

A frost forecast and a greenhouse of pelargoniums, fuchsias, or cacti is an air temperature you can check, not a seed tray with a bigger plug.

Our top picks

Airmaster Airmaster 520 W frost convector

Best overall

Airmaster Airmaster 520 W frost convector
8.6/10about £25.94
View offer
Morris Morris 2 ft 120 W tube

Rank 2

Morris Morris 2 ft 120 W tube
8.2/10about £29.99
View offer
LightHouse LightHouse 45 W heater

Rank 3

LightHouse LightHouse 45 W heater
8.0/10about £22.99
View offer
On this page
  1. 1.The temperature you are actually holding
  2. 2.Insulate first
  3. 3.Then size the heater
  4. 4.Tube or convector

The temperature you are actually holding

Frost free is not one number. The RHS page on heating a greenhouse puts frost-free guidance at about 4°C, and it prefers an electric heater with a thermostat because paraffin and gas add moisture and combustion gases. The RHS April greenhouse notes ask for at least 5°C for fuchsias and pelargoniums, and at least 12°C for many tropicals. Thompson & Morgan’s winter notes, as used for this comparison, mention about 3°C and a milder 7–10°C band. Do not merge those into one setting. If the plants are pelargoniums and fuchsias, the line you can act on is at least 5°C. Tropicals at 12°C are a warmer house. A seed mat claiming 20–30°C at the compost does not hold these air temperatures.

Insulate first

The RHS spends as much space on bubble wrap as on the heater. Thompson & Morgan says to heat only the area you need, and to ventilate so the house does not sit wet. A single-glazed 6 × 8 ft house with a gap at the door is a different job from the same heater in a lined house. Fit the insulation you will live with, then size the heat. Otherwise you buy watts to warm the garden through the glass.
Airmaster Airmaster 520 W frost convector

Testsieger

Airmaster Airmaster 520 W frost convector
8.6/10about £25.94
View offer

Then size the heater

A writer’s rule of thumb, not an RHS table, is about 1 kW for a small 6 × 8 ft greenhouse after insulation. A min/max thermometer outranks that figure. Mats here are about 12–50 W. The tubes are 45–120 W. The Airmaster is 520 W. All of those sit under 1 kW. The THERMOFORGE claims about 1,000 W and prints 120 V, so it is the wrong supply for a UK socket.

Tube or convector

The Morris 2 ft tube is £29.99, 120 W at 240 V and IP55, with a claimed cover of 4 m² and a 90° cut-out. amazon.co.uk shows 4.1 from 500 reviews. Reviews say there is no dial, and several say 120 W does not warm a room. A claimed 4 m² is not a 6 × 8 ft house. The Airmaster FW400 is £25.94 and 520 W, with a printed thermostat of 5–30°C and a stated cover of up to 10 m². amazon.co.uk shows 4.3 from 173 reviews. Some buyers say a 6 × 8 ft house stayed above freezing on a low setting. Others call it a small-room heater. The spec says indoor and IP20, while the blurb names greenhouses. The LightHouse is about 45–50 W, with reports of about 1–3°C in a box or a shed. A seed mat does not hold the air either, which seedling heat mat or greenhouse heater separates.

Common questions

What temperature is frost free for a UK greenhouse?
The RHS heating guidance puts frost-free at about 4°C. Thompson & Morgan’s winter notes, as used here, also mention about 3°C. Those are air temperatures you check with a thermometer. No heater in this set is shown to hold every greenhouse there.
What do pelargoniums, fuchsias, and tropicals need above that?
The RHS April notes say at least 5°C for fuchsias and pelargoniums, and at least 12°C for many tropicals. Thompson & Morgan also uses a milder 7–10°C band. At least 5°C is a frost watch for those plants. At least 12°C is a warmer house, and a 120 W tube is even less likely to hold it in a 6 × 8 ft greenhouse.
Is a 45 W or 120 W tube enough for a 6 × 8 ft house?
Treat both as background heat. LightHouse reviews cite about 1–3°C in a small box or a shed, with no thermostat. The Morris is 120 W, claims about 4 m², and reviews say it does not warm a room. Your thermometer outranks a wattage guess.
Which heater has a thermostat, and which is IP55?
The Airmaster prints a thermostat of 5–30°C at 520 W, and the spec says IP20 and indoor. The Morris 120 W tube prints IP55 and a 90° cut-out, and reviews say there is no dial. IP55 tolerates a splashier spot. A cut-out is not a dial set to 5°C. LightHouse reviews say there is no thermostat at all.
Can a seedling heat mat protect plants from a frost night?
No. A mat of about 12–20 W warms compost in a tray. It does not hold the air around pelargoniums or fuchsias. The RHS uses mats for seed, around 18°C in the compost, which is a different job from a frost-free greenhouse at about 4°C.
Why check the heater with a min/max thermometer?
A dial and a review can both be wrong for your house. One buyer may report a 6 × 8 ft greenhouse above freezing, and another may call the same model a small-room heater. A min/max thermometer at plant height, away from the blast, shows the lowest air temperature the plants had.
What must you not do with paraffin, gas, or a wet heater?
The RHS notes that paraffin and gas add moisture and combustion gases, so an electric heater with a thermostat is the controllable option in that guidance. Do not wet an electric heater, and do not stand an IP20 convector in water because a blurb mentions a greenhouse. If it smells of burning, unplug it and stop.

Related pages

How to use a seedling heat mat

Plant heater buying guide

Plant heaters

Best plant heater: windowsill top pick in this set

Seedling heat mat or a greenhouse heater for the UK

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)