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How to use a seedling heat mat

Updated: October 2026

You have a tray of tomatoes or chillies, or a lidded propagator, and the risk is that the compost cooks or water reaches a live appliance. Put the heat under the tray, keep water off the appliance, and switch off anything you cannot set. The Royal Horticultural Society says many seeds germinate around 18°C, and the packet comes first. The listings are in our [plant heater comparison](/hub/plant-accessories/plant-heater). The Products Compared editorial team used them, and RHS sowing guidance, on 16 October 2026. A qualifying purchase can earn us a commission at no extra cost to you.

You have a tray of tomatoes or chillies, or a lidded propagator, and the risk is that the compost cooks or water reaches a live appliance.

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
HBN HBN mat with thermostat

Rank 3

HBN HBN mat with thermostat
8.8/10about £27.99
View offer
On this page
  1. 1.Under the tray, not in the compost
  2. 2.If there is no thermostat
  3. 3.Probe, lid, and water
  4. 4.Smoke, a dead mat, and when to stop

Under the tray, not in the compost

Lay the mat flat on a dry surface and sit the seed tray on it. The element warms the compost from below. Do not bury it, fold it into wet mix, or use it as a room heater. The Levson page prints IP65 and still says not to treat the mat as a room heater. Waterproof with no code, as on the plain HBN mat, means you can wipe the surface. Leave the mat uncreased. Do not cover the mat with a towel, and do not stack a second tray on it. The EarlyGrow already has the heater in the base, so do not add another mat underneath.

If there is no thermostat

Several mats claim a band and give you no dial. The plain HBN mat claims about 24–30°C at 17.5 W. The Levson claims 20–30°C at 20 W. The EarlyGrow holds about 16–19°C at 12 W, nearer the RHS figure of about 18°C, and you still cannot change it. Reviewers of the unregulated mats say seedlings cooked because the mat stayed on. One United Kingdom Levson review says the mat stayed cold after a single use. If there is no thermostat, you are the control. Switch the mat off if the compost is hot or the seedlings look scorched. A plug-in timer only breaks an always-on supply into sessions. It is not a setpoint. Do not leave an unregulated mat on overnight and assume the claimed band will hold 18°C.
HBN HBN 17.5 W seedling mat

Testsieger

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

Probe, lid, and water

Put the probe into the compost at seed depth, not on the mat and not in the room air. The HBN combo claims 5–42°C, a probe, and error codes E1, E2, and E3. Reviews say a setpoint did not always stick, so check with a separate thermometer on the first day. The WADEO set claims 5–45°C and one controller for two mats. Reviews say the soil overshoots, and both mats share that setpoint. On a lidded propagator, open the vents once seed has germinated. EarlyGrow reviewers say headroom is tight and the capillary mat can be too wet. Top up the reservoir before the mat dries, and stop before it floods. Lift the tray, water it, let it drain, and put it back. The RHS says not to wet heaters. Keep a greenhouse heater off the wet floor, on a socket with a 30 mA RCD.

Smoke, a dead mat, and when to stop

Unplug at once if a heater smokes or smells of burning. LightHouse reviews include smoke, a fallen end cap, and units that stopped heating. Morris reviews include smoke on first switch-on. Let it cool, do not open the casing, and use the returns route. Do not run the THERMOFORGE on UK mains: that listing prints 120 V. A new greenhouse supply, a burning smell, a cracked plug, or an unknown RCD is an electrician’s job. New fixed wiring in England and Wales is Building Regulations Part P work. A mat does not frost-proof a greenhouse. Those targets are on the greenhouse frost heater page. Which listing to buy is on best plant heater.

Common questions

Where does a seedling heat mat sit, and what must not go on the element?
The mat lies flat under the seed tray, on a dry surface. Compost, a wet saucer, a folded towel, and a stacked second tray do not go on the element. A lidded propagator already has its heater in the base.
What do you do with a mat that has no thermostat?
You switch it yourself. The plain HBN 17.5 W mat and the Levson 20 W mat claim a band and give you no dial. Turn the mat off if seedlings scorch. A timer only interrupts the supply. It does not hold 18°C.
Where does the probe go when a controller is in the box?
Push the probe into the compost at seed depth, not onto the bare mat or into the room air. On the first day, check with a separate thermometer. Reviews of the HBN combo say a setpoint did not always stick, and WADEO reviews say the soil can overshoot when two mats share one controller.
How do you water a heated tray without wetting the heater?
Lift a tray off a bare mat, water it, let it drain, and replace it. On the EarlyGrow, top up the reservoir so the capillary mat does not dry, and stop before it floods. Reviewers found that mat too wet. Do not pour water so it runs under the heater. The RHS says not to wet heaters.
What should you do if a heater smokes on first use?
Unplug it and leave it to cool. Do not open the casing or switch it on again to see if the smell goes. LightHouse and Morris reviews both include smoke on first use. Return the unit, and call an electrician if the plug is cracked. A larger wattage is not the next step while a faulty heater is still plugged in.
When is this an electrician’s job rather than a setup job?
A new greenhouse supply, a burning smell, a cracked plug, or an unknown RCD is an electrician’s job. Outdoor sockets should be on a 30 mA RCD. New fixed wiring in England and Wales falls under Building Regulations Part P. Sowing depth and vent position are the part you can do at the tray.

Related pages

Plant heater buying guide

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Best plant heater: windowsill top pick in this set

Greenhouse frost heater for pelargoniums and fuchsias

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)