Key takeaways
- The pot adds no heat; it only soaks up the candles' warmth and gives it out slowly
- A few tealights give about as much heat as a small light bulb
- That's not enough to keep a whole greenhouse frost-free
- Unattended naked flames near canes, labels and staging are a real fire risk
- Burning candles in a sealed space produces carbon monoxide
- A thermostatic electric heater, insulation or fleece are safer choices
Flower pot heaters aren’t worth the risk. The pot adds no heat of its own, a few tealights give very little warmth, and naked flames left alone in a greenhouse can start a fire. A thermostatic electric heater, good insulation or a layer of fleece will do far more, far more safely.
This guide looks at the viral tealight trick. For every proper heating option, see the greenhouse heating guide.
What is a flower pot heater?
It’s a home-made frost-buster, not a product. You stand a few tealights on a metal tray or tin, then turn a terracotta pot upside down over them. Some versions nest two pots. The clay warms up and gives off gentle heat.
The appeal is obvious: a few pence of candle wax instead of an electric heater. The videos rarely mention how little heat it makes, or what happens if something goes wrong.
This is the classic set-up. The clay warms up and glows with heat, but every bit of that warmth comes from the candles.
Do flower pot heaters actually work?
Only a little. The pot can’t create heat. All the warmth comes from the candles, and they give out the same amount whether the pot is there or not.
What the pot does is soak up that heat and release it slowly from a bigger surface. That feels steadier, but it isn’t more. A handful of tealights gives about as much heat as a small old-fashioned light bulb.
That might take the chill off a tiny cold frame or the air just around the pot. It won’t keep a whole greenhouse above freezing on a frosty night.
Are flower pot heaters a fire risk?
Yes. A flower pot heater is several naked flames, often left alone, in a space full of things that burn. Think wooden staging, bamboo canes, paper labels, plastic trays, bubble wrap and bags of compost.
Tealights have their own hazard. Packed closely together, they can melt into one pool of wax that flares up as a single large flame. Knock the tray and the flames land among dry material with nobody there to notice. Fire services have warned against leaving candles burning unattended for exactly this reason.
Crowd tealights together and the wax can pool and flare into one big flame. Not something to leave alone overnight.
Can a flower pot heater cause carbon monoxide?
Yes. Every flame makes carbon monoxide, a gas you can’t see or smell. A few candles are a small source, but a small sealed space changes things.
Gardeners often seal greenhouses with bubble wrap for winter, then light candles inside. Fumes can build up and harm anyone who steps in to check the plants. In a porch, shed or conservatory joined to the house, the risk is greater. Never burn candles anywhere people or pets sleep, and always leave a vent open around any flame.
What to use instead of a flower pot heater
Safer options remove the loose flame altogether.
A thermostatic tube heater only switches on when it’s needed, with no flame and no fumes. Have an electrician fit a weatherproof supply.
- Thermostatic electric heater: a tube heater or greenhouse fan heater only runs when the temperature drops. It needs an outdoor-rated, RCD-protected supply put in by an electrician.
- Paraffin greenhouse heater: if there’s no power, a heater built for greenhouses is far safer than loose tealights. It still needs ventilation.
- Insulation: bubble wrap on the glass keeps in the heat you have. Leave vents clear. See how to insulate a greenhouse.
- Fleece: a layer over tender plants on cold nights gives a few degrees of protection. The garden fleece guide explains which grade to use.
- Move plants: bring the most tender ones indoors on the coldest nights.
The guide to protecting plants from frost covers more ways to get tender plants through winter.
Is there ever a safe way to use one?
Only in a very small way, and only while you’re there. For a single shelf, porch or cold frame on one frosty evening, a couple of tealights under a pot will lift the temperature slightly for a few hours.
If you try it:
- Use two or three tealights, not eight.
- Stand the tin on a stable, fireproof surface, well away from anything dry.
- Keep a vent open.
- Stay nearby, and snuff it out before you leave or go to bed.
For a few tender plants, a cold frame is often the better answer. The guide to cold frame gardening shows how to overwinter plants with no heat at all.
A closed cold frame keeps young plants out of the worst of the frost. No flames, no fumes, no worries.
Flower pot heater mistakes to avoid
- Expecting it to heat a greenhouse. It can’t, so tender plants still get caught out.
- Leaving it burning overnight. Unattended flames are the biggest risk.
- Packing in lots of tealights. More flames mean more flare and more fumes.
- Sealing every gap. Fumes build up with no vent open.
- Putting it near dry material. Canes, labels, compost bags and wooden staging all burn.
Before winter really bites, read our guide to securing a greenhouse in wind, as gales do more damage than frost. More winter jobs are on our how-to hub.
Frequently asked questions
Do flower pot heaters actually work?
They give a little warmth, but the pot creates no extra heat. The candles supply all of it. The clay just stores it and lets it out slowly, which is fine for a tiny space but not a greenhouse.
Are flower pot heaters a fire risk?
Yes, unattended tealights are a real fire risk. Grouped candles can pool wax and flare. A knocked tray near canes, paper or wooden staging can easily start a fire.
Can a flower pot heater cause carbon monoxide?
Yes, burning candles produce carbon monoxide. In a small sealed space it can build up. It has no smell, so leave a vent open and never use one where people or pets sleep.
What can I use instead of a flower pot heater?
A thermostatic electric greenhouse heater is the safest choice. Without power, use a paraffin heater made for greenhouses, with ventilation. Insulation and fleece help a lot too.
How many tealights does a flower pot heater need?
Viral versions use four to eight, but more flames mean more risk. Extra candles don’t make it a real greenhouse heater. If you try one, use a few, stay nearby and never seal the space.
Our team produces practical UK gardening guidance from source research, contributor records and established horticultural practice. Editorial standards are overseen by Matt W.
