During spring and autumn, many gardeners face the same challenge: warm days followed by cold nights. In unheated plastic or foil greenhouses, temperatures can drop below the comfort zone for plants, especially tomatoes.
In such situations, a full electric greenhouse heater is not always necessary. For light frost, a simple and well-known method can help – heating the greenhouse with candles and a clay pot.
How does the clay pot heating method work?
Clay is a natural material that absorbs and stores heat very effectively. When candles burn underneath a clay pot, the pot heats up and slowly releases the stored warmth into the surrounding air.
This process helps reduce temperature fluctuations inside the greenhouse and lowers the risk of frost damage during cold nights.
In practice, this method can extend the life of tomato plants in a plastic greenhouse by around 1–2 weeks, provided that daytime temperatures remain warm and night frost is mild.
What do you need?
- 2 clay flower pots (one larger, one about 5 cm smaller in diameter)
- A metal rod or threaded bolt with nuts
- 4 metal washers
- 2–4 thick tea lights or pillar candles
- 3–4 stones or bricks for stability
- A clay saucer or other fireproof base
How to build the candle heater
- Insert the metal rod through the drainage hole of the larger clay pot and secure it with nuts and washers.
- Attach the smaller clay pot upside down on the rod, leaving a gap between the pots to allow warm air circulation.
- Place the candles on a fireproof base.
- Arrange stones or bricks around the candles and carefully place the clay pot construction on top.
Important safety precautions
- Always place the heater on a stable, non-flammable surface
- Ensure proper ventilation inside the greenhouse
- Keep all flammable materials away from open flames
- Do not leave the setup unattended for long periods
When is this method suitable?
The candle and clay pot heater works best for:
- small plastic or foil greenhouses
- short periods of mild frost (around 0 to –2 °C)
- spring and autumn season extension
It is not a replacement for a professional greenhouse heater during severe or prolonged frost, but it can be an effective temporary or supplementary solution.
Conclusion
Heating a greenhouse with candles and a clay pot is a smart, low-cost and electricity-free way to protect tomatoes from light frost. Used correctly, it helps extend the growing season and protect your harvest when temperatures drop at night.
