A wonderful DIY idea: With a candle and a suitable pot, you can create a clay pot heater in no time at all, which you can use as a frost monitor for your greenhouse. With these instructions you can do it.
One thing right away: miracles should not be expected from this improvised frost guard. Nevertheless, the clay pot heater is usually sufficient to keep small greenhouses frost-free. In principle, all clay pots without glaze or paint are suitable. From 40 centimeters (16 in) in diameter, the heat can then quietly come from two or more candles, so the homemade frost guard is more effective.
Proper frost guard, which can be purchased as a device, is usually an electrically operated fan heater with built-in thermostat. As soon as the temperature drops below freezing, the devices jump on automatically. Unlike these electric frost monitors, of course, the DIY version does not work automatically: if a frosty night is coming, the candles must be lit by hand in the evening for frost protection. However, the improvised clay pot heater has two advantages: it consumes neither electricity nor gas, and the cost of purchase is much lower.
What do you need for the clay pot heater?
Pillar or Advent wreath candles are perfect for the clay pot heater. They are inexpensive and often burn for days depending on height and thickness. Table candles or even tea lights burn out too quickly and you would have to constantly replace them. If the pot is too small, the candle may become soft from the radiating heat and will have a short burn time.
You can also melt down leftover candles and use them to make new thick candles especially for your clay pot heater. In that case, you should simply pour the wax into a shallow, wide tin can or small clay pot and place the thickest wick possible in the center. The stronger the wick, the larger the flame and the more heat energy will be released during combustion.
To match the required number of clay pots and candles to your own greenhouse, you need to experiment a little. Of course, the heating capacity of the frost monitor also depends on the size and insulation of the greenhouse. The candles cannot heat up against leaky windows in winter, and the glass or foil house must not be too large either.
Building a frost guard: Step by step
Place the candle in the designated place and put the clean clay pot over it. If possible, the candle should not be directly under the water drainage hole, but warm the wall of the pot. Otherwise, much of the heat energy will escape into the surrounding air. The goal is that the burned clay is heated as much as possible, because then the particularly effective radiant heat is created.
Higher candles heat almost only the bottom of the pot. Then the water drainage hole can become a problem. Cover it with a clay shard to keep the heat in the pot and keep it from getting too hot directly above the frost guard. A small stone under the pot or a few slits cut into the rim of the pot with a parting grinder will provide the necessary air supply. If the flames do not get enough oxygen, they will only flicker and soot heavily.
Important tips for clay pot heating
Although the clay pots only become very warm and not hot, as a precaution nothing flammable should be on top of them. After all, it should only be warm in the greenhouse and not red hot. Of course, this also applies to the inside of the pot, which must be clean. Because plant or even substrate residues can certainly ignite and then together with the candle wax even generate quite large flames in the frost guard. It is essential that the candles are placed on fireproof supports such as bare soil or stone slabs. In order for potted plants overwintering in the greenhouse to benefit from the radiant heat, they should be placed with some safe distance around the frost guard.
Classically, the frost monitor is used in the greenhouse when overwintering potted plants or growing young plants. However, you can also place the improvised heater in a sheltered place on the terrace, in case the potted plants there are surprised by early night frosts.