Gardeners Are Using This DIY Terracotta Pot Hack to Heat Their Greenhouses – But Does It Actually Work?
The viral clay pot heater hack is taking the internet and greenhouses by storm, but does this DIY trick really work? A scientist turned gardener weighs in.
Clay pot heaters have become an online phenomenon. Tons of people, many gardeners among them, claim all you need to heat your house or greenhouse is a few terracotta pots and a handful of candles. But what's the truth behind these claims?
Can you really heat a greenhouse with a terracotta pot and some candles? It's an appealing idea because most gardeners have these common and inexpensive supplies on hand. But whether or not you can actually use them to keep the plants in your greenhouse warm is questionable.
Temperature control is one of the key factors issues in successful greenhouse gardening. A greenhouse is a great way to extend the growing season and to grow more plants year-round, so being able to heat one cheaply and easily is a very attractive idea. I'll put my knowledge as a former scientist to test out the the clay pot heater hack and dig into the risks and limitations of this viral trend.
What You Will Need
If you want to make a clay pot heater for yourself to see if it can add a little more heat to your greenhouse, here are the common garden tools and supplies you will need to try it.
- Unglazed, unpainted terracotta pots, either one large pot or one smaller pot nested in a larger one. You can find tons of terracotta pots on Amazon.
- A pillar candle or a few tea light candles, which are also available on Amazon.
- Bricks or other non-flammable materials to prop up the pots.
- Something to block the drainage hole in the pot, like a terracotta saucer, a coin, or some aluminum foil.
How to Make a Clay Pot Heater
The idea behind this viral gardening hack is that the terracotta will concentrate and then radiate heat from the candle. While it doesn’t quite work that way, this small heater can raise the temperature of a small space by a few degrees. Here's how to try it for yourself.
Turn the pot upside down over a candle or a few small candles. Set the pot on top of a pair of bricks or a metal stand. This allows air in to keep the flames lit. Make sure the materials you use and the surface on which you place the heater are not flammable.
Cover the drainage hole on the top of the pot with the saucer from the pot or something metal, like a quarter. Again, this material should not be flammable. Covering the drainage hole keeps the heat from escaping through the top of the heater.
Do Clay Pot Heaters Work?
You might see some claims online that a simple clay pot heater can heat a whole room or a sizable greenhouse. These claims are exaggerated and falsely assume that the DIY heater is actually generating more heat than it really is.
In reality, the candles you use in the heater only generate a set amount of heat or energy. One candle can produce about 40 watts or 100 BTUs per hour. Placing it under a terracotta pot won’t increase this number.
The terracotta pot changes how the energy from the candle is distributed. In a small, insulated space, this can lead to an increase in the overall temperature of 2 to 5 degrees Fahrenheit. For plants in a greenhouse on a cold winter night, this could be the difference between surviving and succumbing to cold and frost.
Unfortunately, greenhouses are notoriously drafty and poorly insulated. If you have a small, well-constructed greenhouse or a covering for the space that insulates it well during colder nights, a terracotta heater could help you protect delicate plants and seedlings. But don't expect it to heat your greenhouse the same way as a conventional greenhouse heater, like this one from Amazon.
Are Clay Pot Heaters Safe?
You might be thinking that if one terracotta heater can raise the temperature a few degrees, several heaters or more candles would be even better. We caution you not to go overboard with this DIY project, as it can easily become dangerous.
One risk is that moisture in the terracotta, when heated, can expand and cause the pot to rupture or shatter. If you are next to it when this happens, you could be struck or pierced by a sharp piece of terracotta.
Another risk is fire. Even if you’re careful by using non-flammable materials, something called the flashover effect could lead to a very dangerous grease fire. Too many candles gathered together in a confined space can cause them to reach a point called the auto-ignition temperature. At this temperature, all of the pooled wax ignites, leading to a grease fire that can spread quickly and be difficult to put out.
Finally, there is the risk of carbon monoxide poisoning. A terracotta pot heater works best in a small, insulated space, but this raises the possibility of gathering carbon monoxide in that space. Candles produce carbon monoxide which normally disperses enough that it's not an issue. But if you are using several candles in a space with no or little airflow, it can build up to potentially dangerous levels.
Should you use a terracotta heater in your greenhouse? The answer is maybe. Done safely and in a smaller greenhouse, it could save some of your more delicate plants. It can be particularly useful when you’re sprouting tender seedlings in late winter and the weather turns unexpectedly cold. But don’t rely on this type of heater to warm your entire greenhouse or to save all your plants.
If you do try this trend, be safe and follow these guidelines to prevent a fire and the buildup of harmful gases. If you’re not sure it will be safe in your space, it’s best to use other more reliable heating methods.
