Is a Greenhouse Heater Necessary? : The Truth About Greenhouse Heaters

You’ve either built or invested in a greenhouse and now you’ve found that it’s still not as warm as you want it in your greenhouse. You might be wondering if buying a greenhouse heater is necessary.

In general, yes, greenhouse heaters can be necessary for certain climate zones and for plants that can’t deal with colder temperatures on their own. 

There are four types of climate zones: polar, temperate, subtropical, and tropical. The polar climate is where temperatures will be at their coldest. In the polar climate zone, temperatures can drop below -50° F, making it not a great place for greenhouses.

In the temperate climate zone, average temperatures will range from 32° F to 68° F. Since greenhouses are hotter than outside temperatures (more on this below), this can be a pretty good zone to grow in, especially further to the south. In the subtropical zone, the average temperature will be above 50° F most of the time, another great area for greenhouses. 

The last climate zone, tropical, has average temperatures of 64.4° F during the coldest month of the year. The tropical climate zone will be too hot for greenhouses in most cases, and they definitely won’t require a heater.

Where they will require a heater is in the polar, temperate, and subtropical zones. Not always, depending on your situation, but this is where a little consideration comes in. Each of those three zones covers a lot of land horizontally and vertically, meaning that one area’s fall and winter will differ from another’s. There can also be unusually cold winters to account for too.

In addition, what if you’d like to grow certain plants that aren’t sustainable with the temperatures in that particular zone? Or, even if they are, what if you’d like to grow them for a longer period? An unheated greenhouse might not be able to sustain them. With a greenhouse heater, though, they can.

How Does a Greenhouse Work?

So, how does a greenhouse work anyway? Well, it’s well known that plants need water, nutrients, heat, and light to survive. A greenhouse provides the last two with its see-through exterior and enclosed interior.

A greenhouse’s exterior is usually made of glass or plastic sheeting, allowing sunlight to filter through. That sunlight is then absorbed by something dark, such as plants or flooring. This process is why it’s not a good idea to wear dark clothes on a sunny day. 

Next, some of that absorbed light becomes heat. The heat rises and fills the greenhouse. Since the greenhouse doesn’t let it go anywhere, it warms up and stays warmed up.

Temperature Difference Inside & Outside a Greenhouse

In general, greenhouses can get up to 30° F hotter than the temperature outside. Of course, they don’t always get this hot, but they can. Most greenhouses should be between 75-85° F during the day since many plants require those temperatures to survive. So, even if the greenhouse was 30° F hotter, meaning the lowest temperature the outside could be is 45° F.

Cost of a Greenhouse Heater

Greenhouse heaters will cost anywhere from approximately $30 to over $300. Of course, these are based on current items and are only estimates. If you’re looking for some of the best options available, check out this article on the best greenhouse heaters.

When it comes to monthly costs, they will depend on the size of your heater and how long you plan to run it each day. To calculate exact costs, you’ll need a formula. Start with how many watts the heater is, which manufacturers should indicate on product pages or manuals. Next, multiply that number by how many hours you plan to use the heater each day.

Once you have that number, divide it by 1,000 to get your kilowatt-hours per day. Then, multiply that number by the days you plan to use the heater per year. That’s how much energy the heater’s going to use each year.

To get the yearly cost, you’d take that number and multiply it by cents per kilowatt-hour, which varies from area to area. This information is provided on many government or government organization websites. To get the monthly cost, take that number, divide it by 12, and you have your monthly cost.

How Safe are Greenhouse Heaters?

Greenhouse heaters are as safe to use as any other electrical device. Unfortunately, that also means that, like any other electrical device, they do have the potential to cause a fire. More so because they actively give off heat. With that in mind, make sure the greenhouse heater is on a safe surface, away from anything it shouldn’t be too close to, and your fire risk will be minimal. For more information, take a look at our in depth look at greenhouse heater safety.

4 Ways to Heat a Greenhouse Without Electricity

Ok so maybe you’ve determined that supplemental heat in your greenhouse is necessary.

Maybe you’ve considered buying a greenhouse heater, done a bit of research, and wanted to know your other options that won’t cost you an arm and a leg in energy bills. Whatever your reason, here are some different ways to heat a greenhouse without electricity:

  • Add Another Layer Of Insulation

Windows or plastic sheets can only do so much to keep the heat inside a greenhouse, so why not add another layer? One thing you can add is bubble wrap. Yes, you read that right. Bubble wrap!

If you decide to go with bubble wrap, look for horticultural bubble wrap. Or, if that’s not an option, go with one that has bigger bubbles. Those are the key to what makes it good insulation.

  • Seal Off Cracks And Gaps

The more heat you can keep inside the greenhouse, the higher its temperature, and the more likely it’ll stay that temperature. Unfortunately, even the best-built greenhouses will have cracks and gaps. So, find a good-quality sealing strip that’ll suit your needs, and go to town.

  • Build A Heat Sink

Heat sinks are also known as thermal mass. They store heat during the day and release it during the night. Thankfully, they sound more complicated to build than they are. Some easy options are darker-colored brick walls or water drums painted black. 

You can even paint a number of gallon plastic water bottles black and stack them on the wall that gets the most direct sunlight.

  • Put A Compost Pile In

As things decompose during the composting process, they release heat. You can build a composting area yourself or purchase a bin to put it all in.

If you’re looking into ways to heat your greenhouse, you likely already have a greenhouse in your yard. If not, be sure to check out our recommendations for the best winter greenhouses on the market.

A Useful Video on a DIY Greenhouse Heater