Great Solutions for How to Heat a Greenhouse During Winter

When it comes to having a greenhouse, one of the biggest expenses is usually not the greenhouse itself, but heating it. Believe it or not, the greenhouse industry spends about 70% of its operating cost on heat! I, for one, don’t want to spend that much money.

So let’s talk about the best solutions to the dilemma of how to heat a greenhouse during the winter without breaking the bank. From natural methods that use a thermal mass source to conventional greenhouse heaters, you can customize a solution that works best for you!

Extend Your Gardening Season

Greenhouse gardening can be so rewarding and fun. I absolutely love having a greenhouse for starting seeds indoors and growing year-round!

But even if you don’t have the luxury of a greenhouse, there are ways to keep gardening in the winter, like using frost blankets for plant protection, winter sowing, and building a DIY cold frame.

Why You Need to Regulate Greenhouse Temperature

If you are blessed with a greenhouse, you probably already know that you need a heat source (besides the sun). As long as a greenhouse has adequate ventilation, the daytime high temperatures are usually not the issue; it’s the low nighttime temperatures that pose a problem for many growers.

Some of the benefits of heating a greenhouse to create a stable environment include: 

• Optimal Growth - Regardless of what you are growing, big temperature fluctuations will stress plants and seedlings, causing poor or erratic growth.

• Germination Rates - Cold temperatures lead to infrequent germination for many kinds of seeds.

• Heat/Cold Damage - High heat and extreme cold can both damage or kill your plants.

• Increased Options - A properly heated greenhouse enables you to grow a much wider variety of plants. 

Tracking Greenhouse Temperature

It is important to track the high and low temperatures in your greenhouse so you know what steps to take. The goal is to keep the temperature in your greenhouse consistently in the optimal range if at all possible, especially by reducing temperature fluctuations between daytime and nighttime.

This is why I recommend getting a thermometer for your greenhouse. It doesn’t have to be anything special or expensive (a basic hanging thermometer can work), just something that reliably tells you what the temperature is, so you can track your temperature data and know when to heat and when to vent. 

Pro Tip: If you have a retail greenhouse or need to monitor temperature when you are not in the greenhouse, try a Temp Stick. With it, you can track the temperature and humidity in real time, 24 hours a day, no matter where you are. You can also set alerts that come right to your phone for nighttime security or for when you’re gone.

The Shape of Your Greenhouse Matters

When looking for a greenhouse kit to buy, you’ll see several main styles of greenhouses: gable-roofed greenhouses, round hoop-house greenhouses, and Gothic style greenhouses. You may be surprised to learn that the shape of a greenhouse makes a huge difference in how much heat it can trap!

A gabled greenhouse has a conventional peaked roof that comes down to vertical sidewalls. A hoop-house greenhouse has an arched shape, like a semi-circle. A Gothic style greenhouse is a hybrid between the two, having a high peaked roof that curves down into steep sidewalls.

The sides of a peaked roof are angled towards the sun which allows more sunlight into the greenhouse and traps that energy inside causing it to bounce around instead of escaping back out. A round or arched roofed greenhouse loses more heat because the energy swirls around the edges (instead of bouncing around), coming in contact with the edge of the greenhouse longer.

If you live in a place that gets really cold, you should choose a greenhouse that has a peak in the roof (either the gabled or Gothic style) because it will trap more heat and allow snow to slide off. 

Best Greenhouse Heating Solutions

There are various methods for heating a greenhouse. While warming a greenhouse naturally is often the most cost-effective solution, it is usually less effective than heating it with a heater.

For a long time, I used only natural methods to heat the greenhouse, but I eventually settled on using a combination of natural heating solutions and a localized heat source to get the job done. 

Here are some ideas to help you select the best greenhouse heating method or combination of methods so you can have great success with greenhouse gardening, too: 

• Thermal Mass

• Solar Heat Sink

• Geothermal 

• Localized Heat Source

How to Heat a Greenhouse Naturally

Most natural heating methods use thermal mass to heat a greenhouse. Thermal mass is an object’s ability to absorb heat from the sun during the day, hold the heat, and release the heat at night. 

Objects with high thermal mass (like brick, concrete, or water) can store large amounts of heat energy and change temperature slowly. In contrast, an object with low thermal mass (like the air inside a greenhouse) changes temperature quickly. 

Placing objects with a high thermal mass inside the greenhouse creates thermal equilibrium, the process by which two objects transfer energy until they both reach a common temperature. Objects that are heated up by the sun during the day will, in turn, heat up the air in the greenhouse during the night. 

Thermal Mass of Soil

Soil and plants are a great thermal mass source! Even in an outdoor garden, garden soil gets warm throughout the day and releases heat throughout the night. 

Soil and plants in the greenhouse do the same thing. The more plants and soil you have in your greenhouse, the more their thermal mass will keep the greenhouse warm during the night.

Fabric grow bags are a great choice for greenhouse growing. They are black, which helps absorb heat, and they are durable, reusable, movable, and encourage root pruning to stimulate growth. Check out the MIgardener store for grow bags in every size, even up to large 100-gallon grow bags!

Solar Heat Sink

A solar heat sink generates thermal mass for your greenhouse using water tanks. Water is highly effective at retaining heat. But, water tanks can only heat the greenhouse as much as the water is warmed by the sun during the day.

Black water barrels or tanks are the preferred choice for a solar heat sink because black absorbs heat more than white or other colors. Black barrels (or ones that are painted black on the outside) will maximize heat absorbtion.

Place the water barrels in strategic locations where they can soak up as much sun as possible without shading your plants. The general rule of thumb is that you will need 4 to 5 gallons of water to heat one square foot of greenhouse. 

Gravel or Stone

A gravel or stone floor will hold onto heat as well. We placed a 3-inch layer of gravel across the entire greenhouse floor to act as a thermal mass and eliminate the mud.

Geothermal

As the name implies, the geothermal method of heating a greenhouse uses heat from the earth. This has been done for hundreds of years by digging out an area to recess the greenhouse into the side of a hill or down into the ground. 

By digging down deeper than the frost line, you can reach temperature-stable soil that stays permanently thawed. (The frost line is the how far down the ground can freeze in the winter; here in Michigan, that is about 4 feet.) 

That thawed soil will always be warmer than 32°F and so it will naturally heat your greenhouse when the outside temperature is below freezing. 

Using Localized Heating in a Greenhouse

I highly recommend utilizing some or all of the natural heating methods listed above. They are a great way to save money and grow greener. But you may still end up needing to supplement with a localized heat source as I do. 

When heating our retail greenhouse, I faced the dilemma of not being able to use a regular greenhouse heater because I didn’t have access to a large propane tank or natural gas. I also couldn’t use a wood-burning heat source due to government regulations in the area where the greenhouse was located. 

After posting my dilemma online, a lot of people recommended getting a 125,000 BTU forced-air job-site heater, and it works great! The heater has a thermostat, so it kicks on and off during the night to stabilize the greenhouse's temperature, keeping it right where I want it. 

The main concern I hear about using a job-site heater or greenhouse heater is carbon monoxide. I encourage you to read all the warnings on any heater you use and take the matter seriously. 

Just to be safe, we did install a carbon monoxide detector, but my level of concern about it is very low. Our greenhouse has so much ventilation and the heater is rarely running when we are in the greenhouse during the daytime. 

Best Temperature For a Greenhouse

The best temperature for a greenhouse depends on what you are growing. Brassicas like 55° to 65°F, while tomatoes and peppers like warmer temperatures. I keep our greenhouse at around 70°F to 75°F, which allows me to grow a wide variety of crops.

As you get later into the growing season, you need to take less and less action to heat a greenhouse because the outside temperature is warmer.