If you need more space for growing plants in the winter, a greenhouse seems like a logical choice. However, there is still the problem of germinating seeds and keeping plants alive in the winter cold.
So, how do you heat a greenhouse? An electric heater or warming mat will heat a greenhouse with electricity. You can also use a gas heater or wood stove to heat a greenhouse without electricity. Using water barrels, stones, and insulation can all help to retain heat in your greenhouse.
Let’s take a closer look at how these greenhouse heating methods work, and which ones might be the best option for you.
How To Heat A Greenhouse
There are many options for heating a greenhouse in the winter. We’ll start off with methods that use electricity, but if your greenhouse is “off the grid”, then skip to the next section.
Use An Electric Heater To Heat Your Greenhouse
You can buy all sorts of electric heaters on Home Depot, Lowe’s, Walmart, or Amazon websites. However, the heating capacity and prices will both vary considerably.
For instance, you can find portable electric heaters that are suitable for residential use for as little as $25. However, these heaters are only meant to “take the chill off” by warming a bedroom by a few degrees.
These small heaters are not really built for constant use to keep a greenhouse at 60 or 70 degrees Fahrenheit when the outside air is freezing.
You would be better served with a heater that is specifically built for heating a greenhouse in the winter. This will require a heater with higher output (higher BTUH).
The higher the BTUH (British Thermal Units per Hour), the more heat energy a heater can put out, allowing it to heat a larger or colder space.
For comparison, let’s compare two heaters from greenhousemegastore.com.
A portable electric heater goes for $65 and has a BTUH rating of 5120/4436 (input/output).
On the other hand, a Modine H60 heater goes for $732 and has a BTUH rating of 60,000/48,000 (input/output). It weighs 80 pounds.
This translates to about 12 times the cost, for about 12 times the heating capacity (that is, you get what you pay for).
Note that the Modine heating units and others like it require a vent pipe. They should be installed by a certified HVAC technician, which adds to the cost if you choose to heat your greenhouse with this method.
One advantage of electric heaters is that they will always work, as long as you have electricity connected and the power doesn’t go out.
One disadvantage of electric heaters is the potential for uneven heat distribution. An electric heater works by convection. This means that it warms the air in a room, which then moves around the room to heat up other objects.
Use An Infrared Heater To Heat Your Greenhouse
Another option to heat your greenhouse using electricity is by using an infrared heater. An infrared heater uses radiation to warm plants (similar to how the sun warms them up), as opposed to the convection used by other electric heaters.
When choosing a heater, make sure to purchase one with the right safety features.
For example, choose a model with a metal grille to prevent the heating element from coming in contact with flammable objects. If possible, choose a model that also has overheat protection, which turns off the unit if it gets too hot.
One idea is to buy an infrared heater that can be mounted on the ceiling or supports of your greenhouse. This will give the same effect as a “miniature sun” inside the greenhouse. You can check out a ceiling mount portable infrared heater on Home Depot website.
You can also choose a unit that stands on the floor, such as this infrared heater on the Amazon website.
Use A Warming Mat For Seed Germination
One other option to make your greenhouse warm enough for germination is a warming mat. These mats are often used to keep a small amount of soil warm enough for seeds to germinate.
These mats are not really meant to heat the entire greenhouse, but rather, to raise the temperature of the soil enough so that seeds that need more heat can germinate properly, or so that young seedlings can survive until spring.
You can check out a seedling heat mat on the Johnny’s Selected Seeds website.
How To Heat A Greenhouse Without Electricity
Maybe your greenhouse does not have electricity and you don’t want to run an extension cord outside. In that case, there are still some options to heat your greenhouse.
Use A Gas Heater To Heat Your Greenhouse
You have the option of using a heater powered by gas instead of electricity to heat your greenhouse. There are gas heaters that use propane or kerosene, and you can often choose vented or non-vented models.
The obvious advantage is that you don’t need to provide electricity to run a gas heater.
One disadvantage of a gas heater is that you will need to keep it filled with fuel – more so in the coldest months when they will be running almost constantly.
Another disadvantage of gas heaters is that some of these models will need to be vented outside. This adds another potential source of heat loss in your greenhouse.
Use A Wood Or Pellet Stove To Heat Your Greenhouse
If you don’t want to deal with gas, a wood or pellet stove is another option to heat your greenhouse.
One advantage of a wood stove is that you can use branches or cord wood harvested from your own property.
One disadvantage of a wood stove is the smoke that is produced, along with the obvious danger of burning down your greenhouse due to open flame.
Make A Compost Pile In Your Greenhouse To Heat It Up
A compost pile can heat up to a temperature of 170 degrees Fahrenheit (76.7 degrees Celsius). When a large compost pile really gets going, it can generate enough heat to warm up the air inside your greenhouse.
You can continue to add both brown (carbon-rich) and green (nitrogen-rich) materials to your compost pile throughout the winter to keep the bacteria working to produce heat.
I would not rely on this method alone to heat your greenhouse in the winter. However, when combined with some of the other suggestions outlined here, a compost pile can mean the difference between survival and death for the plants in your greenhouse.
Build Your Greenhouse On The South Side Of Your House
This trick is not as useful if your greenhouse is already built. However, if you are thinking of rebuilding or upgrading to a larger greenhouse, then it may be worthwhile.
When you build your greenhouse right next to the south side of your house, you get two benefits.
First of all, the greenhouse will get the maximum amount of sun (assuming you are in the northern hemisphere).
Second, the greenhouse will absorb some of the heat from your house, allowing you to save on heating costs for the greenhouse itself (electricity or gas).
How To Retain Heat In A Greenhouse
It’s one thing to heat up your greenhouse, but keeping the heat inside is just as important. Here are some ways to retain the heat that you generate inside your greenhouse.
Use Water Barrels To Retain Heat In Your Greenhouse
Placing water barrels in your greenhouse will help to retain heat, especially if you paint the barrels black.
When the sun hits the barrels, it will warm up the barrel itself and the water inside. As the sun goes down and temperatures drop, the warm water will start to release its heat to the air inside the greenhouse.
As an added bonus, these barrels can be used for easy watering of seeds, seedlings, and plants in your greenhouse. Just make sure to keep the barrels full to make sure that you get the maximum benefit from the sun’s energy.
Use Stones Or Cinder Blocks To Retain Heat In Your Greenhouse
You can also use stones or cinder blocks to retain heat in your greenhouse. Although not as effective as water, stone will still retain some heat from the sun and release it as temperatures drop after nightfall.
Either stones or cinder blocks will work (the darker the better).
Use Insulation To Retain Heat In Your Greenhouse
By insulating the walls of your greenhouse, you can retain the heat energy from the sun.
Bubble wrap is one good choice to insulate against the cold air outside and to trap heat inside. Traditional bubble wrap used in packaging and shipping can work, but you can find bubble wrap specifically for greenhouses at garden supply centers or online.
You can also use reflective foil insulation from Amazon, which contains air bubbles but also reflect more of the radiant heat in a greenhouse.
Use Caulking To Seal Cracks And Openings In Your Greenhouse
A quick way to keep your greenhouse warmer is to us caulking to seal any cracks or holes from nails, screws, or at joints where the greenhouse was put together.
If you need to vent a heater, make sure to caulk around the opening to prevent cold air from coming in.
Use Row Covers Or Cloches To Cover Plants Inside Your Greenhouse
If the above steps just aren’t enough by themselves, then you can use row covers or cloches to offer another layer of protection to the plants in your greenhouse.
A row cover is made of fabric and is used to protect an entire row of plants by insulating them from cold. A cloche is often made of plastic and is used to protect one small plant or seedling from the cold (it can also retain moisture).
For more information, check out my article on how to protect your plants from cold and frost.
Conclusion
By now, you have some idea of how to heat your greenhouse, whether you have electricity connected or not. You also know how to retain the heat that you generate from the sun or from electricity.
I hope this article was helpful – if so, please share it with someone who can use the information. If you have any questions about how to heat a greenhouse, please leave a comment below.