Does Your Greenhouse Need a Heater? How to Know

Greenhouses are fantastic additions to any home garden. While the flowers and produce that grows in the ground are subject to weather conditions, a greenhouse allows you to keep things growing all year long. Maybe you have a lovely greenhouse already in place, or possibly you’re looking to install one soon. Either way, you may be wondering how to get a temperature that is adequate for healthy plants.

Does your greenhouse need a heater? Greenhouses allow gardeners to grow plants during cooler temperatures by trapping the sun’s energy inside and then utilizing the soil beneath to trap radiant warmth. However, some plants require even more heat to thrive, so you may need a heater.

The great thing about greenhouses is that, when they are well-engineered, they can provide a humid and warm climate perfect for keeping plants cozy and happy, even in the dead of winter. However, if you live in a particularly frigid zone, or if you have especially finicky plants, you may need to employ the services of a heating element to boost the greenhouse’s power.

How to Know if Your Greenhouse Needs a Heater

Photosynthesis requires not only sunlight but also energy in the form of heat to work. So, while your plants may be fine most of the time without a heater, there may be signs that you need to give your buds a warm boost.

You Live in a Cooler Climate

This first sign is fairly obvious. If you live in Zones 1 through 6 (and even parts of Zone 7), the temperature during the winter will definitely kill your plants. As a rule of thumb, if you want to grow food throughout the winter, and it’s cold out, you need a heater. 

Please note this may only extend your growing season by six weeks on either end. Where you normally might be able to plant in May, you might instead start plants in the greenhouse in March. And where you might have had a final harvest in September, you could be able to keep things going until November. 

Beyond that, you will need to also employ a grow light because your daylight hours are too short to support plant life.

However, the addition of a heating element will serve to balance out temperature fluctuations between the day and night. Its secondary function, in early spring, will be to provide tender seedlings energy for growth.

Your Seedlings Keep Dying

That brings us to our second reason you may need a greenhouse heater: your seedlings aren’t surviving. A greenhouse on a temperate day may be well into the 100ºs during the day while it drops to freezing at night. If this sounds painful to you, imagine those little baby seedlings. 

Your greenhouse can use the sun’s energy alone to warm the greenhouse by 20 to 30ºF. At night, there is no sun to heat, so the greenhouse temperature will match the outside. However, winter vegetation germinates best between 55 and 70ºF, and warm-season plants will germinate best between 70 and 85ºF.

So, the best thing you can do is to start your seedlings in the house.

Once they are ready for the move from a seed pod to a starter pot, you can put them on a germination mat or set them up in the greenhouse near a heatsink. During the day, the excess heat will be drawn into the heatsink, and at night the radiant stored heat will leak out over your plant babies. We’ll talk more about that below.

It’s also worth noting that if you are starting bulb flowers for cutting, like dahlias, a germination mat can be employed beneath the bulbs to force growth quickly. And so, on that note, let’s move on to plant growth.

Your Plant Growth Has Slowed Significantly

Like we mentioned above, most plants grow best at around 70ºF. As the seasons change, you may notice that growth as dropped off or slowed down. Since you may need your vegetables to continue feeding your family, you need constant growth. How can you encourage this?

By installing a heat source of some sort, you can provide the heat energy plants need to photosynthesize, and therefore, grow. As we mentioned previously, you may need to also employ a grow light to supplement poor or lacking sunlight. 

Track lighting that combines UV bulbs with radiant heat bulbs could be a start to solving your issues. And that, of course, brings us to the variety of options for heating your greenhouse.

Common Types of Greenhouse Heating Systems

We’ve scoured a number of greenhouse gardening sources to tally up the best advice across the board. So, below are the most frequently noted heating systems. 

If your greenhouse is quite large or your winters are particularly harsh, you may need a couple of these options in tandem. However, for a small greenhouse in a mild winter, it shouldn’t take too much effort to warm it up.

Add a Layer of Insulation

Something as simple as lining the inside of your greenhouse with a layer of bubble wrap will provide additional insulation. The clear bubble wrap still lets light in. However, the bubbles of air hold heat very well. Remember to leave hinges and louver windows uncovered so they can be opened when you do need ventilation.

You can also insulate the foundation of your greenhouse with a Styrofoam board, in something called a “Swedish skirt:”

  • Lay the foam board horizontally from the base of the foundation, extending 4 feet out.
  • Dig the ground out so the foam slopes down away from the greenhouse. 
  • You’ll then cover it back up and let it do its job. 

This works by trapping the geothermal heat under the greenhouse where it can work toward warming the structure.

Take the additional steps of sealing up any cracks and caulking up joints. The better your greenhouse is sealed up, the more heat it will retain inside as day fades into night. 

Natural Heat Sources

This first section is dedicated to heat sources that require no electricity or power source. These options us the natural energy of the sun or earth to protect your greenhouse tenants.

Heat Sinks with Black Containers 

One easy way to heat a greenhouse is by setting up a heat sink. A heat sink, also known as thermal mass, works by storing thermal energy from the sun during the day and releasing that radiant energy at night. 

Many greenhouse enthusiasts employ 55-gallon water drums painted matte black. Some report lining up and stacking several barrels along the north wall for optimal energy absorption. Others place the barrels at each corner of the house.

A smaller-scale option is to take 1-gallon jugs from milk or tea, paint those black, and fill them with water. These smaller heat sinks are placed at the roots of 5-gallon potted plants to create personal heaters.

Compost Heating 

Hopefully, as a gardener, you’ve already begun composting your own food waste to save money on plant nutrition. All you have to do is move your compost pile into the greenhouse, or at least near to the exterior wall.

On its own, compost puts off nearly 100ºF temperatures. This is thanks to the chemical breakdown of the scraps in a high nitrogen environment. You can drive the heat of your compost pile up by including more grass clippings, fruit or vegetable scraps, or chicken manure. 

To efficiently make your compost heap into a heat source, you need to place it properly

  • One option is to dig a trench through the center of the greenhouse and fill it with the compost. Lay planks or pallets across to create a walkway while still allowing airflow to the pile. 
  • Another option is to use a black 55-gallon drum. This will provide radiant heat from the chemical reactions but will also serve as a mild heat sink.
  • Yet another way to use compost to heat your greenhouse is to place the compost heap near an exterior wall. Then, lay copper tubing through the pile and into the greenhouse around the plant beds. By subsequently running air or water (water is more efficient) through the tubing, it will collect the heat from the pile and circulate it through the greenhouse.

Geothermal Heat Pump 

While geothermal heat does draw energy from the earth, it will require a compressor and a pump to draw warmth up and through your greenhouse. You can install a solar panel to provide the energy needed to avoid tapping into your home’s electricity. 

This is also a better option if you haven’t constructed your greenhouse just yet. You will dig down into the earth where the temperature is stable, between 3 and 35 feet to get below the frost line. Then, you’ll run pipes along this depth and blow air through them. The warmer temperatures underground will heat the air, so it’ll blow warm air into the greenhouse.

Ground to Air Heat Transfer (GAHT)

Along the same lines is a GAHT system. It functions along the same principles of the black barrels in that it drives excess heat into the ground for storage where it will be recirculated into the greenhouse once the temperatures drop. This allows the system to not only keep the greenhouse warm but also cool it off to prevent superheating. 

This system also requires power to run a thermostat. The system will recognize if it is too hot inside and will suck that hot air into the ground where it will cool and vaporize. This has the added benefit of removing excess humidity from the air and moisturizing the soil.

As a heater, though, the thermostat will trigger a fan to suck the cool air of the greenhouse through the same pipe system in the ground to be warmed in the ground and blown back out to warm the plants.

Localized Power Heaters

Transitioning from natural energy to natural energy combined with electricity to other sources of powered heaters, let’s make a quick stop in the middle to talk about localized heating. This section is especially for a greenhouse where specific plants need a little more warmth than others. The first option is, of course, the black gallon jugs for thermal mass. But below are powered options.

Heat Lamps 

There are a couple of kinds of heating bulbs. Red or blue spectrum (link to Amazon) both work to provide radiant heat for your plants.

  • From a fair distance, aim the lights toward the section of plants that need warmth. 
  • They can get pretty hot though, so use the bulbs with a fan to provide circulation. 
  • You could also combine the lamps with black jugs of water to amplify the effect.

Heating Mats, aka Germination Mats

This type of localized heater is best for seedlings or bulbs that need starting. Some even come with a thermostat to optimize control like this one on Amazon. 

  • Place the tray of seedlings over the heating pad to stimulate growth. 
  • If you have several trays needing additional heat, place together with a heating pad.
  • Tent frost netting over them to create a mini-greenhouse within your greenhouse. 

Power Space Heaters

In the harshest of climates, a powered heater may be employed. You might consider an installed heating/cooling system if you have a very large greenhouse that you plan to operate year-round. However, most small scale gardeners should consider smaller heating elements.

Electric Space Heaters 

Many greenhouse owners make use of electric space heaters to quickly bring the temperature up. However, space heaters need to be used in addition to thermal mass. On its own, the heater only radiates heat into the air. Without heat storage, the energy is immediately lost. Use barrels or jugs of water to absorb the heat and continue warming the greenhouse.

Additionally, consider the safety of running your power source out to the greenhouse. You don’t want to trip the circuit or expose electricity to water. If possible, install a solar panel to run any electrical appliances in your greenhouse.

Propane Space Heaters 

Since they don’t require electricity, you might consider a propane space heater. These are fine in a pinch. Please note, though, that they have a lot of drawbacks. 

  • These get really hot, for one thing.
  • Give them plenty of space to prevent fires. 
  • Place the propane heater on level ground to prevent tipping
  • Note that heaters put off carbon monoxide, so you need to ventilate your greenhouse properly 

Overhead Mounted Radiant Heaters

These are designed for garages, workshops, and outbuildings. They are connected to a natural or propane gas supply line and controlled by a thermostat. This is what I use as the main heat source in my seedhouse. The one currently working for me is the Enerco Heatstar 25000 btu garage heater I purchased from Amazon. It was easy to install, doesn’t require a direct vent or electricity (for either the heater or included thermostat).

If you have the necessary supply line, this just might be the way to go – I’ve been quite pleased with mine. Just make sure you install it following the instructions regarding necessary clearances and in accordance with any municipal building codes. If you’re interested, you can read this article that goes into more details how I installed mine.

Wood Stoves 

A wood stove gives off the most heat and can be really cheap if you have your own source of wood to burn. Especially with the costs of electricity and gas, it’s ideal if you can fuel a heat source for free. 

One added benefit of a wood or pellet stove to heat the greenhouse is that you can control the level of heat by the size of the fire. Like other space heaters, you’ll need to consider fans or heat sinks to circulate and hold the heat. 

It does still need ventilation to minimize carbon monoxide. And you should also consider the material of your greenhouse roof. Glass greenhouses should not cause a problem. However, because the chimney on a wood stove is very hot, it’s best not to use a wood stove in a plastic pane greenhouse. You may accidentally melt a hole right through the roof.

Getting Started With a Greenhouse

If you’re wintering plants in a greenhouse, growing vegetables, or starting seedlings, there is a chance you may need additional heat. However, if you also live in a cool climate, for example, in Zones 1 through 7, you will definitely need to heat your greenhouse over the winter. Harsh climates with little sun should also employ grow lamps to provide adequate light and heat for their plants to grow. 

Milder Areas

Once you’ve determined your need for a heater in the greenhouse, take a look at your options. In milder areas may get by with a few gallon jugs of water, painted black, placed around the bases of your plants, and a layer of bubble wrap along the interior walls. A well-placed compost heap could round out an energy-efficient heating system. 

Areas With Extreme Cold and Heat

However, in areas that experience very hot summers and very cold summers, a geothermal system will both heat your greenhouse in the winter and cool in the winter. Whatever your situation, consider opportunities to make use of natural energy from the sun or the earth. These will not only reduce your costs but will continue to run even if the power goes out. 

Safety First

If you do choose to run a heater that requires power, be very careful with your sources. Electrical extension cords should be run safely. Check that there is no exposed wire or fraying, and do not overload the circuit. For propane-powered heaters, set them on a perfectly level surface with plenty of space to prevent fires. Have a ventilation fan or an open window to prevent carbon monoxide build-up, which could become toxic. 

And no matter what, enjoy the process of growing your favorite plants even when Mother Nature says you can’t.

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