Most people assume heating a greenhouse means running a big electric heater all winter and watching their power bill climb. That’s one way to do it, but it’s not the only way, and it’s usually not the smartest way.
I’ve kept my greenhouse above freezing through Zone 5 winters using methods that cost between nothing and about $60. Here’s everything I’ve tried, ranked from free to cheap.
First: Do You Even Need Supplemental Heat?
This is the question nobody asks first, and it should be your starting point.
What are you growing? Hardy greens like spinach, kale, lettuce, and Swiss chard survive down to 25 to 28 degrees Fahrenheit. They don’t need a heated greenhouse. They just need protection from wind and heavy frost.
Tomatoes, peppers, and basil die below 32 degrees. Tropical plants need 50+ degrees at all times.
If you’re growing cold-hardy crops, insulation alone might be enough. If you’re overwintering tender plants or starting seedlings in February, you’ll need some heat source.
Here’s a rough guide to minimum temperatures:
- Hardy greens (kale, spinach): 25 to 28 degrees F
- Cool-season crops (lettuce, peas): 28 to 32 degrees F
- Warm-season starts (tomatoes, peppers): 45 to 50 degrees F
- Tropical plants (citrus, hibiscus): 50 to 55 degrees F
Know your target temperature before you spend money on heating.
Free Methods
Thermal Mass (Water Jugs)
This is the oldest trick in greenhouse growing, and it works. Fill dark-colored containers with water and place them where they’ll catch direct sun during the day. The water absorbs heat while the sun is shining and slowly releases it after dark.
Paint plastic jugs, bottles, or drums flat black. Black absorbs the most solar energy. Line them along the north wall of your greenhouse (where they won’t shade your plants) or tuck them under benches.
How much thermal mass do you need? A rough rule: 2 to 3 gallons of water per square foot of greenhouse floor. For a 6x8 greenhouse (48 square feet), that’s about 100 to 150 gallons of water. That sounds like a lot, but ten 5-gallon buckets gets you to 50 gallons, and you can fit those under a bench easily.
Thermal mass won’t keep a greenhouse warm through a weeklong cold snap with no sun. But on a typical winter day where it’s 50 degrees and sunny, then drops to 25 at night, the water releases enough stored heat to keep the air 5 to 10 degrees warmer than outside. That’s often the difference between frost and no frost.
Compost Heat
A pile of actively decomposing compost generates heat. The center of a hot compost pile reaches 130 to 160 degrees Fahrenheit. You can use this to warm a small greenhouse.
There are two approaches. The simple one: put a compost bin inside the greenhouse. A 3x3 foot bin of actively turning compost will raise the ambient temperature a few degrees.
The more effective approach: build an insulated compost box and run water tubes through the pile, circulating the warm water to heat the greenhouse floor or a growing bench. This is a real project (see “compost heating system” for detailed builds), but it works and produces both heat and finished compost.
The catch: compost heat requires active management. You need to turn the pile, maintain the right moisture and carbon-to-nitrogen ratio, and add fresh material every few weeks. If the pile goes cold, your heat source is gone.
Bubble Wrap Insulation
This one’s dead simple. Line the inside walls and ceiling of your greenhouse with large-bubble bubble wrap. You want the stuff with 1-inch bubbles, not the small packing material kind. Greenhouse-grade bubble wrap with UV stabilizers is best, but regular bubble wrap from a moving supply store works for a season or two.
Bubble wrap adds an insulating air layer that reduces heat loss significantly. Some studies show it cuts heat loss by 30 to 50 percent. The trade-off is slightly reduced light transmission, but it’s minimal with large-bubble wrap.
Attach it with clips to the frame (don’t tape it, because you’ll want to remove it in spring). Focus on the north wall and the roof first, since those lose the most heat.
This costs nothing if you save packing materials, or about $20 to $30 for a roll of greenhouse-grade insulation.
Cheap Methods ($20 to $100)
Electric Space Heaters with Thermostat
If you need reliable, set-it-and-forget-it heat, a small electric space heater with a built-in thermostat is the easiest option. Get a ceramic heater rated for 500 to 1,500 watts. Set the thermostat to your target minimum (usually 35 to 40 degrees to stay above freezing with a margin of safety).
A good ceramic heater costs $30 to $60. Running it costs about $0.50 to $1.50 per night, depending on your electricity rate, outside temperature, and how well your greenhouse is insulated.
For a 6x8 greenhouse in Zone 5 to 6, expect to spend $20 to $40 per month on electricity during the coldest months if you’re keeping it above freezing. If you’re trying to keep it at 50+ degrees for tropicals, double that.
The critical detail: use a heater with a thermostat, not one that just runs at full blast. You want it to cycle on and off as needed. Running a heater continuously wastes electricity and can overheat the space during sunny daytime hours.
Also, use a GFCI-protected outlet or extension cord. Water and electricity are both present in a greenhouse, and a GFCI is a basic safety requirement.
Heat Mats for Seedlings
If you’re not heating the whole greenhouse, just keeping seedlings warm, heat mats are the most efficient option. A 10x20-inch seedling heat mat costs $20 to $40 and uses about 17 watts. That’s pennies per day.
Heat mats warm the soil to about 10 to 20 degrees above ambient air temperature. Put your seed trays on the mat, and the seeds get the soil warmth they need to germinate even when the air is cool.
This is perfect for starting tomatoes, peppers, and other warm-season crops in an otherwise unheated greenhouse. The air can be 45 degrees, but the soil in the trays stays at 60 to 65 degrees, right in the germination sweet spot.
Get a thermostat controller with your heat mat. Without one, the mat just runs at full power constantly. With a thermostat, you set the target soil temperature and the mat cycles on and off. A basic thermostat controller is $15 to $25.
Paraffin (Kerosene) Heaters
These are popular in the UK and getting more common in the US. A small paraffin greenhouse heater costs $30 to $50, burns about $1 of fuel per night, and produces enough heat for a 6x8 greenhouse.
The advantages: no electricity needed, so they work in greenhouses without power. They also produce CO2 as a byproduct, which plants love.
The disadvantages: they produce moisture too (burning fuel releases water vapor), which can increase humidity and disease pressure. They need refueling. And you’re dealing with an open flame in a small enclosed space.
I used one for two winters. It worked fine. I switched to electric because I didn’t want to bother with fuel. If your greenhouse doesn’t have an electrical outlet, a paraffin heater is a reasonable solution.
The Insulation-First Approach
Here’s the principle that saves the most money: reduce heat loss before adding heat sources.
Every dollar you spend on insulation reduces how much you need to spend on heating. A well-insulated greenhouse with no heater stays warmer than a poorly insulated greenhouse with a heater running.
Priority order for insulation:
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Seal gaps. Check every joint, panel connection, door seal, and vent. Greenhouse kits are notoriously leaky. Use weatherstripping foam tape on doors and vents. Seal panel joints with aluminum tape or silicone.
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Insulate the north wall. The north wall never gets direct sun. Cover it with rigid foam insulation board (1 to 2 inches thick) on the inside. This wall is pure heat loss, so insulating it costs you nothing in light and saves significant heat.
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Add bubble wrap. Cover the remaining walls and ceiling.
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Insulate the floor perimeter. Bury 2-inch rigid foam board around the outside perimeter of the greenhouse, 12 to 18 inches deep. This stops cold from the ground from wicking into the greenhouse through the foundation.
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Add a thermal curtain. A sheet of greenhouse plastic or a thermal blanket hung horizontally below the roof peak reduces the volume of air you need to heat. Lower volume means less heat needed. Roll it up during the day, drop it at night.
After doing all of this, you might find you don’t need a heater at all for keeping hardy crops alive. Or that a small heater runs much less often, saving you electricity month after month.
Monitoring Temperature
You need to know what’s happening in your greenhouse when you’re not there. A cheap wireless thermometer with an indoor/outdoor sensor costs $10 to $20 and sends temperature readings to a display in your house.
Better options include WiFi-enabled sensors (about $30 to $50) that send alerts to your phone when the temperature drops below a threshold. The Govee and SensorPush brands are popular and reliable.
My Recommended Setup for Most People
For a 6x8 greenhouse in Zones 5 to 7, keeping above freezing through winter:
- Bubble wrap insulation on north wall and ceiling ($20)
- Seal all gaps with weatherstripping and foam tape ($10)
- Five or six black 5-gallon buckets of water along the north wall (free)
- One ceramic space heater with thermostat as backup for the coldest nights ($40)
- Wireless thermometer to monitor ($15)
Total cost: about $85. Monthly electricity for the heater: $10 to $30. That’s enough to overwinter most perennials, keep hardy greens growing, and start seedlings in late winter.