Heating a greenhouse can extend the growing season, protect plants from cold weather, and allow you to grow plants that wouldn’t survive your climate. With the right heating system, you can turn your greenhouse into a productive, year-round growing space. Here are 8 effective ways to heat your greenhouse:

1. Use Passive Solar Heat

Passive solar heating takes advantage of sunlight to warm your greenhouse. Here are some tips:

Orient your greenhouse to maximize sun exposure. Face the long side of the greenhouse south, with the entrance on the north side.
Use materials like stone, concrete or water barrels to absorb and store heat from the sun. Position these “thermal mass” materials along the back wall of the greenhouse.
Insulate the north wall and use double-glazed glass to reduce heat loss.
Install vents for ventilation at the top to allow warm air to escape.
Whitewash the exterior during summer to reflect sunlight and reduce overheating.
Add a solar curtain to provide shade and insulation at night. Roll it up during the day.

Passive solar heat can raise temperatures 10-20°F on a sunny winter day. It’s simple, free and an easy way to supplement other heating systems.

2. Install Electric Heat Mats

Electric heat mats or cables provide safe, even heat directly to plants’ root zones. They come in various sizes to fit under benches or seed trays.

Benefits:

Energy efficient – only heat a small zone, not the entire airspace.
Prevent root damage by avoiding cold soil.
Flexible installation under benches, beds or in seedling areas.
Thermostats maintain optimal soil temperatures.
Safe for continual use.

Look for waterproof heat mats designed for greenhouses. Combine mats with insulation beneath and around them to hold heat in the soil.

3. Use Electric Space Heaters

Portable electric space heaters are an affordable way to heat a small greenhouse. Options include:

Fan-forced heaters – Best for evenly heating the air in a confined space.
Ceramic heaters – Provide infrared radiation to heat plants and surfaces.
Oil-filled radiator heaters – Release heat slowly and steadily.

Tips for using space heaters:

Select a model with automatic shut-off and tip-over protection.
Size appropriately for the space, usually requiring 10 watts per square foot.
Raise heater off the floor and point away from plants.
Use a thermostat to regulate temperature.
Vent to prevent buildup of fumes.
Use heavy-duty extension cords as needed.

Space heaters should only supplement other heating systems in greenhouses. They are not cost-effective for heating entire structures long-term.

4. Install Gas Unit Heaters

Gas-fired unit heaters are a common heating choice for larger greenhouses. They burn propane or natural gas to generate heat.

Key features:

Forced-air circulation warms the space quickly.
Permanent installation for whole greenhouse heating.
Thermostat regulates temperature.
Flexible sizing from 10,000 to 150,000 BTUs.
Chimney vented outside.

Maintenance tips:

Inspect regularly for leaks or malfunctions.
Keep area around heater clear.
Service annually.

Unit heaters provide powerful, consistent heat. But fuel costs may be higher than electric options. Consider combining with solar or geothermal heat sources.

5. Try Geothermal Heating

Geothermal systems use stable underground temperatures to heat greenhouses. A network of pipes called a “loop” is buried underground, circulating water or antifreeze through the pipes.

How it works:

The liquid absorbs heat from underground and carries it via the pipes into a heat exchanger.
The exchanger transfers heat into the greenhouse air circulation system.
In summer, the system runs in reverse to cool the greenhouse.

Benefits:

Extremely energy efficient – uses 25-50% less power than conventional systems.
Provides consistent, even heating year-round.
Long lifespan with minimal maintenance required.
No outdoor equipment is visible.

The underground loop installation does require more upfront investment. But geothermal heat offers great long-term payoff for large greenhouses.

6. Burn Wood for Radiant Heat

Wood stoves and heaters are a classic option for heating greenhouses through burning wood or pellets.

Types:

Wood stoves – Freestanding stove that heats via radiant energy and some convection.
Rocket mass heaters – Combustion chamber surrounded by thermal mass to radiate gentle warmth.
Wood-fired boiler – Heats water that circulates to radiators in the greenhouse.

Tips for operation:

Use well-seasoned wood or pellets for clean, efficient burns.
Install securely on a nonflammable base.
Keep proper clearance from combustible materials.
Install a chimney through the roof for ventilation.
Let unit burn hot twice a day for complete combustion.

Wood heat works well as a self-sufficient greenhouse heating method. But it requires frequent maintenance and monitoring for safety.

7. Harness Solar Thermal Energy

Solar thermal systems harness the sun’s heat directly to warm your greenhouse. Here’s how they work:

Solar collectors with glass panels trap heat from sunlight.
The collectors contain pipes with circulating fluid to absorb the heat.
The hot fluid flows into a storage tank.
When needed, a heat exchanger transfers warmth from the hot fluid into the greenhouse air or water system.

Advantages:

Free renewable energy from the sun.
Significantly lower heating bills.
Pairs well with radiant floor heat.
Long lifespan, low maintenance.

Solar thermal power takes sunlight’s raw heat and puts it to use. It provides 50-80% of a greenhouse’s heating needs in many climates.

8. Insulate and Use Thermal Curtains

Improving insulation and using thermal curtains are inexpensive ways to boost greenhouse heat retention.

Insulation tips:

Add insulation inside north wall and under floor.
Use double-glazed or double polycarbonate panels.
Caulk and weatherstrip all cracks and gaps.
Add an insulated greenhouse door.

Using thermal curtains:

Use curtains with aluminized fabric that reflects heat back inside.
Install automated curtains to easily close at night.
Curtain across roof peak allows warm air to pool in growing space.
Can reduce heat loss by up to 20%.

Proper insulation and thermal curtains work hand-in-hand with heating systems to maximize efficiency. They pay for themselves quickly in heating cost savings.

Combining Heating Methods

The most effective approach for a productive greenhouse is combining multiple heating techniques:

Passive solar – Provides free daytime warmth.
Insulation – Holds in heat at night.
Active system – Heats on sunless days and during cold seasons. This could be gas, electric, geothermal or wood-fired.
Supplemental heat – Mat heaters for seedlings or other critical zones.

Layering these methods provides reliable year-round warmth tailored to your climate and greenhouse size. Heating your greenhouse doesn’t have to be costly if you use the right combination of techniques.

Frequently Asked Questions

How cold can my greenhouse get at night?

Ideally, the temperature inside your greenhouse should not drop below 45°F at night during winter months. Extended cold below this can damage tender plants. Heating systems should maintain 45-60°F minimums for most overwintering plants.

What size heater do I need?

Choose a heating system sized for your greenhouse’s square footage. A common guideline is to have 10 watts of heating power per square foot of area for hobby greenhouses. Larger commercial greenhouses often need around 5 watts per square foot.

How much does it cost to heat a greenhouse?

Heating costs vary widely based on climate, size, fuel source, and efficiency. Electric heat can range from $50-$100+ per month. Gas ranges from $150-$600+ monthly. Improving insulation can reduce costs significantly. Combining passive solar, geothermal or wood heat helps lower fossil fuel usage.

Should I only run my greenhouse heater at night?

Nighttime heating is crucial, but don’t switch off the heat completely in the day. The temperature difference between inside and outside should not exceed 10-15°F. Keep daytime temperatures at the minimum needed for your plants, and reduce to optimal night minimums.

Can I reuse heat from my home to warm my greenhouse?

You can capture waste heat from a furnace, water heater, or chimney flue to transfer into your greenhouse. Run indoor ventilation ducts into an isolated greenhouse crawlspace. Or use a heat exchanger on flues to heat water in hydronic radiators. This “cogeneration” reuse is an effective heating solution.

Conclusion

Heating allows greenhouses to stay productive year-round. A combination of passive solar gain, insulation, and active heating systems works best for controlling microclimates. Carefully weigh costs, efficiency, and maintenance when selecting the right heating approach. With a well-planned heating strategy, your greenhouse can thrive no matter how cold it gets outside!

How to Heat a Greenhouse: 8 Ways