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A greenhouse allows gardeners to grow vegetables year-round, keep tropical plants in bloom, or start seedlings early in the growing season. But not all greenhouse owners know how to keep a greenhouse warm in winter. The matter of how to heat a greenhouse depends on climate, the greenhouse’s size, and what’s growing inside.
A winter greenhouse creates a microclimate with sufficient sunlight, temperature, and humidity to successfully cultivate plants through all seasons. Most crops thrive in temperatures between 64 and 75 degrees Fahrenheit, although many prefer a warmer environment of 80 degrees. The optimal daytime humidity level for most greenhouse plants is around 80 percent. In cold regions, the following greenhouse heating methods can help produce these optimal conditions for growing or overwintering plants. Choosing the right kind of greenhouse insulation can help keep plants warm while reducing heating costs.
1. Solar-Powered Heating
A solar heater for greenhouse usage can be difficult to find and expensive to operate because of excessive energy demand. It would take 15 100-watt solar panels to generate enough energy for a 1,500-watt heater—so many that they could prevent direct sunlight from reaching the greenhouse, which also might not be strong enough to support the weight of all those panels. This would require placing solar panels on the ground or using existing panels.
Aside from the greenhouse heater itself, a full solar-powered package would include photovoltaic (PV) solar panels, batteries, a solar charger, and an inverter. Heating costs depend on the size of the greenhouse, the amount of sun the location receives, and the overnight temperatures.
Solar panels can last 20 to 25 years (or more) and typically require little to no maintenance, other than routine periodic cleaning, unless damaged by hail, wind, or lightning.
Best For: Solar power is an extremely pricey greenhouse heating system that works best in milder climates that receive a lot of sunshine.Cost: A solar-powered heating setup can run as much as $12,000 for enough equipment (panels, battery, inverter) to provide enough power to heat a 150-square-foot space. Installation costs can run $7 to $9 per watt.
2. Passive Solar Heating
Another way to heat a greenhouse in winter using the sun’s power is passive solar heating: Collecting heat from the sun during the day and releasing it into the greenhouse at night—the basic concept behind greenhouse design.
If you’re building a new greenhouse, here are some effective greenhouse ideas for capturing and storing heat:
- Sinking the floor below the frostline
- Adding thermal mass, such as black 55-gallon water tanks, along the north wall (plan on 2.5 to 5 gallons per square foot and stack them in direct sun)
- Adding extra insulation on the north side
- Painting the south interior wall with white or reflective paint to provide more sun to plants, and painting other walls black to absorb warmth
- Adding a second layer of polycarbonate, or even bubble wrap, to hold warmth inside
- Creating a hot bed or composting pile to generate heat
- Adding a concrete or brick floor to capture heat
Best For: While passive solar heating carries no energy cost, it won’t be sufficient to keep a greenhouse warm enough in winter when there’s snow or extreme temperatures.
Cost: Costs can vary widely, depending on the materials and methods that are chosen.
Our Recommendation: Black 55-gallon drums at Uline, priced from $95. These dent-resistant, corrosion-free barrels can be used indoors or out.
3. Geothermal Heating
A geothermal greenhouse incorporates piping buried at least 6 to 12 feet below the soil. Air or liquid travels through the piping, capturing heat from the ground and carrying it to a heat exchanger that extracts the heat. Closed-loop systems typically use propylene glycol, methyl alcohol, or antifreeze. Open-loop systems use water pumped from a pond, lake, or well. Pipes can also be coiled beneath a compost pile.
A heat pump is required to maintain the high temperatures needed in a year-round greenhouse. Geothermal heating can be an affordable way to heat a greenhouse; the payback period can take just 3 to 7 years.
Best For: Geothermal systems are effective in cold climates. Horizontal-loop systems require a large expanse of land, and vertical-loop systems require extensive digging, which can add to the cost of geothermal heating.
Cost: Geothermal systems typically cost $5,000 to $9,000 per ton (12,000 BTUs/hour). The average price for a 4-ton system (enough to heat a 2,000 square-foot house) ranges from $15,000 to $20,000. Installation runs $8,000 to $24,000, ductwork another $2,000 to $6,000, and yardwork $5,000 to $15,000. Permits can add $100 to $1,000. A heat pump costs $4,000 to $7,000, and an air handler to move air around the space runs about $2,000 to $3,000.
4. Electric Forced-Air Heating
Perhaps the safest, easiest greenhouse heaters to use are electric heaters, as long as the greenhouse has a power supply. These heaters require no ventilation, but users do sacrifice cost for simplicity, as some of the heat generated by that electricity will be lost through the greenhouse glass, largely through conduction. Also, fans are recommended to move the warm air around the space, which can add to the expense.
Many forced-air electric heaters are affordable and fairly inexpensive to operate, depending on the climate, but they may be best for a smaller greenhouse.
Best For: This easy, safe option is best for smaller greenhouses and areas with occasional cold snaps. Otherwise, the energy costs can become exorbitant.
Cost: Many forced-air heaters can be found for under $100, but the cost increases along with the square footage of the greenhouse they’re expected to heat.
Our Recommendation: Palma greenhouse heater by Bio Green at Amazon for $98. This portable, energy-efficient unit heats up to 120 square feet quickly and offers protection from overheating.
5. Infrared Heaters
Radiant energy from infrared heaters powered by electricity is another option for greenhouses, particularly when growing bedding plants. Infrared rays transfer heat to objects in the space, such as plants and benches, through radiation in a manner similar to sunlight. They distribute heat evenly, without drafts like those produced by forced-air and convection heating.
These systems can be more economical because they heat plants, not the surrounding air. But since the air isn’t heated, greenhouses with infrared heaters often feel cold. As well, they may not maintain the required nighttime temperatures for tender plants. For large greenhouses, multiple heaters should be staggered to provide even heat.
Best For: Infrared heaters are an economical option that works best with low-growing or bedding plants, particularly in small greenhouses.
Cost: Infrared heaters start at just under $100 but can reach thousands of dollars for large greenhouses.
Our Recommendation: Dr. Infrared Heater at Amazon for $105. It’s lightweight and portable, with built-in overheat protection, a precision temperature sensor, and an adjustable thermostat.
6. Oil-Filled Heaters
Electric-powered oil-filled radiator-style heaters heat greenhouse air by convection. The oil sealed in the unit is heated, and then that heat is transferred to the air. They don’t put out a lot of heat, but they do produce a steady flow that can be regulated by a thermostat. They are safe to use in a greenhouse, but they can take longer to heat up than infrared or forced-air heaters.
Their advantages include a lower operating cost than typical forced-air electric heaters, which makes them a good choice for greenhouses in cold climates that need heat overnight and on cloudy days. Additionally, oil-filled heaters don’t dry out the air like forced-air heaters do, and because they don’t have a negative impact on humidity, they can help stabilize the temperature and maintain the recommended controlled environment.
Best For: Oil-filled heaters are an economical source of constant radiant heat that doesn’t dry out the air.
Cost: Expect to pay between $100 and several hundred dollars, depending on the size of the unit needed to provide sufficient heat.
Our Recommendation: Dreo Oil-Filled Radiator Heater at Amazon for $135. Three heat settings, plenty of safety features, a 24-hour timer, and quiet operation make this an efficient and appealing choice.
7. Kerosene Heater
For greenhouses without electricity, kerosene heaters are a practical option. As a bonus, it usually costs less to heat with kerosene than electricity, which is a particularly important consideration in climates where the heater has to run continuously to keep overnight temperatures high enough for plants.
Although they can generate a lot of heat, the temperature will diminish with distance from the heater. It’s best to place the heater in the center of the space for even heat distribution. Note that the heater can melt polycarbonate or PVC sheeting on the greenhouse if placed too close. Also, don’t place kerosene heaters too close to plants and keep them safely away from combustible items, although some units are equipped with safety devices that shut off the heater if it’s tipped over or overheats. Because kerosene heaters release carbon monoxide and other toxic fumes, outside ventilation is necessary.
Best For: Kerosene heaters are an option for greenhouses without electrical power, but exercise caution, ensure proper ventilation, and keep the heater away from surfaces that could melt or catch fire.
Cost: The price of kerosene heaters varies according to size, but they generally start around $200.
Our Recommendation: Dyna-Glo Indoor Kerosene Convection Heater at Northern Tool for $189.99. Sized to heat a 1,000-square-foot space with 99.9 percent efficiency, this unit provides 8 to 12 hours of warmth on one 1.9-gallon tank. Automatic pushbutton ignition and one-touch extinguisher make it simple to use, while its tip-over shutoff and cool-top design enhance safety.
8. Propane Heaters
The affordability of propane heaters and their capacity to provide consistent heat with low emissions make them a popular option for greenhouses. However, they may not be safe for glasshouses, conservatories, or other indoor spaces unless ventilated. Unless they are vent-free units, propane heaters must have proper ventilation. Left unvented, they may exhibit inefficient combustion, which results in emissions being exhausted into the greenhouse, leading to risk for plants because of pollutants and moisture.
Whether the unit is installed or portable, a propane heater should be turned off when no one is in the space. They must be placed on a level surface away from combustible materials. It’s crucial that they are correctly sized for the space and offer features such as a low oxygen sensor, overheat protection, and automatic shutoff.
Best For: Propane heaters provide consistent heat, but should be used in small spaces, preferably only when someone is present. They should be well-ventilated or vent-free.
Cost: Propane heaters can easily range from a couple of hundred dollars to thousands, based on BTUs. To determine how many BTUs are needed, it’s necessary to calculate the square footage and the insulation type of a greenhouse.
Our Recommendation: Mr. Heater at Amazon for $208. This vent-free, freestanding, or wall-mounted propane heater is suitable for indoor use. An automatic low-oxygen shutoff system keeps everyone safe, while the thermostat for automatic temperature control keeps everyone comfortably warm.
FAQ
Q. What is the best method for heating a greenhouse?
Electric heating is the safest way to heat a greenhouse. Adding fans evenly distributes the heat. Gas heating is the least expensive option, but requires venting.
Q. How can I heat my greenhouse in winter without electricity?
Without electricity, it’s best to make the most of the sun’s energy. If you’re building new, start by sinking the greenhouse into the ground. If you’re working with an existing greenhouse, insulate heavily (seal up cracks, add bubble wrap), position thermal mass objects (large water barrels, brick or stone planters) to absorb and release heat, make compost in the greenhouse, and use light paint to reflect and dark paint to absorb heat.
Q. What can I grow in an unheated greenhouse over the winter?
Root crops such as carrots, beets, radishes, and turnips can survive freezing temperatures. Onions, garlic, leeks, parsnips, and rutabagas may also survive in an unheated greenhouse.
Q. What temperature should a greenhouse be in winter?
Optimal temperatures are generally 65 to 70 degrees Fahrenheit during the day, and no lower than about 45 degrees at night, but it depends on the plants that are growing in the greenhouse.
Q. Should I open my plastic greenhouse during the day?
If temperatures warrant it, opening doors and vents on sunny days allows fresh air to circulate. Just remember to close them at night.
