Heating Your Greenhouse in Winter: What Are the Options?

November 27, 2024

Customers frequently ask about heating a commercial greenhouse to start spring sales plants in January or February. While some plants may survive in an unheated greenhouse, they may sit dormant for lack of heat and light. However, heating a greenhouse isn’t necessarily worth the cost for every garden center and it may be unnecessary in your climate or if your main goal is to overwinter perennials.

Let’s go over:

What it will cost to heat a commercial greenhouse
Factors to consider when deciding whether heating the greenhouse is worthwhile for your business
Alternatives to heating a greenhouse
Heating methods: passive solar, electric, gas
Grow lights

What Will Heating a Greenhouse Cost?

The cost of heating a greenhouse depends on several factors, including its size, the type of covering material used, the quality of insulation, outdoor temperatures and windspeed, and cloud cover.

Calculate the average cost of heating your greenhouse by first gathering the following information:

Lowest outdoor air temperature (night and day)
Target greenhouse air temperature (night and day)
Greenhouse surface area
U-value of the material covering the greenhouse

To maintain a target growing temperature of 65℉ with an outdoor temperature of 30℉ in an A-frame greenhouse with polycarbonate walls, use this equation:

BTU/hr= U (material heat transfer) x A (surface area) x 𝝙T (indoor/outdoor temperature difference)

Calculate your greenhouse’s surface area with these equations:

AFrame=2(width x height)+2 (length x height)+2(width of roof x length)+(width x height of roof)

Hoop House= 𝝅/2(width x length) +𝝅(width24)

U-values for various surface materials are available in this table. Double polycarbonate or double plastic film is more effective at insulating than single glass, plastic film, polycarbonate, or fiberglass. A lower U-value indicates better insulation. The U-value for single plastic film is 1.25, versus 0.8 for double plastic film or 0.6 for double acrylic or polycarbonate.

So, for an A-frame greenhouse with a surface area of 2,375 (15 feet wide x 50 feet long), covered with double plastic film with an outdoor temperature of 30℉ and a target temperature of 65℉, the equation to calculate BTU/hr will look like this:

BTU/hr= 0.8 x 2,375 x 35 =66,500 BTU/hr

One gallon of propane is about 91,500 BTUs per gallon. Heating the greenhouse in the above example will use about a gallon of propane per 1.5 hours, or 16 gallons a day. This example doesn’t account for passive heat storage or temperature fluctuations since you should prepare to heat your greenhouse without relying on variable factors. Always calculate for unusually low outdoor temperatures since a bit more capability in your heater is better than insufficient capacity.

Is Heating Your Greenhouse Worthwhile?

Before investing in insulation, improved greenhouse materials, or a heating system, consider whether you will sell enough plants from winter production to make heating your greenhouse worth the money. Apart from fuel costs, you’ll need to invest in a heating system to produce actively growing plants in winter and possibly even grow lights depending on your location. In climates with mild winters, solar heat may be sufficient to protect plants in well-insulated greenhouses.

Consider how many plants you will produce and sell and what type of plants you’re trying to grow. If your main focus is overwintering perennials, cheaper alternatives to greenhouse heating are worth considering. If you want to have annuals and vegetables as early as possible, you will likely need some heat to get those ready early.

Alternatives to Heating Your Greenhouse

Cold frames are a cheaper way to protect cold-tolerant plants.

Thermal Blankets can protect container plants from below-freezing temperatures outdoors in the winter. Options include Microfoam polypropylene, polyethylene foam with UV-resistant P/E, polyethylene foam attached to UV-treated polyethylene film, and fleece materials. This inexpensive means of protecting plants in or outside the greenhouse may work well for businesses with mild winters and few freezing days or nights.

Deep Winter Greenhouses are specifically constructed with a particularly angled south-facing wall prepared to collect maximal solar heat and solid, insulated remaining walls to hold the heat in. This type of greenhouse is technical, only works for low-light plants, and isn’t suitable for summer production. For most commercial growers, a DWG isn’t the right solution. Growers focused on sustainable practices or edible crops may want to consider this investment.

Heating Options

While choosing a heating system, consider how your greenhouse holds heat and how outdoor temperatures, wind, and cloud cover will influence the greenhouse temperature. Begin by preparing the space to hold heat efficiently.

Use greenhouse covering material with a low U-value, such as double polycarbonate, double acrylic, glass with thermal blankets, or double plastic film. Materials with UV inhibitors last much longer.

Insulate your greenhouse carefully and thoroughly, caulking or weather-stripping gaps, doors, and vents. Rigid foam insulation for temporary overnight use could be a cost-effective way to retain heat, although it may be too labor-intensive.

Consider passive heat storage, such as water tanks or rock beds, which store heat captured during the day and release it at night. Passive heat storage is probably most interesting to anyone looking for sustainable or alternate fuel systems and plays a role in Deep Winter Greenhouse practices.

Gas hot air heaters may use natural gas or propane, come in many sizes, and have options for wall-mounting or freestanding units. They are relatively low-maintenance and cost anywhere from a few hundred to a few thousand dollars, depending on the size, BTU output, efficiency rating, and style of the unit.

Hot water heaters or radiant heat boilers can provide uniform temperature control in the greenhouse. Most midsized and larger commercial greenhouses are utilizing hydronic heating for their greenhouses. These systems are typically the most expensive to install but offer the most automation and efficiency.

Electric heaters are not cost-effective in larger greenhouses but are often found in smaller hobby houses due to their low input cost. Infrared heaters are potentially more efficient than electric hot air units.

Wood heat is an option as a central or supplemental heat source in greenhouses. However, it requires frequent attention and creates a less controllable heat-regulation scenario, especially in small greenhouses. Oftentimes, the labor costs of wood heat will offset the fuel savings of gas. Many growers are finding it cheaper to run gas boilers instead of wood simply because of the high cost of labor.

Pellet stoves can be expensive but are an option, especially if pellet availability is better than other fuel sources in your area.

Kerosene heaters (along with oil and diesel) are only recommended in adequately ventilated areas. Sufficiently maintained, portable kerosene heaters are excellent for emergency use.

Choosing a high-efficiency heater may cost more upfront but save money in the long run. Heat the space with at least two separate heating units so you’re not left entirely in the cold if a unit needs to be repaired in the dead of winter. Choose heaters that can provide slightly more heat than you strictly need. Planning for the unexpected could be the difference between saving and losing your crop.

Grow Lights

Since winter brings shorter days in addition to colder temperatures, actively growing plants in your heated greenhouse may also require supplemental lighting. Grow lights are often also used to extend day length to help perennials initiate or put off flowering. While home growers often use fluorescent lighting for germination, high-intensity discharge lamps, and LEDs are most common and effective in commercial greenhouse use.

High-pressure sodium lamps are an efficient option, available in various wattage levels from 400-1,000. Metal halide lamps provide a broader light spectrum and may be superior for crops that depend heavily on supplemental light.

LEDs (Light-Emitting Diodes) are more efficient and long-lasting than other lighting sources.

To decide how many fixtures you’ll need, consider how much additional light your crops need to grow and how to provide uniform light across the bench at an appropriate intensity.