How to Heat a Greenhouse in the Winter

Are you a gardener who always feels reluctant to put away your trowel and gloves for the wintertime? Do the chilly days and cold nights of autumn find you daydreaming about what you’ll plant next spring? If this sounds familiar, it may be time for you to consider the possibility of setting up your own greenhouse. It doesn’t have to be an expensive project, and you can’t put a price tag on the satisfaction you’ll feel when you bring fresh vegetables and flowers to your table in freezing weather. Let’s take a look at how easy and affordable it is to enter the world of year-round gardening.

Each climate zone has native plants that are adapted to the local day-length and air temperature ranges. Greenhouses are a way of creating a micro-climate that’s different from the outdoors, so that a gardener who lives in a place with cold winters can grow delicate plants that are native to a warmer region. When you have a greenhouse, it’s like importing a tiny bit of the tropics into your yard. Once you understand the principles of heating greenhouses and managing their lighting and air flow, your gardening possibilities are virtually endless.

Basically, a greenhouse is an enclosed environment that’s at least partially constructed of translucent material. Plastic or glass greenhouse walls allow the sun’s electromagnetic rays to enter the structure and raise the air temperature, while keeping this warm air from escaping. Ever since translucent materials were invented, people have been using them to aid in growing tender plants.

The concept of heating greenhouses was first recorded in Korea in the 1400’s, as people in that cold country realized that they could add to the sun’s heat and open up more growing possibilities. Throughout the centuries the understanding of winter greenhouse technology improved, and farmers were able to precisely control the temperature, humidity, and chemical composition of the greenhouse atmosphere. Greenhouse heating systems eventually became automated, with digital controls and carefully regulated air circulation, and today’s global agricultural industry relies heavily on them.

 Although the word “greenhouse” usually refers to a structure you can walk into, the basic technology applies to cold frames and hoop houses. The most casual gardener may put row covers over a set of new lettuce transplants, if a springtime cold snap is predicted. Cold frames are essentially miniature greenhouses created in a single planting bed, by covering it with a glass or plastic roof.

The food supply in northern countries benefits greatly from greenhouse technology, as fresh vegetables and flowers from large greenhouse complexes are flown around the world. For a home gardener, having access to a greenhouse in winter provides two basic benefits:

• It extends the growing season for native plants. If you live in the Pacific Northwest, for example, the short cool summers can make it difficult to ripen home-grown tomatoes. A greenhouse allows you to start seedlings while there’s still frost outside. After that, it will protect them from becoming mildewy during rainy summer days, and enable you to keep on picking ripe red tomatoes long after October’s icy weather has blackened the outdoor vines.

• A greenhouse lets you grow a wider variety of the world’s plants. For northern vegetable gardeners, it’s always a challenge to ripen warm-weather crops like melons and eggplant. Most citrus trees are out of the question in a region with cold winters, while they can thrive in containers in your greenhouse. And we haven’t even mentioned the flowers that you’ll be enjoying all winter long! If you install a greenhouse heating system and some grow lights, you’ll be able to expand your floral repertoire to produce summer flowers anytime you want. Everyone with a special botanical interest, whether you’re an orchid fancier or an herbalist, will benefit from the expanded opportunities offered by a greenhouse.

Almost every gardener has suffered the disappointment of having tender plants ruined by extreme precipitation. After a heavy rain, your tulips may be bent to the ground, and a layer of snow can destroy an entire garden full of fragile spring flowers. With a greenhouse, you’re in charge of the environment. You can add an electric heater, or just work with solar heating and various natural heat sources to create the exact growing conditions that you want.

If you want to start out small with year round gardening, the good news is that you have lots of easy options for greenhouse heating. The smallest types of greenhouses are simply individual beds that you have turned into cold frames in order to protect a few delicate plants. With this ultra simple set-up, some plastic sheeting or glass windows resting on top of the bed will nicely protect your plant babies from a cold winter. It’s a good idea to construct these so that the roof or row cover can be lifted up if the sun gets too warm.

You can keep your small greenhouse warm by positioning it so that its north wall is against the south side of your home. That way, the greenhouse forms a south facing extension of your home, and it also creates a heat sink for your home, helping you keep your heating bills low. (A heat sink simply means something that absorbs heat when it’s available, and then slowly releases it over time.)

 Solar energy, direct from the sun passing through the walls,  is the first and most important heat source for a plastic or glass greenhouse. The electromagnetic radiation of the sun will heat all the objects inside the greenhouse, including any large thermal mass that you may have put there. Large 55 gallon barrels filled with water are one example of such a thermal mass that many gardeners use. Another way to maximize the warm air that you can capture inside your greenhouse during a cold winter is by using bubble wrap to insulate the translucent walls of your greenhouse. You probably have bundles of this common, inexpensive material around your house from mail order parcels, and it’s perfect as greenhouse insulation.

Keeping your small greenhouse warm isn’t expensive, and an electric heater with a thermostat or automatic timer may only be needed for a few hours during cold nights. The simplicity of this type of heating system makes it easy for gardeners who have to be away during the workdays. You can simply set a thermostat on a small space heater and walk away, knowing that it will only turn on when it’s really needed. Furthermore, today’s heaters come with reliable safety features, including automatic shut-off functions if they tip over or get too hot. Taking advantage of new digital greenhouse heater technology means that you’re approaching your gardening project with the savvy of a real farmer.

When we think about the options for heating greenhouses, it’s good to start with the free ones. Once you’re taking advantage of all the sustainable ways to keep warm air in your greenhouse, you can add an electric heater if needed as a final occasional option.

As we mentioned above, the best way to capture solar energy is to make sure the south facing side of your greenhouse is in direct sunlight.  In the northern hemisphere, the winter sun is always in the southern part of the sky.

Once you’ve positioned your backyard greenhouse to capture all that free winter sunlight (when it’s available), the next step is to find sustainable ways of holding onto the heat you’ve gathered. A thermal mass is some large object that is able to absorb and hold a large amount of heat. One popular way that gardeners prevent abrupt temperature drops after sunset is by putting several 55 gallon drums of water in their greenhouse. Another way is to build the north wall of the greenhouse (which does not usually get direct sunlight) out of a good insulating material like cinder blocks. These blocks will absorb warmth from the hot air in the greenhouse during the day, and then slowly radiate it through the night. Heat sinks such as large volumes of water, stone, cement, and so on are good ways to minimize the heat loss on cold nights.

The next step to efficient heating is to make sure you’re not losing any warmth through air leaks. Although a certain amount of air circulation inside a greenhouse is needed, you want to control the air flow through a series of intentional vents or openings. You don’t want to just inadvertently let it leak out and create cold spots in your greenhouse. As we mentioned before, bubble wrap is a great solution when it comes to insulating clear glass greenhouse walls, or doubling up against plastic sheeting. The air in the bubbles has excellent insulation qualities, just like a puffy down parka insulates your body.

Another approach to insulation is to add some row covers of horticultural fleece over the plants in your greenhouse.

The methods reviewed above can be surprisingly effective to keep your greenhouse warm, and you may not have to add any actual heat sources if you live in a somewhat temperate climate.  Just using some bubble wrap insulation and a few full water barrels may be sufficient to protect tender plants if your area doesn’t go below freezing. If anything, you may find that you have to open up vents and let the hot air out, so you don’t cook your plants. Think of how hot a car gets when it’s parked in the sun, even on a pretty mild day.

The next kind of heating system that you can consider for your greenhouse is the natural free heat that’s generated by a compost pile. If you’re a serious enough gardener to be considering a small greenhouse, you undoubtedly already compost kitchen waste and other organic material. Cornell University is working to help commercial farmers use compost as a major heat source for barns and large greenhouses, but you can benefit from even a fairly casual composting pile. A pile that’s just 10 gallons, or two large plastic buckets full, can heat up to over 120 degrees Fahrenheit.

If you’re unsure about bringing any contaminants into your greenhouse, you should know that fly larvae, weed seeds, and thermosensitive pathogens all die off at temperatures above 104 degrees. This initial hot stage is called the “thermophilic phase,” and it can last for weeks or even months. It’s a good idea to turn your compost pile once a day for a few days, to make sure the cooler peripheral areas have a period of time in the center of the pile. The compost pile delivers its heat to the surrounding environment through conduction, convection, and radiation, and the moisture from a properly wetted compost pile also adds valuable humidity to the greenhouse air.

As we move through the various greenhouse heater options, the next heating system to consider are heaters that burn some kind of fuel. In the closed greenhouse environment, combustion heaters introduce the challenge of dealing with carbon monoxide. Construction experience is necessary in order to build a safe vent or chimney out through the structure of a plastic or glass greenhouse. Here are your options for a heater that burns fuel:

• Gas heater: Unless you’re building a greenhouse that’s integral with your home (such as converting a sunporch), it’s unlikely that you’ll have access to the supply of natural gas from your utility company. You can purchase tanks of propane, and hook them up to a propane heater inside your greenhouse. Safe venting of propane is critical to expel gases, while an adequate supply of oxygen is needed in order for the gas to burn properly.

• Kerosene or paraffin heater: These heaters work by lighting a wick that draws liquid fuel up from a holding vessel. They usually include some type of safety feature that extinguishes them if they fall over, but they are still considered to pose a significant fire risk. Because these heaters release air pollutants (carbon monoxide, nitrogen oxides and sulfur dioxide, as well as polycyclic aromatic hydrocarbons and phthalates), they have to be operated in an area where adequate outside airflow is provided.

• Wood stove: Small wood stoves are a greenhouse heating option that doesn’t rely on fossil fuels. If you have access to firewood, this is a sustainable way to add a hit of occasional heat to your greenhouse. However, wood stoves are not the best greenhouse heat source if your region has cold winters, because they are very labor-intensive. In addition to procuring and hauling firewood, you have to tend the fire every hour or so in order to keep it burning well. You’ll also need to make sure that the chimney (which can get very hot) is adequately insulated at the point where it passes through the structure of your greenhouse.

• Rocket mass heater: Many homesteaders designing large greenhouses as part of a sustainable way of life are excited about the energy efficiency of this heat source. Rocket mass heaters are built as part of the original architecture of a building. They involve a high-temperature combustion chamber in which wood is burned, which is integral to a large brick or masonry mass. The combustion gases are channeled through the masonry, so that the entire thermal mass heats up and radiates its heat outwards for many hours.

The best heating system for your greenhouse is one that will be responsive to moment-by-moment temperature needs. Growing tender plants year-round in your greenhouse means that you will only need heat during certain parts of the year. Passive solar energy and maybe a warm compost pile, combined with bubble wrap insulation, are often sufficient to keep a backyard greenhouse warm almost all the time. For those extra cold nights when you need to protect your delicate plants from freezing, an electric heater is a safe, energy-efficient solution.

 During sub-freezing weather, it can be counterproductive to try to heat an indoor space like a greenhouse with a gas heater that needs venting. Those outdoor heat solutions are excellent for open-air spaces like campsites, but they are less practical when you’re focused on creating an indoor microclimate. Affordable electric heaters are available with sensitive thermostats, so they only run when the temperature drops below a certain point.

Every greenhouse owner has to contend with the issue of air circulation. If the air in a greenhouse is too still, it encourages the growth of mold and mildew. Moisture collects on foliage and other surfaces, and doesn’t easily evaporate in a humid environment when there is no breeze. During the warmer parts of the year, this problem can be solved by opening vents in the roof and letting the warm air rise and be released, drawing cooler air in through vents close to the floor. In the midst of a cold winter, however, you’ll have everything closed up tight to keep the warmth inside, and mold can become a serious problem. Using an electric heater with a fan is a great way to keep air circulating in your greenhouse even on the coldest winter days. Heaters intended for garage and workshop spaces can provide a steady air flow throughout a large greenhouse, eliminating cold spots and maintaining a constant temperature.

If you have a backyard greenhouse, you need to feel confident that any heat source you use will be entirely safe. It’s important to choose an electric greenhouse heater equipped with shutoff features that are activated if the engine overheats. In rural areas, you may have animals that find their way into your greenhouse, and you need the assurance that your electric heater will automatically turn off if it’s tipped over or if something is pushed against it. Sophisticated thermal cut-off systems are important to your peace of mind.

The answer to this question is an unequivocal “Yes!” When direct sunlight enters the south and west facing walls of a greenhouse, you run the risk of literally cooking your plants. This is why every greenhouse (and even every small cold frame) includes vents to allow hot air to escape at the top, while cool air can be let in through openings at the bottom. Even if the calendar tells you it’s winter, it’s possible for bright sunshine to overheat your greenhouse. Large commercial greenhouse operations rely on digital controls to open and close vents in response to temperature fluctuations, but as the owner of a small greenhouse, you’ll need to just check on air temperature yourself. Especially in mild California-type climates, you may need to think about setting up an electric fan even in winter, in order to create a cross breeze and protect your flowers and vegetables from overheating.

A backyard greenhouse lets you garden in a different microclimate, and during dry summer days this may mean that you create a cooler, more humid environment for your thirsty plants. Shade cloth over the greenhouse is useful to lower the temperature, while a misting fan can create good air flow and provide the moisture levels that tropical plants crave.

As winter approaches, you don’t have to abandon your garden. With a solar-focused greenhouse design and safe, energy-efficient supplemental heat, you can enjoy the luxury of fresh flowers, foliage, and fruit all year round.