Do Greenhouses Need Heaters?

Are you having a hard time saying goodbye to your garden? If you live in a climate zone where winter brings short days and cold nights, your outdoor gardening options are reduced to a few frost-hardy overwintering species. Setting up a greenhouse allows you the pleasure of year round gardening. It’s a wonderful feeling to be able to make a salad in mid-winter from your own fresh lettuce, or to pick fresh flowers to grace your holiday table. If you’ve been held back from this delightful experience because you thought that a greenhouse heating system would be unaffordable, it’s definitely time to take a second look. Greenhouse heating can be supplied from a combination of sources, and some of them are even free! Take a look at how easy it is to heat your greenhouse.

What Are Greenhouses?

Basically, a greenhouse is any transparent structure that provides plants with a better growing environment than is available outdoors. Sometimes supplemental lighting and heating are added, to counteract short daylight hours and cold winter temperatures. Greenhouses can be made of glass or clear plastic, and they can be as large or small as you want. Giant commercial greenhouses cover acres of land, and their interior climate is controlled by sophisticated digital automation. On the other end of the spectrum, home gardeners sometimes build cold frames or hoop houses around individual garden beds, extending the growing season for a few tender plants. Many garden supply companies sell greenhouse kits, with prices that start at less than a hundred dollars. You can also find plans online for building greenhouse frames, which you can complete with your own plastic sheeting or glass.

Quick Introduction to How Greenhouses Work

Greenhouses are made of transparent (or translucent) material so that the sun’s radiation can enter them. This radiation is absorbed by the plants and earth in the greenhouse, which then release warmth inside the structure. Since at least some of that warm air is prevented from leaving, the interior environment stays warmer than the outdoors. However, you can’t simply close up your plants in a glass or plastic bubble; ventilation is just as important as heating, for the following four reasons.

• Temperature: If your greenhouse doesn’t have adequate openings to release excess warmth, it can easily overheat. Think about how hot a parked car can get, even on a temperate day.

• Pathogens: A whole host of fungus, pests and plant diseases can get started in a moist, stagnant environment. Circulating air prevents many of these problems, partly by preventing excess moisture buildup.

• Air composition: Plants consume carbon dioxide during the day and oxygen during the night time. You’ll want to give your greenhouse plants an ample supply of both.

• Pollination: Many plants are pollinated by wind. You don’t need to create an actual breeze, but you do need enough air flow to carry the tiny grains of pollen from one blossom to another.

The Question of Light

The farther north you travel, the shorter the days become in winter. Warmth is not the only factor that plants need in order to grow year round; they also need a certain minimum day length. Grow lights provide the wavelength of light that plants need for photosynthesis. Since grow lights imitate the sun’s radiation, they are different from ordinary household light bulbs. Professional farmers and growers dive deeply into the technology of grow lights, and this has become a complex science. A backyard gardener looking to set up a simple greenhouse, however, can get all needed lighting supplies and instruction from an indoor gardening supply store.

Why Do Greenhouses Need Heat?

Plants are constantly undergoing respiration and photosynthesis. These are chemical reactions, which need a certain amount of energy in order to happen. The most basic energy comes in the form of heat. If the temperature falls too low, all the vital processes of life slow down. If you live in a sunny place with only moderate temperature drops in winter, you may be able to get by with an unheated greenhouse. The radiation provided by sunlight would be enough to raise the air temperature inside, and your largest task might be to make sure that your greenhouse doesn’t overheat on bright winter days. However, when it’s below freezing outside, the mere presence of clear plastic or glass walls won’t magically keep your greenhouse warm enough to protect your plants.

Types of Greenhouse Heating

Since you have so many options for heating your greenhouse, we’ll start with the non-powered ones. The first set of these are passive, just making the best use of the sun’s natural radiation.

Location

In the northern hemisphere, winter sunlight comes from the south. In Canada and the northern states, therefore, it’s essential that the translucent portions of your greenhouse should all be south facing. The north wall won’t ever receive direct sunlight, so it can be made of wood or even built against the south side of your home.

Insulation

To save energy, greenhouses can be insulated just as your home can. The only difference is that greenhouse insulation has to be transparent. Bubble wrap is a great solution, since it provides an insulating blanket of air and is very clear. Horticultural bubble wrap will last longer than the kind you use for mailing packages, because it is more durable and UV resistant. You can also use bubble wrap insulation to protect outdoor flower pots to prevent them from freezing and cracking.

Another approach to insulation is to focus on individual planter beds. Even within your greenhouse, you can use row covers or horticultural fleece to slow heat loss from the soil at night. In most cases, these should be removed during the daytime.

Thermal Mass

Another energy-efficient way to heat your greenhouse is to use 55 gallon drums of water as a heat sink. (“Heat sink” just means anything that absorbs heat.) This large volume of water has significant thermal mass, which means it is able to hold heat and then slowly release it. During the day, the water in the barrels heats up because the surrounding air is warm. At night, the water slowly cools down, releasing its warmth into the greenhouse air.

Compost as a Heat Source

If you’re a serious enough gardener to want a greenhouse, it’s quite likely that you already compost your kitchen waste and garden trimmings. The process of decomposition generates heat, and a well-kept compost pile can maintain a temperature of 100 to 160 degrees Fahrenheit. You can simply make a compost pile in one corner of your greenhouse, or you can dig a compost trench that extends for the entire length of the structure. Large greenhouses sometimes even run water pipes through the compost pile, circulating the warm water throughout the entire structure.

Heating Your Greenhouse by Wood

Some farmers and gardeners who want a serious source of hot air for their winter greenhouse may not have an available source of electricity or gas to power a space heater. In such cases, it’s possible to use a wood stove as a source of heat, simply stoking the fire in response to each day’s weather. Wood stoves aren’t viable in a lightweight plastic greenhouse, however; they require more substantial architecture that can safely accommodate their chimney pipe. Rocket mass heaters are an interesting variation on regular wood stoves. These super-efficient wood-burning stoves are built in to a massive chamber of cob, stone, or brick. The heavy material acts as a thermal mass, releasing gentle heat long after the actual fire has gone out. Any heat source that uses combustion runs the risk of releasing dangerous carbon monoxide (CO), however, so it’s important to install a CO detector.

Heating Greenhouses With Electricity is Easy

Electric greenhouse heaters have the advantage of being easy, safe and reliable.

A space heater with a fan offers the additional benefit of providing healthy air circulation, so that there’s less risk of mold and fungus developing on plants. Furthermore, the air flow throughout the structure will eliminate cold spots, as the hot air rises from floor level and is eventually vented through openings in the ceiling.

 Portable garage heaters are perfect for greenhouses, because they include a thermostat with an automatic shutoff, and they can be easily mounted out of the way on a wall bracket. A small greenhouse can easily be heated by a ceramic heater, which comes with a programmable timer so that you never have to worry about whether you turned it off. Electric heaters can also be combined with all the passive construction elements mentioned above, making a well-insulated winter greenhouse very inexpensive to heat.

 If you’ve always wanted to try your hand at greenhouse growing, or if you feel that having tender new greens and fresh flowers would make your winter be happier, don’t hesitate to jump in and give it a try. Affordable greenhouse kits and efficient electric heating can get you started on this rewarding hobby.