Sometimes heating your greenhouse is necessary. Greenhouses are wonderful places, especially in the spring when benches are filled with brilliant green starts, and in the summer, its doors and roof vents propped open, with cucumbers trailing from the ceiling and tomatoes ready for picking.
But in winter? Not so much. Overwintering herbs and potted plants cluster together for warmth. A few brown, leafless cucumber vines hang from an overhead trellis. Kale and spinach are over-picked and the seeds you planted have yet to sprout.
It’s a winter-time fact in most parts of the country: greenhouses, even those that might be attached to the house or garage, need some kind of heat source (of course, supplying appropriate light is equally important).
My first, only partially realized, greenhouse up in the rain forests of Washington’s Olympic Peninsula needed heating just to keep the humidity down. But in most parts of the country, cold is the problem.
You may have built your greenhouse with visions of supplying your family with fresh, year-round greens. But winter growth and germination are difficult when soil temperatures seldom climb out of the low 50-degree level.
Sure you can use an electric heating mat to encourage germination. But even the hardiest green grows slowly — very slowly — when nighttime air temperatures plunge.
If this seems to be a problem for your greenhouse, our guide to heating a greenhouse is everything you need to keep your passion for gardening alive this winter.
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History of Heating Greenhouses
Heating greenhouses was first reported in Korea in the 15th century when residents in that chilly country recognized they could add to the sun’s heat and expand growth opportunities.
Throughout the years, farmers refined their grasp of winter greenhouse technology, allowing them to accurately manage the temperature, humidity, and chemical composition of the greenhouse atmosphere.
Greenhouse heating systems subsequently became automated, with digital controls and precisely regulated air circulation, and they are now heavily used in the global agriculture economy.
Although the term greenhouse normally refers to a walk-in building, the underlying principle also applies to cold frames and hoop houses.
If a spring cold snap is expected, even the most casual gardener may cover a row of young lettuce transplants using row covers. Cold frames are essentially small greenhouses formed by enclosing a single planting bed with a glass or plastic canopy.
Why Have a Heated Greenhouse in the Winter?
If you’re someone that loves gardening all year round and love planning your plants and vegetables for next spring on cold nights in fall, you should consider setting up a greenhouse!
If a gardener has a greenhouse, he or she can add a few months to the growing season every year. There are also a few greenhouse gardeners that can grow year-round without installing a heating system.
However, in most climates, nighttime temperatures grow too cold for most plant species throughout the winter months. Most greenhouse enthusiasts will need to install some form of heating system in order to maintain the greenhouse operating all year.
Plus, knowing how to properly heat your greenhouse can be useful in other situations as well, especially if you want to grow some exotic plants, but live in an area with cooler temperatures.
Understanding How Greenhouses Lose Heat
Keeping a greenhouse at a specific temperature is a straightforward matter of thermodynamics where heat loss must equal heat gain.
The plants in the glasshouse will freeze or fry if that equation tips too far in one direction or the other. And a greenhouse can lose heat in a variety of ways.
Heat is lost through conduction as it travels into and out of the glasshouse’s components, such as the doors, fans, metal purlins, and glazing.
Other types of heat loss include infiltration and exfiltration. This occurs when heat physically escapes through empty areas, such as vents or gaps in building materials.
Even in a tightly sealed greenhouse, infiltration and exfiltration can account for up to 10 percent of heat loss.
Radiation is the last type of heat loss. Most greenhouse materials, with the exception of glasshouse glazing constructed of polyethylene film, do not allow radiant energy to flow through.
Most of the time, as a gardener, you will try to stop this heat loss by adding more heat or taking steps to stop it from happening in the first place.
But there are instances when you need to cool a greenhouse down on purpose to avoid problems with overheating. Evaporative cooling, ventilation, and fans can rapidly reduce the temperature within a glasshouse.
Ways to Heat a Greenhouse
There are many ways of heating a greenhouse, but in this article we’ll focus on the most common ways you can accomplish it easily at home.
But remember that plastic sheeting or a tarp should not be used as it will only amplify the cold and its effects, rather than warm up your greenhouse.
When picking a method of heating your greenhouse, consider the size of your greenhouse and the cost of the type of heating method.
But whether you have a small greenhouse or a large one, we’re sure you’ll find a great method here. So, if direct sunlight isn’t enough to heat up your greenhouse, here are ways to do it yourself:
Add Thermal Mass
Before you think about any heating system, it’s important to think about how to catch the heat already in the system. Take steps to increase the thermal mass in your greenhouse.
Materials with high thermal mass catch and store the heat energy from the sun slowly during the day and release it slowly when temperatures fall at night. (The ground to air heating described above is, in essence, a way to refine and manage this natural energy flow. But there are simple and easy ways to take advantage of the same effect in a smaller way.)
Materials with high thermal mass include soil, clay, stone, water, bricks, and ceramics.
By placing more of these materials in a greenhouse, we can catch and store more energy and regulate temperatures. The more thermal mass you can add, the cooler the space will remain in summer, and the warmer it will be in winter.
Use a Germination Mat
The germination mat is one kind of way to bring the temperatures you need to your greenhouse. There are as many ways of heating your greenhouse as there are greenhouses, and some of the new energy-conscious heating techniques (fuel is expensive!) are promising if not proven.
When trying to find the right germination mat, look for one that comes with a programmable thermostat like this one from Vivosun.
Try a Horticultural Fleece
On really cold nights, drape a layer or two of frost protection fleece over your plants to provide additional protection without turning up the heat. Remember to remove the fleece during the day to ensure that your plants receive adequate light and airflow.
Consider a Trench
One fuel-avoiding, sustainable-friendly method is to build a trench down the center of your greenhouse and, after covering it with palettes or some cobbled walk-way, make compost in it. This might be limited to a small hole in the center of a hobby-sized greenhouse.
Even at that, the compost will help moderate temperatures in the greenhouse and you’ll always have a ready supply of garden gold. And the daytime temperatures in the greenhouse should encourage your compost to heat up. Click on our composting guide to learn more.
Now – full disclosure – we haven’t tried this. Our greenhouse was attached to the south-facing side of the house in an attempt to get what little sun we had to give us some solar-generated temperature gain.
And it worked, a bit, even on some cloudy days, the concrete grow box and stone walkway serving as heat sinks, giving off warmth to the bathroom and the kitchen porch at night. But an open pile of compost situated (technically) inside the house wasn’t considered to be a good idea.
Hotbeds involve using heat from composting materials in the same way as a trench, but this time we’re going the opposite way.
A hotbed is essentially a raised bed that has been covered in layers of decaying straw, manure, or other organic material, with a thin layer of growing medium like soil or compost on top that can be used to sprout seeds or grow plants.
In this way, it functions as a raised bed by being essentially a compost pile that has been covered with soil or compost. Similar to any other compost pile, a hotbed is composed of organic matter. A decent combination of nitrogen-rich greens and carbon-rich brown materials is ideal.
Try Black 55-Gallon Barrels
Another way of creating a heat sink that will absorb energy during the daylight hours and give it up slowly in the cold dark is to place 55-gallon barrels (or whatever is available and convenient) in corners and other practical locations in the greenhouse. Covering the north wall of your greenhouse is also a good idea.
Make sure that they are painted black for maximum solar gain. Even buckets of water in a hobby-sized greenhouse will moderate temperatures just enough to make a degree or two difference, a difference that might be critical.
We know a guy who wrapped black garbage bags around packaged tube sand and laid those out in his small house. The bags slowly disappeared as he and his son needed more weight in the back of their pickups.
Use Electric Room Heaters
Electric room heaters are the easiest and probably most popular way to heat a winter greenhouse overnight. Be sure that you follow all safety instructions and make sure your heater is stable and away from any flammable material.
Also, take care if you’re running an extension cord out to your greenhouse. Make sure all connections, especially those inside the house, are snug.
Heat circulation is important when using an electric heater. Moving the warm air around will prevent hot spots (and their contrasting cold spots) as well in reducing condensation that heating will encourage. Some heaters have built-in fans, some need additional circulation..
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Hot Water Heating Systems
Using hot water heating systems needs careful consideration and planning, but they are quite effective when done correctly.
In principle, these systems employ a boiler to heat water, which is then pumped throughout the greenhouse through a network of pipes.
As the hot water flows through the pipes, heat is released, which warms the surrounding areas and materials. A central controller is utilized to make necessary adjustments.
Even vaporized water can be utilized in the hot water system; liquid water isn’t necessary. In these systems, steam is created by the boiler in place of hot water, and pressure rather than pumps are used to move it through the pipes. Steam will heat your glasshouse more effectively than liquid water since gases have more energy than liquids.
The ability to direct heat exactly where you want it is another remarkable feature of hot water systems. Pipes can be run beneath or on top of the floor, directly beneath growing benches, along walls, or above the plants through ceiling attachments.
Hot water is one of the most energy-efficient systems because with the right piping location, plants may be maintained warm enough without losing too much heat to voids.
There are numerous potential fuel sources for a hot water system, including natural gas, propane, wood, coal, solar energy, and other forms of electricity, each having its own advantages and disadvantages.
Unit heaters are the most basic type of heat source that a home gardener can install in a greenhouse.
It’s a terrific option for greenhouse owners who want something less complicated than a hot water system. Unit heaters, which are often fuelled by natural gas, propane, or oil, are small, economical, and trustworthy.
In essence, unit heaters like gas heaters burn fuel, which produces hot exhaust that passes through metal components known as heat exchangers.
A fan located behind the device pulls air from the greenhouse and forces it through the heat exchanger. The heat exchanger’s warmth enters the air and circulates throughout the area.
Because these heaters require such a significant amount of oxygen to function, an air line that extends to the exterior of the greenhouse can also be connected to the heater. This prevents the oxygen contained within the greenhouse from being depleted at an absurdly rapid rate.
Unit heaters can be installed anywhere in the greenhouse, often hanging from the ceiling, but hot water systems are more difficult to reconfigure on the go.
It is important to keep in mind that the energy efficiency of heating with hot air is lower than that of heating with hot water.
Radiant heaters work by heating an aluminum tube to about 900 degrees Fahrenheit, at which point it starts to give off infrared radiation.
When a nearby reflector sends the radiation outward, it hits plants and surfaces, which soak it up and turn it into heat. This heat then spreads throughout the greenhouse.
Surfaces warmed by radiant heaters are often warmer than the surrounding air, resulting in very cost-effective heating.
Installation costs can be high, and it’s important to put them in the right place to avoid cold spots. But if the cost and planning involved are not an issue for you, this is a great option you should consider.
We’ve been surprised at the number of greenhouse operators using wood heat to warm their buildings. With propane becoming more and more expensive, wood and pellet stoves, again with provided circulation, are operating effectively and more cheaply than other fuels.
Large, commercial-sized greenhouses are finding wood a viable alternative to expensive gas and petroleum products. When installing a wood stove in the greenhouse be sure to follow all your local code requirements.
Stand-alone pellet stoves are especially easy to load and operate and most come with some kind of temperature control. Some have blowers to circulate heat.
I don’t think I would put a wood stove in a plastic-covered greenhouse. Stovepipes can get very hot and the risk of melting or igniting Visqueen and other plastic covers seems great short a stove vented out through a masonry foundation or some other such careful planning.
I had hoped to put a small, slow-burning wood stove in my attached glass greenhouse, both to heat the growing things and the rooms on that side of the house; one of the unrealized dreams in my greenhouse.
What about a plastic-covered greenhouse? Insulation is a great way to conserve heat without expending fuel. I’ve seen it recommended that those with Visqueen-covered houses line the inside of their plastic with bubble wrap.
Now, again, I can’t say we’ve tried this but it makes perfect sense that the bubble wrap, with its large bubbles of air, would preserve indoor heat while letting sunlight through.
Hmmm…anybody else try this or have other ideas on heating greenhouses? You know we’d love to hear about it.
In the meantime, here’s a guide to heating greenhouses from those resourceful, greenhouse-happy British. Notice they put the bubble wrap idea first. If you don’t have a greenhouse and the idea seems impractical for whatever reason . . . how about a hot box cold frame?
How Warm Should a Greenhouse Ideally Be?
Of course, in addition to knowing how to heat a greenhouse, you must also understand how warm a greenhouse should be.
In general, a minimum temperature of around 37 degrees Fahrenheit is enough for most tender plants.
However, they can still pose problems if they become damp. So, shoot for 45 degrees Fahrenheit or 50 degrees Fahrenheit if you want to feel more at ease.
Remember that there is such a thing as an overheated greenhouse. Generally speaking, 90 degrees Fahrenheit is regarded as being too warm, so you will need to provide ventilation and shade to protect the plants.
Using a thermometer to check on your greenhouse every day will let you know if you need to lower or raise the temperature. Heating systems with built-in thermostats can help with this
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