Well, it’s hard to believe, but it’s that time of year again. Hopefully you got a chance to clean your greenhouse this fall and check out all of your systems before the winter chill has set in. Now is time to concern yourself with heating the greenhouse for the winter. A lot of people think that the greenhouse will hold the heat it builds up during the day all through the night. That could not be further from the truth. A greenhouse will lose most of its heat through radiation cooling once the sun goes down. This happens because the plants, containers, beds, hydroponic systems, etc. in the greenhouse are warmer than the outside. They will lose heat by emitting infrared radiation through the greenhouse glazing.
Of course the very first thing you should do is determine what temperature you need to maintain in the greenhouse at night. A good (and inexpensive) way to monitor this is to get a max/min thermometer. This tells you how low the temperature gets at night and how warm it is during the day. They are very easy to reset for the next day’s reading.
You need to look at your plants and their requirements. There are a lot of people, even in the colder regions who grow in their greenhouses in the winter without heat. If you are growing crops such as broccoli, cabbage, lettuces, carrots, turnips, beets, radishes or other cold hardy plants you may only need to keep the frost and bitter cold from your plants. Read this article for more information on how to grow winter vegetables in a greenhouse.
A lot of things can be done in your greenhouse to increase the temperature slightly without the use of commercial heaters. You can “double wrap” them. By this I mean you can put cold frames or mini high tunnels, mini greenhouses inside your large greenhouse. This will give them an extra layer of insulation. If you have electricity you can also add heat cables to the ground inside these mini structures. That may be all you need to grow these cold hardy crops. You should be growing in the ground rather than in containers in the winter greenhouse, as the containers will not stay as warm as a bed of dirt.
There are some solar methods that can be utilized to maintain the heat. The simplest and most common is by thermal mass. The easiest way to do this is to place barrels under your benches and/or gallon jugs near your plants. They should be painted black and filled with water. Once the sun goes down, they will release the heat they have built up and help maintain the greenhouse temperature. This is a good article that discusses how to heat a greenhouse with thermal mass.
Another way to help retain some of the heat at night is to use a solar blanket. This can be as simple as a heavy blanket purchased at the local big box store. Hang it on the inside of the ceiling and pull it shut at night to help hold the heat.
Some people place their compost piles right inside of their greenhouse for supplemental heat. These piles will reach as high as 150 degrees F when they are working properly. Some people consider them a fire hazard and will place their compost piles outside and pump water through the pile to be heated and into the greenhouse. If a compost pile is functioning properly there should be no odor. If your compost is stinky like rotting eggs or a musty smell you should add more browns and if it is not hot enough add more greens to the pile.
If you are trying to grow tomatoes, or other crops that are out of season in the winter you will need supplemental heating. This can be supplied by electric, natural gas or propane heaters. I have used the small electric ceramic heaters in my hobby greenhouse, but have never been that thrilled with them. We all know that water and electricity do not mix. So, we built shelves up high out of range of our watering system to keep them dry, but I didn’t find this to be very effective heating. Since warm air rises we were losing most of our heat to the top of our greenhouse.
There are larger electric heaters that are designed to hang from the ceiling of the greenhouse. Still, I don’t like where they are located and how the warm air is going to rise. You can aim them down lower using their louvers, but that only works to an extent. There are natural gas and propane heaters available. We have been using the Southern Burner heaters for quite a while now. They are great because you can put them under a bench and not lose any valuable space in your greenhouse. The heat will circulate throughout the greenhouse at an even temperature, plus they don’t require any electricity. The natural gas or propane heaters are designed as a vented or non-vented model. We have always used non-vented, but these will not meet code in all states so check before purchasing and using one. Even though they are non-vented they still require a fresh air intake. Personally I don’t feel that this is a safety issue (in a greenhouse) unless you plan to sleep in your greenhouse, but the code is the code. And we should always follow it for our own safety.
There are many different ways to use your greenhouse in the winter. Your first consideration should always be what the needs are of the crop you are growing. Once you have decided that you just need to figure out the systems needed to create those conditions. Let’s not forget that the winter greenhouse is also a great place to go out, sit, enjoy, read a book and catch some rays.
Tammy Wylie has been using, selling and installing greenhouses since 1993. She is a lifelong gardener and the owner of AdvanceGreenhouses.com. She blogs about different growing methods at GrowYourOwnFoodAnywhere.com.