A greenhouse, absent heating, is not inherently capable of avoiding frosts. The problem is not during the day, but rather overnight when there’s no sun energy to help warm the greenhouse. To avoid frosts within the greenhouse, we have to heat the greenhouse to at least a minimal temperature.
Before we get too deep, let’s talk about our goal with heating. We’re not trying to create a tropical world when it’s below freezing outside. Rather, our primary goal is to avoid frost as that’s the “real” enemy of plants. Frost occurs when the ambient air temperature drops below the dew point and both those temperatures are at or below freezing. So, our goal is to simply keep temperatures above the lowest possible dew point where frost could actually occur. (Which is around 36 degrees Fahrenheit)
We’re still relying on our plant’s natural cold hardiness to be out there and “like it,” so to speak. So, early in the season, we’re putting out those cold hardy and frost tolerant plants. As minimum lows rise, the greenhouse eventually becomes safer for frost sensitive and even warm loving plants.
Since we have a small and insulated greenhouse, we can get away with a simple space heater for our minimal heating needs. We’re generally heating against mid-20’s to high 30’s for outdoor temperatures. So we don’t need to move the heating dial all that much. We’re really only trying to raise the temperature about 10 to 15 degrees Fahrenheit.
We generally set our heating temperatures to around 40 degrees Fahrenheit. This is at the temperature probe. Remember, temperatures become striated in a greenhouse, so we want to make sure the low points in the greenhouse still remain above that minimal dew point where frost can occur.
It was really scary for us to try and use electricity to heat our greenhouse, especially given our high energy costs. But, it really wasn’t that bad! For our greenhouse, heating against temperatures of around 25 Fahrenheit takes us about 7 kilowatt hours per day. When outdoor temps rise to the mid 30’s, that drops down to a kilowatt hour a day or even less. Also, we really only have to heat at night since the sun provides us daytime warmth.
As you can see, the scale quickly shifts towards “very cheap” to use heating, compared to grow lights. This scales exceptionally well as you get more and more plants into the greenhouse. Over most of the shoulder season, our heater will typically cycle on and off throughout the night. It’s only when it’s really cold outside that it turns on full blast.
Lastly, you might notice that we have two space heaters and two heating temperature controllers. This is both for redundancy and also electrical safety. Our temperature controllers top out at 10 amps, which is well below a 1500 watt space heater. So, we use two 750 watt space heaters. This means we can experience a total failure and still be OK. Also, if one heater can’t keep up, the second can kick in to help. (Which only happens, in our case, when temps fall below 25 Fahrenheit.)
Unfortunately, all this deep knowledge of how our greenhouse performs is not a simple calculation. We tested it thoroughly and determined the limits. Those limits are more about what we’re willing to spend on energy and less about the technical limits of the systems. Frankly, we’ve never pushed that limit and don’t intend to!
Hopefully you found this deep dive into our greenhouse heating systems helpful!
