Night number one in the greenhouse was a success! Despite temperatures getting down into the mid-20’s, our plants were just fine! So, let’s do a multi-part series on our greenhouse – how greenhouses work, the design and heating systems we use and what our general goals are!
We’ve spent a lot of time pursuing what we’d call a “subarctic grade” greenhouse. How we’d define that is an effective ability to heat it, at a minimal cost. Our goal is to start using our greenhouse about 4 to 6 weeks prior to last frost, once low overnight temps solidly stay in the teens and 20’s.
It’s important to know that greenhouses aren’t inherently immune to frost. The problem isn’t during the day, it’s overnight when there’s no ambient source of heat. The only way to truly ensure frost-free conditions is external heat or enough heat retention to prevent temperatures from dropping to frost forming temperatures.
Not all greenhouses can be heated, at least cost effectively. Heat applied to the ill-prepared greenhouse can result in lighting hundred dollar bills on fire! A huge key to making this cost effective is the insulation we’ve applied to our greenhouse. We’ll dive into this concept more deeply tomorrow.
One does not simply toss a heater into a greenhouse and hope for the best. We spent a lot of time testing our systems, making sure we could maintain habitability for our plants. We also measured heating input costs, against various outdoor temperatures, ensuring we weren’t simply burning absurd amounts of money.
Our goal here isn’t to heat to warm temperatures. Rather, to provide just enough heat to the point where frost cannot form, which is about 36 degrees Fahrenheit. We set our heater to 45 degrees at the probe. Since temperatures stratify (hot air rises), this means we see about 36-38F at the plant level.
We discuss this quite a bit here, but colder temperatures aren’t inherently dangerous to most plants. (Perhaps peppers and basil excluded.) What’s dangerous to plants is that frost. It causes cellular walls to freeze, burst and given enough severity, can devastate the plant. Our goal is solely to prevent that damage.
Since I’m sure many want to know what it costs? Our greenhouse takes about 5 kilowatt hours to minimally heat against low to mid-20’s. (That’s about $1.80 a day, we can’t even run grow lights for that!) It’s about twice that if temps are in the low to mid-teens. But, if we time things right, we start at $2 a day and quickly go to almost nothing as seasonal temperatures rise.
More to come on this topic! We’ll dive into what makes our greenhouse “subarctic grade” and what we think is most important tomorrow!
