So, we very briefly mentioned the other day that we’re riding out our greenhouse as long as we can this season. Let’s talk about the metrics we’re looking at and how we’re the decision to keep going or not.
As we’ve mentioned in the past, we went through the effort of insulating our greenhouse, or basically sealing it with UV rated plastic. This allows us to get a huge amount of heat efficiency and also makes it practical to heat it. We can guarantee minimum temperatures in the low 40’s, even when outdoor temperatures dip into the 20’s. Since it’s relatively air tight, we’re not just heating the outdoors.
We’ve also set up our greenhouse for fairly extensive monitoring of the various inputs. We know how much electricity we spend on using our fans and heaters, plus we can continuously monitor and log the respective temperatures within. With this data at hand, we can make highly informed decisions based on both cost and efficacy.
We know a lot about how our greenhouse performs in the spring, like when we’re hardening off our plants and starting to move things to our greenhouse before frosts clear. But, we don’t know much about the “other” direction, since we’ve often just shutdown our greenhouse when its convenient for us. The goal of this test is to learn what is “reasonable” when it comes to our kit. This is stuff you can’t read in books or learn on a website. You just have to do it.
What we know right now is that with dips into the mid 30’s, we’re spending about .5 to 1.5 kilowatt hours to heat our greenhouse overnight. Translated to cost, that’s between 15 and 45 cents per day. That’s the kind of money where we can afford to do experiments “for the sake of science.” But, there’s definitely a line where the cost inputs become too great.
Honestly, we haven’t yet set a “deadline” or “cost” for when we’ll harvest everything out and shut it down. But, we are reasonable people that don’t want to spend tons of money on heating a greenhouse in the late fall and even early winter. Perhaps our deadline will be when it snows, or maybe it’ll be when our heaters can no longer keep up. Or, maybe we’ll just decide we don’t want to garden in the cold anymore. Our guess is that our hard limiter is going to be when the day time sun can no longer support further maturation of our tomatoes. Basically, that’d be the point where it’s struggling to get above 50F during the day.
The reality is that UV rated plastic and a relatively air tight design is not a miracle. We’re not talking R35 insulation levels! It’s maybe something like R3 at best. But, for a greenhouse that “usually” depends on the sun warming up ambient air temperatures, that can make a huge difference. We often see our greenhouse take many hours after darkness falls before temperatures equalize with the outdoors, to the point where external heat is needed.
We are prepared to move our various gardening equipment into the greenhouse as soon as we do shut it down. We really don’t enjoy gardening once the snow hits the ground and outdoor temperatures fall below freezing, so we want it to be as easy as possible to shut down.
So, we do think it’ll be “soon”…but that could be late September or maybe even early October. A lot of it’s going to depend on how rapidly we start descending into winter and whether we get favorable conditions in the coming weeks!
