Now that temperatures are (finally) starting to get a little more sane here, we can look at starting to harden off some of our more frost sensitive plants like tomatoes and others. We generally like to wait until average day time temperatures start getting into the high 40’s to low 50’s. This usually correlates to the time where overnight temperatures also start to get above freezing.
The reason we wait a little bit to harden off frost sensitive plants is just to reduce the general stress we are subjecting them to. While we’ve done some experiments taking many of them them into the high 30’s and low 40’s, we have found they generally don’t like it. Cold stressing plants isn’t “good” for them, no matter how you slice it. We conducted those experiments to find the general limits of the plants, not necessarily to define how we’d normally treat them.
Just like with our cold hardy and frost tolerant plants, these will be destined for our heated greenhouse after they are hardened off. While we do risk lows of around 45F in there, we do know that this is the lowest they will get. When temperatures warm up a bit more like they have lately here, our greenhouse tends to stay well above this for many hours after the sun goes down. We basically rely on the heat retention of our greenhouse to maintain warmer temperatures for the majority of the time. The coldest of temperatures are typically seen over the wee hours of the morning, just before the morning sun starts heating things up again.
We’ve kind of found that a lot of plant temperature tolerance deals with the “average” of temperatures. For example, if you put a frost sensitive plant outside into 20 degrees, it’s not like it’ll just die right away. It takes many hours for the cold to start the harmful freezing processes. Should you disrupt that process, and bring the plant back into safe temperatures, the plant will largely be OK. It’s not going to like you, but it’ll be OK. This is exactly the same kind of concept we’re applying to our frost sensitive plants in our greenhouse.
Our peppers remain indoors as these are some of our most sensitive plants. We might start to look at hardening them off soon, but we’re sticklers about waiting for days that are clearly above 50F to do so.