I believe I’m one of the few NYC dwellers who maintains several heads of lettuce through early December. The few really chilly nights we’ve had this fall, I’ve brought the containers in from my terrace to my (chilly but not freezing) spare room, and then brought them outdoors once it warmed up.
My question is, now that I’ve found that I can leave the lettuce outdoors when it’s 32, or even 31 or 30, and they recover (probably because the temp. reading is a little higher on my terrace than it is in Central Park–ambient heat from the building or something) but is there an absolute killer number where the plants will just perish, even if the building is giving off a little heat? Will they survive, for example, at 25 degrees Fahrenheit? Is there any way to predict this without simply seeing if they die or not?
It’s dependant on too many factors when you are using a remote point as your temperature gauge. While building heat can be significant, usually the biggest factor in maintaining a higher temperature in these situations is the roof. Without a clear ‘view’ of the night sky temperature is lost much more slowly. Of course this factor is mitigated by even a moderate wind. Rate of heat loss is also dependant on humidity, to a minor extent on air pressure and on several other factors. So your plants might be perfectly alright on calm 25o night with a low pressure and low humidity. On a 30o night with a moderate wind and high humidity they could freeze solid. It’s pot luck.
You didn’t mention a variety. True head lettuce does not handle freezes at all well. Leaf lettuce does the best, esp. the red varieties like Ruby and Red Sail. It also depends on how well they are hardened off. A sudden hard freeze is bad. Several days of progressively cooler weather not so much.
I always have red lettuce that survives each winter, with others like Black Seeded Simpson and Oak Leaf usually surviving. I also mulch with chopped leaves. But then again, the coldest it gets here is usually in the high teens.