Hardening off lettuce
I have lettuce (Tom Thumb and a mesclun mix that includes black seeded Simpson, I tried to pick out the beet seeds but maybe some Tendergreen mustard in there), spinach and kale. Germinated 2 weeks ago, Tom is starting to form true leaves. Looks like spring is coming here now, most of the snow is gone and highs will be in the 40's and low 50's from now on, last night's low was 19 but still have plenty of nights below freezing coming.
It's a little early to start hardening off now, maybe next week, I want to set the plants out mid-April. DH keeps asking about building a cold frame but I don't know about hardening the flats off in that (could have direct-seeded in one if we didn't have the snow cover). I'm afraid a cold frame built out of scrap lumber or concrete block and an old window might still be too cold at night, and no need for it during the day (except as a windbreak).
What do the experts think? I haven't seeded lettuce indoors before, or used a cold frame (again, except a temporary one that was more of a windbreak and heat sink - concrete block with old sliding glass doors on top, south side open) except for tomatoes - much later in season and not overnight.