We've been opening it during the day, maybe sometimes getting to it a bit late, after the sun has hit it for a little while, but last night they were left open and still again this morning there were a few wilted ones...

Sounds like damping off, especially when you say the stem looks like it's been cut. Might be too moist in there if you're growing moss too.

Just did a bit of searching and definitely fits the profile for damping off, is there anything that can be done at this point to save the rest of them?

Were they grown from seed in the cold frame or were they recently transplanted into the cold frame? If transplanted were the seedlings well hardened off? Lettuce takes a few days to establish deep roots when transplanted and needs daily watering until well established.

They look too big to me at this point to be damping off.

They were transplanted around April 16th and only started wilting last week during a hot spell...

OK we can rule that out. If it is only affecting some of the plants it may be that you have a insect pest that my be doing damage to the plant roots. Possibly even a rodent - vole or ground squirrel digging under the bed.

Do they rebound overnight? You might sacrifice one plant and dig it up to see what the root system looks like.

Thanks Jack. There aren't any underground animals to contend with where we are (inner city). Does it help to know that all of the other plants (kale, mache, spinach) in the coldframe are absolutely fine? I'll see if they rebound when the weather cools this evening. I may dig one up and inspect the roots....

When a whole plant keels over suddenly, it's usually due to cutworm in my garden. Pull one up to confirm if the base of a wilted plant have been severed by cutworms or not. If the problem's cutworm, place stout twigs adjacent to the remaining plants.

Daren

Thanks Daren. I just found one lying there on the ground and when I picked it up it was not connected to the root system anymore. I fished around and there were some roots but nothing seemed significant to me. The reason I don't think it's cutworm however is that it takes a very long time for the plant to go from "nice" to "flat", and we even had one rebound briefly from "wilted" to "nice" and then to "flat".