You have to vent, or your crops cook on a 45-degree day
Last lesson was row covers, hoops, wind. Today we're back to the cold frame, because there's a failure mode with these boxes that catches almost everybody the first year, and it's not the one you'd guess.
Everyone worries about the cold. That's the whole point of the box, right, keep the cold out. So people build it, close the lid, and walk away feeling good about themselves. Then a sunny 45-degree day shows up in March and by 2pm the inside of that frame is pushing 100 degrees and everything in there is limp and cooked. Oh man, it's a rough thing to discover. You did all this work to protect the plants from cold, and heat is what gets them.
Here's what actually happens. That window is a greenhouse on a small scale, which is the whole point, but a greenhouse with no airflow doesn't know when to stop. The sun comes in, the glass traps it, and the temperature inside climbs way past what the air temperature outside would tell you. On a still, sunny day, even a cold one, a closed frame can run 40 or 50 degrees hotter inside than out. Your lettuce doesn't care what the forecast says. It cares what it's actually sitting in.
Venting, the actual routine
This isn't complicated, but it does mean you can't treat the frame as something you set and forget.
Prop it open on sunny days, even cold ones. Once the sun's been up an hour or two and it's clear, crack the lid using that prop arm we built in a while back. A few inches is often enough. On a warmer sunny day, prop it most of the way open.
Close it back up before the sun drops. The whole reason you built the frame is to hold heat overnight. If you leave it vented after the sun goes down, you're giving that heat right back and you've gained nothing.
Overcast days need less venting, sometimes none. Without direct sun hitting the glass, the frame won't run away on you the same way. Check it anyway. Don't assume.
Check the thing more than you think you need to. I still go out and stick a thermometer in mine more often than is probably necessary. I'll admit that's a personal quirk, I don't fully trust a soil or air reading until I've checked it twice. But especially your first season, get in the habit of actually looking, not guessing from the kitchen window.
If you're gone all day at work, this is the real limitation of a cold frame versus something with automatic venting. Honestly, most home gardeners don't need automatic venting, that's a bigger investment than the problem calls for. But you do need somebody checking that box on a sunny day, even if it's a neighbor or a kid with a job to do.
Why I keep a frost calendar taped inside the shed
My first year gardening, I planted tomatoes the second week of May because it felt warm enough. It was one of those stretches where the afternoons were pleasant and everybody's yards were getting green and I figured I was past it. Third week of May, a frost came through and wiped out every single plant. Total loss, replant from scratch.
That's the mistake in one direction, trusting a warm stretch too early. Venting is the same mistake pointed the other way. You get so focused on the frost date and the cold snap that you stop paying attention to what a sunny afternoon will do to a closed box. I've kept a frost calendar taped inside my shed door ever since that first May, and it's not really about the calendar date, it's about what it trained me to do, which is check the actual conditions instead of trusting what the season is "supposed" to be doing. Frost dates are a suggestion, not a promise, and so is "it's still cold out, the frame's fine closed up." Neither one replaces walking outside and looking.
Before next time
Next time you've got a cold frame with anything living in it, go check it around 1 or 2pm on the next sunny day, cold or not, and see what the inside temperature actually is compared to what you'd expect. Most people are surprised the first time.