A closed cold frame can easily reach over 120 degrees if sunny especially while the sun is high. One of my cold frames I need to have ventilated if it's sunny and 28 degrees, another is about 35 degrees and the other two frames are ok until about 40 degrees.

Depending on the outside temperature I'll prop the lids either an inch (with a block of wood)or 3 inches (with a brick). Generally by 9 am I open them fully if it's around 40 degrees.

80-85 would be ok for tomato seedlings.

Soil cables are something that's used more during the winter months. I don't use them though they might have some use during spring. The soil stays fairly warm as it is. I have Christmas lights that I use for heat if needed but I've only used them about a dozen times since November.

I've experimented with a few warm weather seedlings this spring ( 2 tomatoes and 2 peppers). The peppers have all but failed and the tomatoes are hanging around ok. I figured it was early for warm weather seedlings in an cold frame but thought I would try. We have had a snowy and cold April. As temps start climbing into the 40s overnight it'll stay closer to 55-60 in the frames at night so it should then be ok for the warm weather stuff.

I have a couple questions regarding the temperatures of 'baby' (younger seedlings, about 2-3 weeks old).

I know that stuff like broccoli, cauliflower, etc. prefer cooler temps upon germination (60-70 F degree range)

However I'm wondering if the temp should be kept warmer when they're younger until the plant becomes more established or if I can lower the temp to 60-70 range for young seedlings.

Would the 60-70 temp range for younger seedlings (even cold weather stuff with the exception of endive, sorrel, arugula, escarole, and lettuce) be too cold?

I also have the opposite question regarding the warmer end of the scale for younger seedlings; would it be too hot for cool weather crops.

How long should the 80-85 degree temp range be kept for pepper tomato and eggplant seedlings? (how old, how many true leaves should they have etc.)

Or is the 80-85 temp range the ideal growing temp range for them or is that too hot?

Also what is the growing temp range for squash seedlngs I know they prefer warmer temps, but what is the temp range for them? Is 70-80 degrees good; or is 80+ too hot for them?