That's a loaded question. It depends on your latitude, whether there are clouds that night, soil temperature, wind (try to shelter it), location (south-facing is best). Only a thermometer will tell you, and the difference will vary with time of year. Also, what type of film? If using plastic, I recommend 6 mil. Is there wood in your cold frame construction or all plastic? A cold frame with lumber as some of the walls will hold more heat at night. With only plastic and PVC, it's more similar to a low tunnel than a real cold frame. Will 50 F outside fry them? If it's sunny and February, yes definitely. It may rise into the 90s or higher inside. If it's December and dark and cloudy, leave it closed. If it's the type of cloudy day that's still bright, open it a bit. Only experience with your setup can tell you exactly how much to open for a situation. At night you will get anywhere from 2 to 12 degrees F of moderation but this really depends on the factors I listed, and more. A low tunnel type of setup gets much colder than a traditional cold frame. You can improve heat retention by another 2-6 degrees by tossing a heavyweight floating row cover fabric or blanket over the whole frame at night. Lastly, don't grow any warm season crops in your cold frame in winter. You probably knew that. For cool season crops, freeze protection is not vital but the bigger the moderation, the more vigorous they will be and the longer they'll survive. They'll easily take 20 F and well below now and then, but if that's *every* night with no recovery in day temps. to at least the 50s, you'll start to see damage. The amount of moderation you get increases with lowering outside temp. If it's 10 F, and you have a blanket-covered, wood-framed cold frame in a sheltered spot, it might be 25 F or higher inside. Possibly. If it's just single layer plastic all around in an exposed place, it might get down to 12 F inside. Do not mulch the soil in fall. Although mulch conserves soil warmth, that very factor leads to very cold air pooling above the mulch on clear nights.

Northerner, I brought in my hot and sweet pepper plants (Bells, Anchos, Italian sweet, DArbol, Poblanos, Serranos) to overwinter, because I prefer not to sow them again if I don't have to-- indoors or out. I'd like to get fruit earlier on all of them next year. Peppers don't reseed for me.(Incidentally, I sowed all of these indoors last year...good thing, because it was an exceptionally cool summer) Tomatoes, on the other hand,reseed like the dickens around here. I usually pull up the volunteers now so I don't get any diseases. When I plant WSed Toms, I traditionally get fruit about two weeks after my SIL (who puts in store-bought plants). Once the weather gets warm, my plants have caught up to hers. And then, I'm getting fruit for at least two weeks after she's pulled her plants out of the ground. I am 3 miles from Lake Ontario, but I'm pretty sure other Zone 5ers that are not so close to a body of water WS Toms too. And if you saw anything on the Weather Channel about our storm last winter, you'll know this is not a warm place. Feb. is traditionally in the negative temps, often doubledigit-negative at night. Bottom line, you should do what you're comfortable with. But if you can spare one Tom seed or two, I suggest at least trying to WS them, to see what happens and compare. I won't waste room indoors on Toms anymore. I save it for the things that don't WS well for me. And some things, I've Spring or Summer Sowed, then overwintered indoors and it works really nice. I did this with Heliotrope, Bay Laurel, Gardenia, and Datura-- things that didn't or wouldn't WS well or at all here. This might also be a way to go for Peppers, although you'd have to plan a year in advance. Linda

When I was in grade school we lived on one site and gardened in another. The site where we lived was too shady for a garden so we made the garden on a another ranch five miles away. In those days you could get your tomatoes off to an early start by using what was called a "Hotcap" over each plant. If the sun was allowed to shine on the "Hotcap" in a few minutes the plant was cooked. We were hard pressed to get over to the garden to remove the hotcaps every sunny morning, and we did not always make it. Al

I am in micro zone 6b and I have 2 in ground tangerine trees that I covered with glass. It was unheated unless the temperature dropped below 15 F. I added a little heat via Light bulbs. In February the temperature went above 100 F 4 time and hit 80 many days. My trees put on 75% of their growth between October through march 3 years in a row. Make sure your roots stay above 55F and they will do fine. I foliar feed 24-8-16 miraclegrow through out the winter. I root fed at half strength every 2 week with warm (100F) water solution to insure the roots stay warm. my ground was well insulated with 6 inches of mulch. Keep the mulch 6 inches from tree trunk and have mouse traps loaded to kill bark eating mice. Use peanut butter bait.8 feet tall, 2.5 year from seed sprouting in ground and covered with glass October till May.