I am using raised beds with plastic covered hoop houses to grow my winter vegetable garden. The plastic is 6-mil and I've sewn-in zippers for easy access.
Right now I have - peas, carrots, broccoli, cauliflower, cabbage, kohlrabi, swiss chard, celery, spinach and a few other greens. I also have some beautiful lush tomato plants with lots of slowly ripening tomatoes. (Picked the last of the eggplant and peppers yesterday.)
Sunday night it got very cold and I carefully covered everything, but I'm sure I didn't open fast enough Monday morning and burned some of the tomato leaves. I also lost one broccoli plant that got WAY too hot.
My question is - Is there a "best temperature" to open the covers?

I have thermometers in some of the beds so I can tell the inside temp.
Thanks for your help.
Gloria