Think temperature! If the temp inside rises too high open them. Close them later in the day so they can warm up. All depends on different factors. Sunny, cloudy, windy,outside air temp. You can put automatic openers on them. They use them in greenhouses. Do a web search for Growers Solution and you will find some there. Depending on what you grow you can add a heat source also. Could be as simply as a light bulb. Some people cover them with blankets on very cold nights. And snow makes a good insulator and the light goes right thru it. For the most part sunny days above 45 degrees can cause temps inside to rise above 90 or 100 and simple cook your plants. Put one of those thermostats that records hi and lo temps in there and see how it changes. I have a remote thermostat that transmits my greenhouse temp to the reciever in the house. This has saved my plants, one night the heat went out and temps dropped. I woke up at 4 AM went to frig for a drink and checked temp. It would have frozen everything. Next on my list is temp alarm.

Hi rb55. Thanks so much for answering! (I was beginning to wonder where everyone is, lol!)

Your answer is full of great info that I will definitely use - I wasn't aware that a 45 degree day could raise the temperature inside a cold frame so much.

I realize now though that I wasn't at all clear in my original post. I wasn't looking so much for info on opening the cold frames for temperature control (although this is something I really didn't think of and now realize is important to know).

What I was wondering is when is the optimum time to open them for harvest, etc. For example, I wanted to cut some greens and also wanted to add some water bottles to help with heat retention. But I wasn't sure when the best time to open the frame would be to do so. The temperatures here dropped so dramatically and so quickly (we were about 15 degrees below normal this past week) and being a newbie to this whole cold frame business, I didn't know if the timing of opening made a difference to the plants and if so, how.

That's dedication, going out at 4AM to save your plants, lol. If my experiment with cold frames goes well this winter, hopefully next year I will expand and perhaps get some of those gadgets too.

Thanks so much for your help.
:)
Dee

Well now that's another story. Harvest time. You are much less limited on when to harvest. On any sunny day the coldframe will heat back up rather quickly. Most winter crops will take a little chill without much problem. Lettuce will actually survive light freezing. The water in the leaves retreats into the stem preventing severe damage. So you can open just about any time, depending on how long you leave them open. The heat in the ground will also warm them up. It is tropical plants that cold air is hard on. And that temperature rise I wrote about varies according to the time of year (angle of sun), wind, and type of covering ect. I stated a high temp. to be safe but 80 is easy to reach.

4 AM? With a couple thousand dollars worth of tropicals, one being my pride and joy Vanda orchid that is now worth several hundred. I ran out in my slippers and the clothes I was sleeping in! Seeing the temp at 36 degrees in the greenhouse will send most any orchid lover into cardiac arrest.....

Roger

Thanks Roger. I had a feeling I was overthinking things. I won't be so worried about it now.

I haven't gotten to orchids yets. I'm still on lettuce, lol. Maybe after I master that, I'll step up, lol. Thanks again for your help!

:)
Dee