Cold frame or solar hot box?

Growing up in upstate NY, every serious gardener had one or more cold frames.  No more economical way to extend your gardening season is available than a cold frame.  These good old boys never bought cold frames, they built them.   Cold frames were what everyone did with old storm windows.  If someone was seen installing fancy new-fangled al-you-min-eeum storm windows, one of the good old boys would pull up in their old pickup and cart away the old storm windows.  The name "cold frame" correctly tells you that there was no heating element used in a cold frame.  It's simply a solar heated box that provides a bit of protection from frosts and cold nights.  Maybe cold frames would be more appealing, and better understood, if they were called solar hot boxes.  The crops that work best are those that are pretty cold hardy, crops that do well in early spring anyways; the cold frame just extends the season.  My usual cold frame inmates are lettuces and spinach grown in window boxes.  The trick is to thin them ruthlessly enough  to prevent competition; remember they sell baby greens for premium prices, so think of it as harvesting.  Here's the basic rule: lettuce seedlings 2" tall should be 2" apart; 4" tall seedlings should be 4" apart, and 8" will allow lettuces to grow to full size.  Green onions are another very easy early crop.  You can raise a lot of them in a small space.  Lettuce seedlings are also started for later transplanting.  In the fall baby bok choi is a favorite often remaining in the cold frame awaiting our pleasure until late November.  People often seem surprised that cold frames really work, but actually over heating in a solar hot box is a potential problem on any sunny day, and they need to be propped open to let some of the heat escape.  In the spring, plants are grown in boxes using a potting mix; the garden soil beneath just stays too cold.  So that will be today's main activity planting spinach, diverse  lettuces, and some romaine and bibb lettuce plantlets.  This will give us salad greens ready for eating at just about the time people without a cold frame will be planting their seeds.