It's estimated that using a coldframe gives you about an extra zone and a half of protection.

Swanz

I like your idea about the black water containers in the frames. I'm doing the same thing and it seems to help. One way to determine that is to have a Max/Min thermometer and record the results. If you have access to straw bales you can surround the frames for added insulation. I have two frames 80" x 36" with white painted 3/4'" foamboard house insulation on the inside that relects light and has an R value of 3 (?) or so. The spinach is doing well and I've just palnted gobs of radishes and some romaine. I made mine from double insulated patio doors hinged on the high side of sloping cedar frames. I built an metal frame with old cast iron window sash weights that acts as a counter weight for the rather heavy door. I reduced the effort to open them from 35 lbs. down to 8 lbs. One way to add more days to the growing season is to cover them with another layer of protection such as a plastic and frame mini greenhouse. I've got more patio doors and I'm thinking about making a temporary greenhouse structure to cover the coldframes in order to grow my beloved veggies. I would also get another frame going because you'll outgrow that one you have now. More is better when it comes to having successive plantings. Check out Eliott Coleman's FOUR SEASON GARDENING. Tons of inspration and cold frame stuff from a master.

The key is to have mature plants once the cold weather hits. The mature plants should remain all winter. Just pick them when its above freezing. Use cold hardy greens and carrots.

Yes - have them mature first. I am learning the hard way that I planted too late, but still I am getting some salad greens etc., and have improved my plans for next year. Coleman's book is helping me too. It is a joy to absorb knowledge from someone who loves their occupation.

Any kind of thermal mass helps (water jugs, brick, earth). They heat up slower and cool off slower than the air so the temps get moderated at both ends. That's why you'll see commercial greenhouses w/ large water barrels, usually w/ a couple of planks on top to form a bench. I use water jugs in my CF. On cold nights an old blanket or 2 on top helps. For especially cold nights you could drop a trouble light in there. You'll get 4-5 degrees warmer w/ a 75 watt bulb. Just make sure you run an outdoor rated cord.