The lettuces, spinach, kale and cabbage would be fine out in a coldframe. I've been growing all 4 of those all winter long. The spinach has been doing quite nice. The warm season stuff cucumbers, tomato, squash and melons are okay in the coldframe but not until 2-4 weeks before your last frost date. If you put them out earlier than that they may get too cold at night and become stunted. Unless maybe you want to have a heated coldframe which is them called a hotbed. I use Christmas lights for extra heat. I thought about the buried cable option but I have a way of accidentally digging up things that are buried. As digdirt said coldframes/hotbeds are something that one needs to monitor every day. I use 4 different methods. The frames wide open for days above 40F, cracked open about 3 inched for days 30 to 40, cracked open 1/2 inch for days 25 to 30F and closed fully below 25F. Just harvested a bunch from this frame. :-)

I doubt you'll prevent them freezing at night on the balcony. You'd best bring them inside. ~tom

The rock wall is a great idea! I hadn't thought of that! The plastic doesn't completely touch the ground on the back side and a rock wall (if I can fit it in there without crushing the asparagus on that side) would close the gap and be a neat way of helping store heat under the hoop cover. Thanks! To keep the temperature of the cold frames (AKA sun boxes) where I want it, I merely slide the glass on top like opening a window a crack or make it a wide opening. This allows the heat to escape. I don't have much going on in them yet. I did plant carrot, radish and spinach seeds this weekend in the shallow ones as an experiment. It is supposed to warm up into the high 60s\70s at the tail end of this week, so it will be interesting if I get some germination with that. I collected more pieces from that old trampoline. The curved portions that make up the giant ring.. as it turns out can also fit together into a U-shape also (albeit curvy), but they are taller, so with more plastic I think I could get away with another hoop house a little taller on the other side. Maybe the big Swiss Chard and garlic chives would like that?

My cold frame is more of a mini greenhouse. It's built from a pressure treated frame, some rebar hammered in the ground, and pvc pipe shoved over the ends of the rebar. The plastic is your run of the mill 6mil contractor's plastic. I expect to get 2 years out of it. By late April, that frame will become a veggie plot. All the pipe, rebar, and plastic will be rolled up and stored in the basement. I made sure during the coldest period to close it up tight so that the heat inside would warm the soil and keep my newly rooted cuttings alive. Now that it's getting warmer, I have to open and close it every day. I have wintersown some things in there though. All veggies. I'll also use it to start my annuals in another week or two. I'll use storage totes to put all the little cups in and bottom water as needed. The wintersown containers I use are open to the elements and offer only a little protection. They don't need to be hardened off at all since I had them in the sun. I only needed to take the tops off a day or two before I planted them out. I don't think I'll be as lucky with the seedlings in the cold frame. They'll need to be given a little more care at night, but that could be the annuals talking. Protected from the elements. These didn't get babied at all. For me, the difference is in cold hardiness.