Survival under plastic

So here are my "poster-seedlings" for hoop houses in the cold. Red Cabbage and Brussels Sprout seedlings raised under lights and on South-facing window sills in the house, in seedling flats and then later transplanted into newspaper "plant bands" in sawed-off milk cartons. Transplanted 3-4-08 into "hoop house" in the garden, mulched with straw and row cover doubled over it at night. Minimum outside temperature since then has been 12.2 degrees F. Minimum temperature inside the hoop house has been 25.2. After the first cold, snowy night, I did decide to add more straw so the little plants were about an inch under straw. Today I unburied them and they took some sun.

Here are more seedlings having their afternoon in the hoop house- Viroflay Spinach, New Zealand Spinach, more broccoli, cabbage, onions, and Brussels Sprouts. They will spend the night inside under lights for part of the time (they get a total of 16 hours a day).
These are Siberian Kale coming up in abundance in a "hoop house". Why am I not surprised that the Siberian Kale is doing well? I got these seeds from "Seeds of Change" FYI

Golden Chard coming up. Oh how do I love Chard!

Hard to see, but this is lots of Orach

coming up (back of leaves and stems

are red). It helps to remember that there are no tender, green things growing outdoors yet other than the grass and some weeds that are just starting to get going. Even the evergreens are looking a bit weary about now. It is March in Boulder after all. None of the above plants I'm growing would be able to survive under current conditions without cover. As I search for seeds in various seed supply catalogues, I am looking not just for frost-hardy plants, but also plants that can hopefully tolerate some heat, because our temperatures vacillate between extremes here in this high, relatively arid climate.